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* gccint: (gccint).            Internals of the GNU Compiler Collection.
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   This file documents the internals of the GNU compilers.

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File: gccint.info,  Node: Patterns,  Next: Example,  Prev: Overview,  Up: Machine Desc

9.2 Everything about Instruction Patterns
=========================================

Each instruction pattern contains an incomplete RTL expression, with
pieces to be filled in later, operand constraints that restrict how the
pieces can be filled in, and an output pattern or C code to generate
the assembler output, all wrapped up in a `define_insn' expression.

   A `define_insn' is an RTL expression containing four or five
operands:

  1. An optional name.  The presence of a name indicate that this
     instruction pattern can perform a certain standard job for the
     RTL-generation pass of the compiler.  This pass knows certain
     names and will use the instruction patterns with those names, if
     the names are defined in the machine description.

     The absence of a name is indicated by writing an empty string
     where the name should go.  Nameless instruction patterns are never
     used for generating RTL code, but they may permit several simpler
     insns to be combined later on.

     Names that are not thus known and used in RTL-generation have no
     effect; they are equivalent to no name at all.

     For the purpose of debugging the compiler, you may also specify a
     name beginning with the `*' character.  Such a name is used only
     for identifying the instruction in RTL dumps; it is entirely
     equivalent to having a nameless pattern for all other purposes.

  2. The "RTL template" (*note RTL Template::) is a vector of incomplete
     RTL expressions which show what the instruction should look like.
     It is incomplete because it may contain `match_operand',
     `match_operator', and `match_dup' expressions that stand for
     operands of the instruction.

     If the vector has only one element, that element is the template
     for the instruction pattern.  If the vector has multiple elements,
     then the instruction pattern is a `parallel' expression containing
     the elements described.

  3. A condition.  This is a string which contains a C expression that
     is the final test to decide whether an insn body matches this
     pattern.

     For a named pattern, the condition (if present) may not depend on
     the data in the insn being matched, but only the
     target-machine-type flags.  The compiler needs to test these
     conditions during initialization in order to learn exactly which
     named instructions are available in a particular run.

     For nameless patterns, the condition is applied only when matching
     an individual insn, and only after the insn has matched the
     pattern's recognition template.  The insn's operands may be found
     in the vector `operands'.  For an insn where the condition has
     once matched, it can't be used to control register allocation, for
     example by excluding certain hard registers or hard register
     combinations.

  4. The "output template": a string that says how to output matching
     insns as assembler code.  `%' in this string specifies where to
     substitute the value of an operand.  *Note Output Template::.

     When simple substitution isn't general enough, you can specify a
     piece of C code to compute the output.  *Note Output Statement::.

  5. Optionally, a vector containing the values of attributes for insns
     matching this pattern.  *Note Insn Attributes::.


File: gccint.info,  Node: Example,  Next: RTL Template,  Prev: Patterns,  Up: Machine Desc

9.3 Example of `define_insn'
============================

Here is an actual example of an instruction pattern, for the
68000/68020.

     (define_insn "tstsi"
       [(set (cc0)
             (match_operand:SI 0 "general_operand" "rm"))]
       ""
       "*
     {
       if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
         return \"tstl %0\";
       return \"cmpl #0,%0\";
     }")

This can also be written using braced strings:

     (define_insn "tstsi"
       [(set (cc0)
             (match_operand:SI 0 "general_operand" "rm"))]
       ""
     {
       if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
         return "tstl %0";
       return "cmpl #0,%0";
     })

   This is an instruction that sets the condition codes based on the
value of a general operand.  It has no condition, so any insn whose RTL
description has the form shown may be handled according to this
pattern.  The name `tstsi' means "test a `SImode' value" and tells the
RTL generation pass that, when it is necessary to test such a value, an
insn to do so can be constructed using this pattern.

   The output control string is a piece of C code which chooses which
output template to return based on the kind of operand and the specific
type of CPU for which code is being generated.

   `"rm"' is an operand constraint.  Its meaning is explained below.


File: gccint.info,  Node: RTL Template,  Next: Output Template,  Prev: Example,  Up: Machine Desc

9.4 RTL Template
================

The RTL template is used to define which insns match the particular
pattern and how to find their operands.  For named patterns, the RTL
template also says how to construct an insn from specified operands.

   Construction involves substituting specified operands into a copy of
the template.  Matching involves determining the values that serve as
the operands in the insn being matched.  Both of these activities are
controlled by special expression types that direct matching and
substitution of the operands.

`(match_operand:M N PREDICATE CONSTRAINT)'
     This expression is a placeholder for operand number N of the insn.
     When constructing an insn, operand number N will be substituted
     at this point.  When matching an insn, whatever appears at this
     position in the insn will be taken as operand number N; but it
     must satisfy PREDICATE or this instruction pattern will not match
     at all.

     Operand numbers must be chosen consecutively counting from zero in
     each instruction pattern.  There may be only one `match_operand'
     expression in the pattern for each operand number.  Usually
     operands are numbered in the order of appearance in `match_operand'
     expressions.  In the case of a `define_expand', any operand numbers
     used only in `match_dup' expressions have higher values than all
     other operand numbers.

     PREDICATE is a string that is the name of a C function that
     accepts two arguments, an expression and a machine mode.  During
     matching, the function will be called with the putative operand as
     the expression and M as the mode argument (if M is not specified,
     `VOIDmode' will be used, which normally causes PREDICATE to accept
     any mode).  If it returns zero, this instruction pattern fails to
     match.  PREDICATE may be an empty string; then it means no test is
     to be done on the operand, so anything which occurs in this
     position is valid.

     Most of the time, PREDICATE will reject modes other than M--but
     not always.  For example, the predicate `address_operand' uses M
     as the mode of memory ref that the address should be valid for.
     Many predicates accept `const_int' nodes even though their mode is
     `VOIDmode'.

     CONSTRAINT controls reloading and the choice of the best register
     class to use for a value, as explained later (*note Constraints::).

     People are often unclear on the difference between the constraint
     and the predicate.  The predicate helps decide whether a given
     insn matches the pattern.  The constraint plays no role in this
     decision; instead, it controls various decisions in the case of an
     insn which does match.

     On CISC machines, the most common PREDICATE is
     `"general_operand"'.  This function checks that the putative
     operand is either a constant, a register or a memory reference,
     and that it is valid for mode M.

     For an operand that must be a register, PREDICATE should be
     `"register_operand"'.  Using `"general_operand"' would be valid,
     since the reload pass would copy any non-register operands through
     registers, but this would make GCC do extra work, it would prevent
     invariant operands (such as constant) from being removed from
     loops, and it would prevent the register allocator from doing the
     best possible job.  On RISC machines, it is usually most efficient
     to allow PREDICATE to accept only objects that the constraints
     allow.

     For an operand that must be a constant, you must be sure to either
     use `"immediate_operand"' for PREDICATE, or make the instruction
     pattern's extra condition require a constant, or both.  You cannot
     expect the constraints to do this work!  If the constraints allow
     only constants, but the predicate allows something else, the
     compiler will crash when that case arises.

`(match_scratch:M N CONSTRAINT)'
     This expression is also a placeholder for operand number N and
     indicates that operand must be a `scratch' or `reg' expression.

     When matching patterns, this is equivalent to

          (match_operand:M N "scratch_operand" PRED)

     but, when generating RTL, it produces a (`scratch':M) expression.

     If the last few expressions in a `parallel' are `clobber'
     expressions whose operands are either a hard register or
     `match_scratch', the combiner can add or delete them when
     necessary.  *Note Side Effects::.

`(match_dup N)'
     This expression is also a placeholder for operand number N.  It is
     used when the operand needs to appear more than once in the insn.

     In construction, `match_dup' acts just like `match_operand': the
     operand is substituted into the insn being constructed.  But in
     matching, `match_dup' behaves differently.  It assumes that operand
     number N has already been determined by a `match_operand'
     appearing earlier in the recognition template, and it matches only
     an identical-looking expression.

     Note that `match_dup' should not be used to tell the compiler that
     a particular register is being used for two operands (example:
     `add' that adds one register to another; the second register is
     both an input operand and the output operand).  Use a matching
     constraint (*note Simple Constraints::) for those.  `match_dup' is
     for the cases where one operand is used in two places in the
     template, such as an instruction that computes both a quotient and
     a remainder, where the opcode takes two input operands but the RTL
     template has to refer to each of those twice; once for the
     quotient pattern and once for the remainder pattern.

`(match_operator:M N PREDICATE [OPERANDS...])'
     This pattern is a kind of placeholder for a variable RTL expression
     code.

     When constructing an insn, it stands for an RTL expression whose
     expression code is taken from that of operand N, and whose
     operands are constructed from the patterns OPERANDS.

     When matching an expression, it matches an expression if the
     function PREDICATE returns nonzero on that expression _and_ the
     patterns OPERANDS match the operands of the expression.

     Suppose that the function `commutative_operator' is defined as
     follows, to match any expression whose operator is one of the
     commutative arithmetic operators of RTL and whose mode is MODE:

          int
          commutative_operator (x, mode)
               rtx x;
               enum machine_mode mode;
          {
            enum rtx_code code = GET_CODE (x);
            if (GET_MODE (x) != mode)
              return 0;
            return (GET_RTX_CLASS (code) == 'c'
                    || code == EQ || code == NE);
          }

     Then the following pattern will match any RTL expression consisting
     of a commutative operator applied to two general operands:

          (match_operator:SI 3 "commutative_operator"
            [(match_operand:SI 1 "general_operand" "g")
             (match_operand:SI 2 "general_operand" "g")])

     Here the vector `[OPERANDS...]' contains two patterns because the
     expressions to be matched all contain two operands.

     When this pattern does match, the two operands of the commutative
     operator are recorded as operands 1 and 2 of the insn.  (This is
     done by the two instances of `match_operand'.)  Operand 3 of the
     insn will be the entire commutative expression: use `GET_CODE
     (operands[3])' to see which commutative operator was used.

     The machine mode M of `match_operator' works like that of
     `match_operand': it is passed as the second argument to the
     predicate function, and that function is solely responsible for
     deciding whether the expression to be matched "has" that mode.

     When constructing an insn, argument 3 of the gen-function will
     specify the operation (i.e. the expression code) for the
     expression to be made.  It should be an RTL expression, whose
     expression code is copied into a new expression whose operands are
     arguments 1 and 2 of the gen-function.  The subexpressions of
     argument 3 are not used; only its expression code matters.

     When `match_operator' is used in a pattern for matching an insn,
     it usually best if the operand number of the `match_operator' is
     higher than that of the actual operands of the insn.  This improves
     register allocation because the register allocator often looks at
     operands 1 and 2 of insns to see if it can do register tying.

     There is no way to specify constraints in `match_operator'.  The
     operand of the insn which corresponds to the `match_operator'
     never has any constraints because it is never reloaded as a whole.
     However, if parts of its OPERANDS are matched by `match_operand'
     patterns, those parts may have constraints of their own.

`(match_op_dup:M N[OPERANDS...])'
     Like `match_dup', except that it applies to operators instead of
     operands.  When constructing an insn, operand number N will be
     substituted at this point.  But in matching, `match_op_dup' behaves
     differently.  It assumes that operand number N has already been
     determined by a `match_operator' appearing earlier in the
     recognition template, and it matches only an identical-looking
     expression.

`(match_parallel N PREDICATE [SUBPAT...])'
     This pattern is a placeholder for an insn that consists of a
     `parallel' expression with a variable number of elements.  This
     expression should only appear at the top level of an insn pattern.

     When constructing an insn, operand number N will be substituted at
     this point.  When matching an insn, it matches if the body of the
     insn is a `parallel' expression with at least as many elements as
     the vector of SUBPAT expressions in the `match_parallel', if each
     SUBPAT matches the corresponding element of the `parallel', _and_
     the function PREDICATE returns nonzero on the `parallel' that is
     the body of the insn.  It is the responsibility of the predicate
     to validate elements of the `parallel' beyond those listed in the
     `match_parallel'.

     A typical use of `match_parallel' is to match load and store
     multiple expressions, which can contain a variable number of
     elements in a `parallel'.  For example,

          (define_insn ""
            [(match_parallel 0 "load_multiple_operation"
               [(set (match_operand:SI 1 "gpc_reg_operand" "=r")
                     (match_operand:SI 2 "memory_operand" "m"))
                (use (reg:SI 179))
                (clobber (reg:SI 179))])]
            ""
            "loadm 0,0,%1,%2")

     This example comes from `a29k.md'.  The function
     `load_multiple_operation' is defined in `a29k.c' and checks that
     subsequent elements in the `parallel' are the same as the `set' in
     the pattern, except that they are referencing subsequent registers
     and memory locations.

     An insn that matches this pattern might look like:

          (parallel
           [(set (reg:SI 20) (mem:SI (reg:SI 100)))
            (use (reg:SI 179))
            (clobber (reg:SI 179))
            (set (reg:SI 21)
                 (mem:SI (plus:SI (reg:SI 100)
                                  (const_int 4))))
            (set (reg:SI 22)
                 (mem:SI (plus:SI (reg:SI 100)
                                  (const_int 8))))])

`(match_par_dup N [SUBPAT...])'
     Like `match_op_dup', but for `match_parallel' instead of
     `match_operator'.

`(match_insn PREDICATE)'
     Match a complete insn.  Unlike the other `match_*' recognizers,
     `match_insn' does not take an operand number.

     The machine mode M of `match_insn' works like that of
     `match_operand': it is passed as the second argument to the
     predicate function, and that function is solely responsible for
     deciding whether the expression to be matched "has" that mode.

`(match_insn2 N PREDICATE)'
     Match a complete insn.

     The machine mode M of `match_insn2' works like that of
     `match_operand': it is passed as the second argument to the
     predicate function, and that function is solely responsible for
     deciding whether the expression to be matched "has" that mode.



File: gccint.info,  Node: Output Template,  Next: Output Statement,  Prev: RTL Template,  Up: Machine Desc

9.5 Output Templates and Operand Substitution
=============================================

The "output template" is a string which specifies how to output the
assembler code for an instruction pattern.  Most of the template is a
fixed string which is output literally.  The character `%' is used to
specify where to substitute an operand; it can also be used to identify
places where different variants of the assembler require different
syntax.

   In the simplest case, a `%' followed by a digit N says to output
operand N at that point in the string.

   `%' followed by a letter and a digit says to output an operand in an
alternate fashion.  Four letters have standard, built-in meanings
described below.  The machine description macro `PRINT_OPERAND' can
define additional letters with nonstandard meanings.

   `%cDIGIT' can be used to substitute an operand that is a constant
value without the syntax that normally indicates an immediate operand.

   `%nDIGIT' is like `%cDIGIT' except that the value of the constant is
negated before printing.

   `%aDIGIT' can be used to substitute an operand as if it were a
memory reference, with the actual operand treated as the address.  This
may be useful when outputting a "load address" instruction, because
often the assembler syntax for such an instruction requires you to
write the operand as if it were a memory reference.

   `%lDIGIT' is used to substitute a `label_ref' into a jump
instruction.

   `%=' outputs a number which is unique to each instruction in the
entire compilation.  This is useful for making local labels to be
referred to more than once in a single template that generates multiple
assembler instructions.

   `%' followed by a punctuation character specifies a substitution that
does not use an operand.  Only one case is standard: `%%' outputs a `%'
into the assembler code.  Other nonstandard cases can be defined in the
`PRINT_OPERAND' macro.  You must also define which punctuation
characters are valid with the `PRINT_OPERAND_PUNCT_VALID_P' macro.

   The template may generate multiple assembler instructions.  Write
the text for the instructions, with `\;' between them.

   When the RTL contains two operands which are required by constraint
to match each other, the output template must refer only to the
lower-numbered operand.  Matching operands are not always identical,
and the rest of the compiler arranges to put the proper RTL expression
for printing into the lower-numbered operand.

   One use of nonstandard letters or punctuation following `%' is to
distinguish between different assembler languages for the same machine;
for example, Motorola syntax versus MIT syntax for the 68000.  Motorola
syntax requires periods in most opcode names, while MIT syntax does
not.  For example, the opcode `movel' in MIT syntax is `move.l' in
Motorola syntax.  The same file of patterns is used for both kinds of
output syntax, but the character sequence `%.' is used in each place
where Motorola syntax wants a period.  The `PRINT_OPERAND' macro for
Motorola syntax defines the sequence to output a period; the macro for
MIT syntax defines it to do nothing.

   As a special case, a template consisting of the single character `#'
instructs the compiler to first split the insn, and then output the
resulting instructions separately.  This helps eliminate redundancy in
the output templates.   If you have a `define_insn' that needs to emit
multiple assembler instructions, and there is an matching `define_split'
already defined, then you can simply use `#' as the output template
instead of writing an output template that emits the multiple assembler
instructions.

   If the macro `ASSEMBLER_DIALECT' is defined, you can use construct
of the form `{option0|option1|option2}' in the templates.  These
describe multiple variants of assembler language syntax.  *Note
Instruction Output::.


File: gccint.info,  Node: Output Statement,  Next: Constraints,  Prev: Output Template,  Up: Machine Desc

9.6 C Statements for Assembler Output
=====================================

Often a single fixed template string cannot produce correct and
efficient assembler code for all the cases that are recognized by a
single instruction pattern.  For example, the opcodes may depend on the
kinds of operands; or some unfortunate combinations of operands may
require extra machine instructions.

   If the output control string starts with a `@', then it is actually
a series of templates, each on a separate line.  (Blank lines and
leading spaces and tabs are ignored.)  The templates correspond to the
pattern's constraint alternatives (*note Multi-Alternative::).  For
example, if a target machine has a two-address add instruction `addr'
to add into a register and another `addm' to add a register to memory,
you might write this pattern:

     (define_insn "addsi3"
       [(set (match_operand:SI 0 "general_operand" "=r,m")
             (plus:SI (match_operand:SI 1 "general_operand" "0,0")
                      (match_operand:SI 2 "general_operand" "g,r")))]
       ""
       "@
        addr %2,%0
        addm %2,%0")

   If the output control string starts with a `*', then it is not an
output template but rather a piece of C program that should compute a
template.  It should execute a `return' statement to return the
template-string you want.  Most such templates use C string literals,
which require doublequote characters to delimit them.  To include these
doublequote characters in the string, prefix each one with `\'.

   If the output control string is written as a brace block instead of a
double-quoted string, it is automatically assumed to be C code.  In that
case, it is not necessary to put in a leading asterisk, or to escape the
doublequotes surrounding C string literals.

   The operands may be found in the array `operands', whose C data type
is `rtx []'.

   It is very common to select different ways of generating assembler
code based on whether an immediate operand is within a certain range.
Be careful when doing this, because the result of `INTVAL' is an
integer on the host machine.  If the host machine has more bits in an
`int' than the target machine has in the mode in which the constant
will be used, then some of the bits you get from `INTVAL' will be
superfluous.  For proper results, you must carefully disregard the
values of those bits.

   It is possible to output an assembler instruction and then go on to
output or compute more of them, using the subroutine `output_asm_insn'.
This receives two arguments: a template-string and a vector of
operands.  The vector may be `operands', or it may be another array of
`rtx' that you declare locally and initialize yourself.

   When an insn pattern has multiple alternatives in its constraints,
often the appearance of the assembler code is determined mostly by
which alternative was matched.  When this is so, the C code can test
the variable `which_alternative', which is the ordinal number of the
alternative that was actually satisfied (0 for the first, 1 for the
second alternative, etc.).

   For example, suppose there are two opcodes for storing zero, `clrreg'
for registers and `clrmem' for memory locations.  Here is how a pattern
could use `which_alternative' to choose between them:

     (define_insn ""
       [(set (match_operand:SI 0 "general_operand" "=r,m")
             (const_int 0))]
       ""
       {
       return (which_alternative == 0
               ? "clrreg %0" : "clrmem %0");
       })

   The example above, where the assembler code to generate was _solely_
determined by the alternative, could also have been specified as
follows, having the output control string start with a `@':

     (define_insn ""
       [(set (match_operand:SI 0 "general_operand" "=r,m")
             (const_int 0))]
       ""
       "@
        clrreg %0
        clrmem %0")


File: gccint.info,  Node: Constraints,  Next: Standard Names,  Prev: Output Statement,  Up: Machine Desc

9.7 Operand Constraints
=======================

Each `match_operand' in an instruction pattern can specify a constraint
for the type of operands allowed.  Constraints can say whether an
operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether
the operand may be an immediate constant, and which possible values it
may have.  Constraints can also require two operands to match.

* Menu:

* Simple Constraints::  Basic use of constraints.
* Multi-Alternative::   When an insn has two alternative constraint-patterns.
* Class Preferences::   Constraints guide which hard register to put things in.
* Modifiers::           More precise control over effects of constraints.
* Machine Constraints:: Existing constraints for some particular machines.


File: gccint.info,  Node: Simple Constraints,  Next: Multi-Alternative,  Up: Constraints

9.7.1 Simple Constraints
------------------------

The simplest kind of constraint is a string full of letters, each of
which describes one kind of operand that is permitted.  Here are the
letters that are allowed:

whitespace
     Whitespace characters are ignored and can be inserted at any
     position except the first.  This enables each alternative for
     different operands to be visually aligned in the machine
     description even if they have different number of constraints and
     modifiers.

`m'
     A memory operand is allowed, with any kind of address that the
     machine supports in general.

`o'
     A memory operand is allowed, but only if the address is
     "offsettable".  This means that adding a small integer (actually,
     the width in bytes of the operand, as determined by its machine
     mode) may be added to the address and the result is also a valid
     memory address.

     For example, an address which is constant is offsettable; so is an
     address that is the sum of a register and a constant (as long as a
     slightly larger constant is also within the range of
     address-offsets supported by the machine); but an autoincrement or
     autodecrement address is not offsettable.  More complicated
     indirect/indexed addresses may or may not be offsettable depending
     on the other addressing modes that the machine supports.

     Note that in an output operand which can be matched by another
     operand, the constraint letter `o' is valid only when accompanied
     by both `<' (if the target machine has predecrement addressing)
     and `>' (if the target machine has preincrement addressing).

`V'
     A memory operand that is not offsettable.  In other words,
     anything that would fit the `m' constraint but not the `o'
     constraint.

`<'
     A memory operand with autodecrement addressing (either
     predecrement or postdecrement) is allowed.

`>'
     A memory operand with autoincrement addressing (either
     preincrement or postincrement) is allowed.

`r'
     A register operand is allowed provided that it is in a general
     register.

`i'
     An immediate integer operand (one with constant value) is allowed.
     This includes symbolic constants whose values will be known only at
     assembly time.

`n'
     An immediate integer operand with a known numeric value is allowed.
     Many systems cannot support assembly-time constants for operands
     less than a word wide.  Constraints for these operands should use
     `n' rather than `i'.

`I', `J', `K', ... `P'
     Other letters in the range `I' through `P' may be defined in a
     machine-dependent fashion to permit immediate integer operands with
     explicit integer values in specified ranges.  For example, on the
     68000, `I' is defined to stand for the range of values 1 to 8.
     This is the range permitted as a shift count in the shift
     instructions.

`E'
     An immediate floating operand (expression code `const_double') is
     allowed, but only if the target floating point format is the same
     as that of the host machine (on which the compiler is running).

`F'
     An immediate floating operand (expression code `const_double') is
     allowed.

`G', `H'
     `G' and `H' may be defined in a machine-dependent fashion to
     permit immediate floating operands in particular ranges of values.

`s'
     An immediate integer operand whose value is not an explicit
     integer is allowed.

     This might appear strange; if an insn allows a constant operand
     with a value not known at compile time, it certainly must allow
     any known value.  So why use `s' instead of `i'?  Sometimes it
     allows better code to be generated.

     For example, on the 68000 in a fullword instruction it is possible
     to use an immediate operand; but if the immediate value is between
     -128 and 127, better code results from loading the value into a
     register and using the register.  This is because the load into
     the register can be done with a `moveq' instruction.  We arrange
     for this to happen by defining the letter `K' to mean "any integer
     outside the range -128 to 127", and then specifying `Ks' in the
     operand constraints.

`g'
     Any register, memory or immediate integer operand is allowed,
     except for registers that are not general registers.

`X'
     Any operand whatsoever is allowed, even if it does not satisfy
     `general_operand'.  This is normally used in the constraint of a
     `match_scratch' when certain alternatives will not actually
     require a scratch register.

`0', `1', `2', ... `9'
     An operand that matches the specified operand number is allowed.
     If a digit is used together with letters within the same
     alternative, the digit should come last.

     This number is allowed to be more than a single digit.  If multiple
     digits are encountered consecutavely, they are interpreted as a
     single decimal integer.  There is scant chance for ambiguity,
     since to-date it has never been desirable that `10' be interpreted
     as matching either operand 1 _or_ operand 0.  Should this be
     desired, one can use multiple alternatives instead.

     This is called a "matching constraint" and what it really means is
     that the assembler has only a single operand that fills two roles
     considered separate in the RTL insn.  For example, an add insn has
     two input operands and one output operand in the RTL, but on most
     CISC machines an add instruction really has only two operands, one
     of them an input-output operand:

          addl #35,r12

     Matching constraints are used in these circumstances.  More
     precisely, the two operands that match must include one input-only
     operand and one output-only operand.  Moreover, the digit must be a
     smaller number than the number of the operand that uses it in the
     constraint.

     For operands to match in a particular case usually means that they
     are identical-looking RTL expressions.  But in a few special cases
     specific kinds of dissimilarity are allowed.  For example, `*x' as
     an input operand will match `*x++' as an output operand.  For
     proper results in such cases, the output template should always
     use the output-operand's number when printing the operand.

`p'
     An operand that is a valid memory address is allowed.  This is for
     "load address" and "push address" instructions.

     `p' in the constraint must be accompanied by `address_operand' as
     the predicate in the `match_operand'.  This predicate interprets
     the mode specified in the `match_operand' as the mode of the memory
     reference for which the address would be valid.

OTHER-LETTERS
     Other letters can be defined in machine-dependent fashion to stand
     for particular classes of registers or other arbitrary operand
     types.  `d', `a' and `f' are defined on the 68000/68020 to stand
     for data, address and floating point registers.

     The machine description macro `REG_CLASS_FROM_LETTER' has first
     cut at the otherwise unused letters.  If it evaluates to `NO_REGS',
     then `EXTRA_CONSTRAINT' is evaluated.

     A typical use for `EXTRA_CONSTRANT' would be to distinguish certain
     types of memory references that affect other insn operands.

   In order to have valid assembler code, each operand must satisfy its
constraint.  But a failure to do so does not prevent the pattern from
applying to an insn.  Instead, it directs the compiler to modify the
code so that the constraint will be satisfied.  Usually this is done by
copying an operand into a register.

   Contrast, therefore, the two instruction patterns that follow:

     (define_insn ""
       [(set (match_operand:SI 0 "general_operand" "=r")
             (plus:SI (match_dup 0)
                      (match_operand:SI 1 "general_operand" "r")))]
       ""
       "...")

which has two operands, one of which must appear in two places, and

     (define_insn ""
       [(set (match_operand:SI 0 "general_operand" "=r")
             (plus:SI (match_operand:SI 1 "general_operand" "0")
                      (match_operand:SI 2 "general_operand" "r")))]
       ""
       "...")

which has three operands, two of which are required by a constraint to
be identical.  If we are considering an insn of the form

     (insn N PREV NEXT
       (set (reg:SI 3)
            (plus:SI (reg:SI 6) (reg:SI 109)))
       ...)

the first pattern would not apply at all, because this insn does not
contain two identical subexpressions in the right place.  The pattern
would say, "That does not look like an add instruction; try other
patterns."  The second pattern would say, "Yes, that's an add
instruction, but there is something wrong with it."  It would direct
the reload pass of the compiler to generate additional insns to make
the constraint true.  The results might look like this:

     (insn N2 PREV N
       (set (reg:SI 3) (reg:SI 6))
       ...)

     (insn N N2 NEXT
       (set (reg:SI 3)
            (plus:SI (reg:SI 3) (reg:SI 109)))
       ...)

   It is up to you to make sure that each operand, in each pattern, has
constraints that can handle any RTL expression that could be present for
that operand.  (When multiple alternatives are in use, each pattern
must, for each possible combination of operand expressions, have at
least one alternative which can handle that combination of operands.)
The constraints don't need to _allow_ any possible operand--when this is
the case, they do not constrain--but they must at least point the way to
reloading any possible operand so that it will fit.

   * If the constraint accepts whatever operands the predicate permits,
     there is no problem: reloading is never necessary for this operand.

     For example, an operand whose constraints permit everything except
     registers is safe provided its predicate rejects registers.

     An operand whose predicate accepts only constant values is safe
     provided its constraints include the letter `i'.  If any possible
     constant value is accepted, then nothing less than `i' will do; if
     the predicate is more selective, then the constraints may also be
     more selective.

   * Any operand expression can be reloaded by copying it into a
     register.  So if an operand's constraints allow some kind of
     register, it is certain to be safe.  It need not permit all
     classes of registers; the compiler knows how to copy a register
     into another register of the proper class in order to make an
     instruction valid.

   * A nonoffsettable memory reference can be reloaded by copying the
     address into a register.  So if the constraint uses the letter
     `o', all memory references are taken care of.

   * A constant operand can be reloaded by allocating space in memory to
     hold it as preinitialized data.  Then the memory reference can be
     used in place of the constant.  So if the constraint uses the
     letters `o' or `m', constant operands are not a problem.

   * If the constraint permits a constant and a pseudo register used in
     an insn was not allocated to a hard register and is equivalent to
     a constant, the register will be replaced with the constant.  If
     the predicate does not permit a constant and the insn is
     re-recognized for some reason, the compiler will crash.  Thus the
     predicate must always recognize any objects allowed by the
     constraint.

   If the operand's predicate can recognize registers, but the
constraint does not permit them, it can make the compiler crash.  When
this operand happens to be a register, the reload pass will be stymied,
because it does not know how to copy a register temporarily into memory.

   If the predicate accepts a unary operator, the constraint applies to
the operand.  For example, the MIPS processor at ISA level 3 supports an
instruction which adds two registers in `SImode' to produce a `DImode'
result, but only if the registers are correctly sign extended.  This
predicate for the input operands accepts a `sign_extend' of an `SImode'
register.  Write the constraint to indicate the type of register that
is required for the operand of the `sign_extend'.


File: gccint.info,  Node: Multi-Alternative,  Next: Class Preferences,  Prev: Simple Constraints,  Up: Constraints

9.7.2 Multiple Alternative Constraints
--------------------------------------

Sometimes a single instruction has multiple alternative sets of possible
operands.  For example, on the 68000, a logical-or instruction can
combine register or an immediate value into memory, or it can combine
any kind of operand into a register; but it cannot combine one memory
location into another.

   These constraints are represented as multiple alternatives.  An
alternative can be described by a series of letters for each operand.
The overall constraint for an operand is made from the letters for this
operand from the first alternative, a comma, the letters for this
operand from the second alternative, a comma, and so on until the last
alternative.  Here is how it is done for fullword logical-or on the
68000:

     (define_insn "iorsi3"
       [(set (match_operand:SI 0 "general_operand" "=m,d")
             (ior:SI (match_operand:SI 1 "general_operand" "%0,0")
                     (match_operand:SI 2 "general_operand" "dKs,dmKs")))]
       ...)

   The first alternative has `m' (memory) for operand 0, `0' for
operand 1 (meaning it must match operand 0), and `dKs' for operand 2.
The second alternative has `d' (data register) for operand 0, `0' for
operand 1, and `dmKs' for operand 2.  The `=' and `%' in the
constraints apply to all the alternatives; their meaning is explained
in the next section (*note Class Preferences::).

   If all the operands fit any one alternative, the instruction is
valid.  Otherwise, for each alternative, the compiler counts how many
instructions must be added to copy the operands so that that
alternative applies.  The alternative requiring the least copying is
chosen.  If two alternatives need the same amount of copying, the one
that comes first is chosen.  These choices can be altered with the `?'
and `!' characters:

`?'
     Disparage slightly the alternative that the `?' appears in, as a
     choice when no alternative applies exactly.  The compiler regards
     this alternative as one unit more costly for each `?' that appears
     in it.

`!'
     Disparage severely the alternative that the `!' appears in.  This
     alternative can still be used if it fits without reloading, but if
     reloading is needed, some other alternative will be used.

   When an insn pattern has multiple alternatives in its constraints,
often the appearance of the assembler code is determined mostly by which
alternative was matched.  When this is so, the C code for writing the
assembler code can use the variable `which_alternative', which is the
ordinal number of the alternative that was actually satisfied (0 for
the first, 1 for the second alternative, etc.).  *Note Output
Statement::.


File: gccint.info,  Node: Class Preferences,  Next: Modifiers,  Prev: Multi-Alternative,  Up: Constraints

9.7.3 Register Class Preferences
--------------------------------

The operand constraints have another function: they enable the compiler
to decide which kind of hardware register a pseudo register is best
allocated to.  The compiler examines the constraints that apply to the
insns that use the pseudo register, looking for the machine-dependent
letters such as `d' and `a' that specify classes of registers.  The
pseudo register is put in whichever class gets the most "votes".  The
constraint letters `g' and `r' also vote: they vote in favor of a
general register.  The machine description says which registers are
considered general.

   Of course, on some machines all registers are equivalent, and no
register classes are defined.  Then none of this complexity is relevant.


File: gccint.info,  Node: Modifiers,  Next: Machine Constraints,  Prev: Class Preferences,  Up: Constraints

9.7.4 Constraint Modifier Characters
------------------------------------

Here are constraint modifier characters.

`='
     Means that this operand is write-only for this instruction: the
     previous value is discarded and replaced by output data.

`+'
     Means that this operand is both read and written by the
     instruction.

     When the compiler fixes up the operands to satisfy the constraints,
     it needs to know which operands are inputs to the instruction and
     which are outputs from it.  `=' identifies an output; `+'
     identifies an operand that is both input and output; all other
     operands are assumed to be input only.

     If you specify `=' or `+' in a constraint, you put it in the first
     character of the constraint string.

`&'
     Means (in a particular alternative) that this operand is an
     "earlyclobber" operand, which is modified before the instruction is
     finished using the input operands.  Therefore, this operand may
     not lie in a register that is used as an input operand or as part
     of any memory address.

     `&' applies only to the alternative in which it is written.  In
     constraints with multiple alternatives, sometimes one alternative
     requires `&' while others do not.  See, for example, the `movdf'
     insn of the 68000.

     An input operand can be tied to an earlyclobber operand if its only
     use as an input occurs before the early result is written.  Adding
     alternatives of this form often allows GCC to produce better code
     when only some of the inputs can be affected by the earlyclobber.
     See, for example, the `mulsi3' insn of the ARM.

     `&' does not obviate the need to write `='.

`%'
     Declares the instruction to be commutative for this operand and the
     following operand.  This means that the compiler may interchange
     the two operands if that is the cheapest way to make all operands
     fit the constraints.  This is often used in patterns for addition
     instructions that really have only two operands: the result must
     go in one of the arguments.  Here for example, is how the 68000
     halfword-add instruction is defined:

          (define_insn "addhi3"
            [(set (match_operand:HI 0 "general_operand" "=m,r")
               (plus:HI (match_operand:HI 1 "general_operand" "%0,0")
                        (match_operand:HI 2 "general_operand" "di,g")))]
            ...)

`#'
     Says that all following characters, up to the next comma, are to be
     ignored as a constraint.  They are significant only for choosing
     register preferences.

`*'
     Says that the following character should be ignored when choosing
     register preferences.  `*' has no effect on the meaning of the
     constraint as a constraint, and no effect on reloading.

     Here is an example: the 68000 has an instruction to sign-extend a
     halfword in a data register, and can also sign-extend a value by
     copying it into an address register.  While either kind of
     register is acceptable, the constraints on an address-register
     destination are less strict, so it is best if register allocation
     makes an address register its goal.  Therefore, `*' is used so
     that the `d' constraint letter (for data register) is ignored when
     computing register preferences.

          (define_insn "extendhisi2"
            [(set (match_operand:SI 0 "general_operand" "=*d,a")
                  (sign_extend:SI
                   (match_operand:HI 1 "general_operand" "0,g")))]
            ...)


File: gccint.info,  Node: Machine Constraints,  Prev: Modifiers,  Up: Constraints

9.7.5 Constraints for Particular Machines
-----------------------------------------

Whenever possible, you should use the general-purpose constraint letters
in `asm' arguments, since they will convey meaning more readily to
people reading your code.  Failing that, use the constraint letters
that usually have very similar meanings across architectures.  The most
commonly used constraints are `m' and `r' (for memory and
general-purpose registers respectively; *note Simple Constraints::), and
`I', usually the letter indicating the most common immediate-constant
format.

   For each machine architecture, the `config/MACHINE/MACHINE.h' file
defines additional constraints.  These constraints are used by the
compiler itself for instruction generation, as well as for `asm'
statements; therefore, some of the constraints are not particularly
interesting for `asm'.  The constraints are defined through these
macros:

`REG_CLASS_FROM_LETTER'
     Register class constraints (usually lower case).

`CONST_OK_FOR_LETTER_P'
     Immediate constant constraints, for non-floating point constants of
     word size or smaller precision (usually upper case).

`CONST_DOUBLE_OK_FOR_LETTER_P'
     Immediate constant constraints, for all floating point constants
     and for constants of greater than word size precision (usually
     upper case).

`EXTRA_CONSTRAINT'
     Special cases of registers or memory.  This macro is not required,
     and is only defined for some machines.

   Inspecting these macro definitions in the compiler source for your
machine is the best way to be certain you have the right constraints.
However, here is a summary of the machine-dependent constraints
available on some particular machines.

_ARM family--`arm.h'_

    `f'
          Floating-point register

    `F'
          One of the floating-point constants 0.0, 0.5, 1.0, 2.0, 3.0,
          4.0, 5.0 or 10.0

    `G'
          Floating-point constant that would satisfy the constraint `F'
          if it were negated

    `I'
          Integer that is valid as an immediate operand in a data
          processing instruction.  That is, an integer in the range 0
          to 255 rotated by a multiple of 2

    `J'
          Integer in the range -4095 to 4095

    `K'
          Integer that satisfies constraint `I' when inverted (ones
          complement)

    `L'
          Integer that satisfies constraint `I' when negated (twos
          complement)

    `M'
          Integer in the range 0 to 32

    `Q'
          A memory reference where the exact address is in a single
          register (``m'' is preferable for `asm' statements)

    `R'
          An item in the constant pool

    `S'
          A symbol in the text segment of the current file

_AMD 29000 family--`a29k.h'_

    `l'
          Local register 0

    `b'
          Byte Pointer (`BP') register

    `q'
          `Q' register

    `h'
          Special purpose register

    `A'
          First accumulator register

    `a'
          Other accumulator register

    `f'
          Floating point register

    `I'
          Constant greater than 0, less than 0x100

    `J'
          Constant greater than 0, less than 0x10000

    `K'
          Constant whose high 24 bits are on (1)

    `L'
          16-bit constant whose high 8 bits are on (1)

    `M'
          32-bit constant whose high 16 bits are on (1)

    `N'
          32-bit negative constant that fits in 8 bits

    `O'
          The constant 0x80000000 or, on the 29050, any 32-bit constant
          whose low 16 bits are 0.

    `P'
          16-bit negative constant that fits in 8 bits

    `G'
    `H'
          A floating point constant (in `asm' statements, use the
          machine independent `E' or `F' instead)

_AVR family--`avr.h'_

    `l'
          Registers from r0 to r15

    `a'
          Registers from r16 to r23

    `d'
          Registers from r16 to r31

    `w'
          Registers from r24 to r31.  These registers can be used in
          `adiw' command

    `e'
          Pointer register (r26-r31)

    `b'
          Base pointer register (r28-r31)

    `q'
          Stack pointer register (SPH:SPL)

    `t'
          Temporary register r0

    `x'
          Register pair X (r27:r26)

    `y'
          Register pair Y (r29:r28)

    `z'
          Register pair Z (r31:r30)

    `I'
          Constant greater than -1, less than 64

    `J'
          Constant greater than -64, less than 1

    `K'
          Constant integer 2

    `L'
          Constant integer 0

    `M'
          Constant that fits in 8 bits

    `N'
          Constant integer -1

    `O'
          Constant integer 8, 16, or 24

    `P'
          Constant integer 1

    `G'
          A floating point constant 0.0

_IBM RS6000--`rs6000.h'_

    `b'
          Address base register

    `f'
          Floating point register

    `h'
          `MQ', `CTR', or `LINK' register

    `q'
          `MQ' register

    `c'
          `CTR' register

    `l'
          `LINK' register

    `x'
          `CR' register (condition register) number 0

    `y'
          `CR' register (condition register)

    `z'
          `FPMEM' stack memory for FPR-GPR transfers

    `I'
          Signed 16-bit constant

    `J'
          Unsigned 16-bit constant shifted left 16 bits (use `L'
          instead for `SImode' constants)

    `K'
          Unsigned 16-bit constant

    `L'
          Signed 16-bit constant shifted left 16 bits

    `M'
          Constant larger than 31

    `N'
          Exact power of 2

    `O'
          Zero

    `P'
          Constant whose negation is a signed 16-bit constant

    `G'
          Floating point constant that can be loaded into a register
          with one instruction per word

    `Q'
          Memory operand that is an offset from a register (`m' is
          preferable for `asm' statements)

    `R'
          AIX TOC entry

    `S'
          Constant suitable as a 64-bit mask operand

    `T'
          Constant suitable as a 32-bit mask operand

    `U'
          System V Release 4 small data area reference

_Intel 386--`i386.h'_

    `q'
          `a', `b', `c', or `d' register for the i386.  For x86-64 it
          is equivalent to `r' class. (for 8-bit instructions that do
          not use upper halves)

    `Q'
          `a', `b', `c', or `d' register. (for 8-bit instructions, that
          do use upper halves)

    `R'
          Legacy register--equivalent to `r' class in i386 mode.  (for
          non-8-bit registers used together with 8-bit upper halves in
          a single instruction)

    `A'
          Specifies the `a' or `d' registers.  This is primarily useful
          for 64-bit integer values (when in 32-bit mode) intended to
          be returned with the `d' register holding the most
          significant bits and the `a' register holding the least
          significant bits.

    `f'
          Floating point register

    `t'
          First (top of stack) floating point register

    `u'
          Second floating point register

    `a'
          `a' register

    `b'
          `b' register

    `c'
          `c' register

    `d'
          `d' register

    `D'
          `di' register

    `S'
          `si' register

    `x'
          `xmm' SSE register

    `y'
          MMX register

    `I'
          Constant in range 0 to 31 (for 32-bit shifts)

    `J'
          Constant in range 0 to 63 (for 64-bit shifts)

    `K'
          `0xff'

    `L'
          `0xffff'

    `M'
          0, 1, 2, or 3 (shifts for `lea' instruction)

    `N'
          Constant in range 0 to 255 (for `out' instruction)

    `Z'
          Constant in range 0 to `0xffffffff' or symbolic reference
          known to fit specified range.  (for using immediates in zero
          extending 32-bit to 64-bit x86-64 instructions)

    `e'
          Constant in range -2147483648 to 2147483647 or symbolic
          reference known to fit specified range.  (for using
          immediates in 64-bit x86-64 instructions)

    `G'
          Standard 80387 floating point constant

_Intel 960--`i960.h'_

    `f'
          Floating point register (`fp0' to `fp3')

    `l'
          Local register (`r0' to `r15')

    `b'
          Global register (`g0' to `g15')

    `d'
          Any local or global register

    `I'
          Integers from 0 to 31

    `J'
          0

    `K'
          Integers from -31 to 0

    `G'
          Floating point 0

    `H'
          Floating point 1

_MIPS--`mips.h'_

    `d'
          General-purpose integer register

    `f'
          Floating-point register (if available)

    `h'
          `Hi' register

    `l'
          `Lo' register

    `x'
          `Hi' or `Lo' register

    `y'
          General-purpose integer register

    `z'
          Floating-point status register

    `I'
          Signed 16-bit constant (for arithmetic instructions)

    `J'
          Zero

    `K'
          Zero-extended 16-bit constant (for logic instructions)

    `L'
          Constant with low 16 bits zero (can be loaded with `lui')

    `M'
          32-bit constant which requires two instructions to load (a
          constant which is not `I', `K', or `L')

    `N'
          Negative 16-bit constant

    `O'
          Exact power of two

    `P'
          Positive 16-bit constant

    `G'
          Floating point zero

    `Q'
          Memory reference that can be loaded with more than one
          instruction (`m' is preferable for `asm' statements)

    `R'
          Memory reference that can be loaded with one instruction (`m'
          is preferable for `asm' statements)

    `S'
          Memory reference in external OSF/rose PIC format (`m' is
          preferable for `asm' statements)

_Motorola 680x0--`m68k.h'_

    `a'
          Address register

    `d'
          Data register

    `f'
          68881 floating-point register, if available

    `x'
          Sun FPA (floating-point) register, if available

    `y'
          First 16 Sun FPA registers, if available

    `I'
          Integer in the range 1 to 8

    `J'
          16-bit signed number

    `K'
          Signed number whose magnitude is greater than 0x80

    `L'
          Integer in the range -8 to -1

    `M'
          Signed number whose magnitude is greater than 0x100

    `G'
          Floating point constant that is not a 68881 constant

    `H'
          Floating point constant that can be used by Sun FPA

_Motorola 68HC11 & 68HC12 families--`m68hc11.h'_

    `a'
          Register 'a'

    `b'
          Register 'b'

    `d'
          Register 'd'

    `q'
          An 8-bit register

    `t'
          Temporary soft register _.tmp

    `u'
          A soft register _.d1 to _.d31

    `w'
          Stack pointer register

    `x'
          Register 'x'

    `y'
          Register 'y'

    `z'
          Pseudo register 'z' (replaced by 'x' or 'y' at the end)

    `A'
          An address register: x, y or z

    `B'
          An address register: x or y

    `D'
          Register pair (x:d) to form a 32-bit value

    `L'
          Constants in the range -65536 to 65535

    `M'
          Constants whose 16-bit low part is zero

    `N'
          Constant integer 1 or -1

    `O'
          Constant integer 16

    `P'
          Constants in the range -8 to 2


_SPARC--`sparc.h'_

    `f'
          Floating-point register that can hold 32- or 64-bit values.

    `e'
          Floating-point register that can hold 64- or 128-bit values.

    `I'
          Signed 13-bit constant

    `J'
          Zero

    `K'
          32-bit constant with the low 12 bits clear (a constant that
          can be loaded with the `sethi' instruction)

    `L'
          A constant in the range supported by `movcc' instructions

    `M'
          A constant in the range supported by `movrcc' instructions

    `N'
          Same as `K', except that it verifies that bits that are not
          in the lower 32-bit range are all zero.  Must be used instead
          of `K' for modes wider than `SImode'

    `G'
          Floating-point zero

    `H'
          Signed 13-bit constant, sign-extended to 32 or 64 bits

    `Q'
          Floating-point constant whose integral representation can be
          moved into an integer register using a single sethi
          instruction

    `R'
          Floating-point constant whose integral representation can be
          moved into an integer register using a single mov instruction

    `S'
          Floating-point constant whose integral representation can be
          moved into an integer register using a high/lo_sum
          instruction sequence

    `T'
          Memory address aligned to an 8-byte boundary

    `U'
          Even register

    `W'
          Memory address for `e' constraint registers.


_TMS320C3x/C4x--`c4x.h'_

    `a'
          Auxiliary (address) register (ar0-ar7)

    `b'
          Stack pointer register (sp)

    `c'
          Standard (32-bit) precision integer register

    `f'
          Extended (40-bit) precision register (r0-r11)

    `k'
          Block count register (bk)

    `q'
          Extended (40-bit) precision low register (r0-r7)

    `t'
          Extended (40-bit) precision register (r0-r1)

    `u'
          Extended (40-bit) precision register (r2-r3)

    `v'
          Repeat count register (rc)

    `x'
          Index register (ir0-ir1)

    `y'
          Status (condition code) register (st)

    `z'
          Data page register (dp)

    `G'
          Floating-point zero

    `H'
          Immediate 16-bit floating-point constant

    `I'
          Signed 16-bit constant

    `J'
          Signed 8-bit constant

    `K'
          Signed 5-bit constant

    `L'
          Unsigned 16-bit constant

    `M'
          Unsigned 8-bit constant

    `N'
          Ones complement of unsigned 16-bit constant

    `O'
          High 16-bit constant (32-bit constant with 16 LSBs zero)

    `Q'
          Indirect memory reference with signed 8-bit or index register
          displacement

    `R'
          Indirect memory reference with unsigned 5-bit displacement

    `S'
          Indirect memory reference with 1 bit or index register
          displacement

    `T'
          Direct memory reference

    `U'
          Symbolic address


_S/390 and zSeries--`s390.h'_

    `a'
          Address register (general purpose register except r0)

    `d'
          Data register (arbitrary general purpose register)

    `f'
          Floating-point register

    `I'
          Unsigned 8-bit constant (0-255)

    `J'
          Unsigned 12-bit constant (0-4095)

    `K'
          Signed 16-bit constant (-32768-32767)

    `L'
          Unsigned 16-bit constant (0-65535)

    `Q'
          Memory reference without index register

    `S'
          Symbolic constant suitable for use with the `larl' instruction


_Xstormy16--`stormy16.h'_

    `a'
          Register r0.

    `b'
          Register r1.

    `c'
          Register r2.

    `d'
          Register r8.

    `e'
          Registers r0 through r7.

    `t'
          Registers r0 and r1.

    `y'
          The carry register.

    `z'
          Registers r8 and r9.

    `I'
          A constant between 0 and 3 inclusive.

    `J'
          A constant that has exactly one bit set.

    `K'
          A constant that has exactly one bit clear.

    `L'
          A constant between 0 and 255 inclusive.

    `M'
          A constant between -255 and 0 inclusive.

    `N'
          A constant between -3 and 0 inclusive.

    `O'
          A constant between 1 and 4 inclusive.

    `P'
          A constant between -4 and -1 inclusive.

    `Q'
          A memory reference that is a stack push.

    `R'
          A memory reference that is a stack pop.

    `S'
          A memory reference that refers to an constant address of
          known value.

    `T'
          The register indicated by Rx (not implemented yet).

    `U'
          A constant that is not between 2 and 15 inclusive.


_Xtensa--`xtensa.h'_

    `a'
          General-purpose 32-bit register

    `b'
          One-bit boolean register

    `A'
          MAC16 40-bit accumulator register

    `I'
          Signed 12-bit integer constant, for use in MOVI instructions

    `J'
          Signed 8-bit integer constant, for use in ADDI instructions

    `K'
          Integer constant valid for BccI instructions

    `L'
          Unsigned constant valid for BccUI instructions




File: gccint.info,  Node: Standard Names,  Next: Pattern Ordering,  Prev: Constraints,  Up: Machine Desc

9.8 Standard Pattern Names For Generation
=========================================

Here is a table of the instruction names that are meaningful in the RTL
generation pass of the compiler.  Giving one of these names to an
instruction pattern tells the RTL generation pass that it can use the
pattern to accomplish a certain task.

`movM'
     Here M stands for a two-letter machine mode name, in lower case.
     This instruction pattern moves data with that machine mode from
     operand 1 to operand 0.  For example, `movsi' moves full-word data.

     If operand 0 is a `subreg' with mode M of a register whose own
     mode is wider than M, the effect of this instruction is to store
     the specified value in the part of the register that corresponds
     to mode M.  Bits outside of M, but which are within the same
     target word as the `subreg' are undefined.  Bits which are outside
     the target word are left unchanged.

     This class of patterns is special in several ways.  First of all,
     each of these names up to and including full word size _must_ be
     defined, because there is no other way to copy a datum from one
     place to another.  If there are patterns accepting operands in
     larger modes, `movM' must be defined for integer modes of those
     sizes.

     Second, these patterns are not used solely in the RTL generation
     pass.  Even the reload pass can generate move insns to copy values
     from stack slots into temporary registers.  When it does so, one
     of the operands is a hard register and the other is an operand
     that can need to be reloaded into a register.

     Therefore, when given such a pair of operands, the pattern must
     generate RTL which needs no reloading and needs no temporary
     registers--no registers other than the operands.  For example, if
     you support the pattern with a `define_expand', then in such a
     case the `define_expand' mustn't call `force_reg' or any other such
     function which might generate new pseudo registers.

     This requirement exists even for subword modes on a RISC machine
     where fetching those modes from memory normally requires several
     insns and some temporary registers.

     During reload a memory reference with an invalid address may be
     passed as an operand.  Such an address will be replaced with a
     valid address later in the reload pass.  In this case, nothing may
     be done with the address except to use it as it stands.  If it is
     copied, it will not be replaced with a valid address.  No attempt
     should be made to make such an address into a valid address and no
     routine (such as `change_address') that will do so may be called.
     Note that `general_operand' will fail when applied to such an
     address.

     The global variable `reload_in_progress' (which must be explicitly
     declared if required) can be used to determine whether such special
     handling is required.

     The variety of operands that have reloads depends on the rest of
     the machine description, but typically on a RISC machine these can
     only be pseudo registers that did not get hard registers, while on
     other machines explicit memory references will get optional
     reloads.

     If a scratch register is required to move an object to or from
     memory, it can be allocated using `gen_reg_rtx' prior to life
     analysis.

     If there are cases which need scratch registers during or after
     reload, you must define `SECONDARY_INPUT_RELOAD_CLASS' and/or
     `SECONDARY_OUTPUT_RELOAD_CLASS' to detect them, and provide
     patterns `reload_inM' or `reload_outM' to handle them.  *Note
     Register Classes::.

     The global variable `no_new_pseudos' can be used to determine if it
     is unsafe to create new pseudo registers.  If this variable is
     nonzero, then it is unsafe to call `gen_reg_rtx' to allocate a new
     pseudo.

     The constraints on a `movM' must permit moving any hard register
     to any other hard register provided that `HARD_REGNO_MODE_OK'
     permits mode M in both registers and `REGISTER_MOVE_COST' applied
     to their classes returns a value of 2.

     It is obligatory to support floating point `movM' instructions
     into and out of any registers that can hold fixed point values,
     because unions and structures (which have modes `SImode' or
     `DImode') can be in those registers and they may have floating
     point members.

     There may also be a need to support fixed point `movM'
     instructions in and out of floating point registers.
     Unfortunately, I have forgotten why this was so, and I don't know
     whether it is still true.  If `HARD_REGNO_MODE_OK' rejects fixed
     point values in floating point registers, then the constraints of
     the fixed point `movM' instructions must be designed to avoid ever
     trying to reload into a floating point register.

`reload_inM'
`reload_outM'
     Like `movM', but used when a scratch register is required to move
     between operand 0 and operand 1.  Operand 2 describes the scratch
     register.  See the discussion of the `SECONDARY_RELOAD_CLASS'
     macro in *note Register Classes::.

     There are special restrictions on the form of the `match_operand's
     used in these patterns.  First, only the predicate for the reload
     operand is examined, i.e., `reload_in' examines operand 1, but not
     the predicates for operand 0 or 2.  Second, there may be only one
     alternative in the constraints.  Third, only a single register
     class letter may be used for the constraint; subsequent constraint
     letters are ignored.  As a special exception, an empty constraint
     string matches the `ALL_REGS' register class.  This may relieve
     ports of the burden of defining an `ALL_REGS' constraint letter
     just for these patterns.

`movstrictM'
     Like `movM' except that if operand 0 is a `subreg' with mode M of
     a register whose natural mode is wider, the `movstrictM'
     instruction is guaranteed not to alter any of the register except
     the part which belongs to mode M.

`load_multiple'
     Load several consecutive memory locations into consecutive
     registers.  Operand 0 is the first of the consecutive registers,
     operand 1 is the first memory location, and operand 2 is a
     constant: the number of consecutive registers.

     Define this only if the target machine really has such an
     instruction; do not define this if the most efficient way of
     loading consecutive registers from memory is to do them one at a
     time.

     On some machines, there are restrictions as to which consecutive
     registers can be stored into memory, such as particular starting or
     ending register numbers or only a range of valid counts.  For those
     machines, use a `define_expand' (*note Expander Definitions::) and
     make the pattern fail if the restrictions are not met.

     Write the generated insn as a `parallel' with elements being a
     `set' of one register from the appropriate memory location (you may
     also need `use' or `clobber' elements).  Use a `match_parallel'
     (*note RTL Template::) to recognize the insn.  See `a29k.md' and
     `rs6000.md' for examples of the use of this insn pattern.

`store_multiple'
     Similar to `load_multiple', but store several consecutive registers
     into consecutive memory locations.  Operand 0 is the first of the
     consecutive memory locations, operand 1 is the first register, and
     operand 2 is a constant: the number of consecutive registers.

`pushM'
     Output an push instruction.  Operand 0 is value to push.  Used
     only when `PUSH_ROUNDING' is defined.  For historical reason, this
     pattern may be missing and in such case an `mov' expander is used
     instead, with a `MEM' expression forming the push operation.  The
     `mov' expander method is deprecated.

`addM3'
     Add operand 2 and operand 1, storing the result in operand 0.  All
     operands must have mode M.  This can be used even on two-address
     machines, by means of constraints requiring operands 1 and 0 to be
     the same location.

`subM3', `mulM3'
`divM3', `udivM3', `modM3', `umodM3'
`sminM3', `smaxM3', `uminM3', `umaxM3'
`andM3', `iorM3', `xorM3'
     Similar, for other arithmetic operations.  

`minM3', `maxM3'
     Floating point min and max operations.  If both operands are zeros,
     or if either operand is NaN, then it is unspecified which of the
     two operands is returned as the result.

`mulhisi3'
     Multiply operands 1 and 2, which have mode `HImode', and store a
     `SImode' product in operand 0.

`mulqihi3', `mulsidi3'
     Similar widening-multiplication instructions of other widths.

`umulqihi3', `umulhisi3', `umulsidi3'
     Similar widening-multiplication instructions that do unsigned
     multiplication.

`smulM3_highpart'
     Perform a signed multiplication of operands 1 and 2, which have
     mode M, and store the most significant half of the product in
     operand 0.  The least significant half of the product is discarded.

`umulM3_highpart'
     Similar, but the multiplication is unsigned.

`divmodM4'
     Signed division that produces both a quotient and a remainder.
     Operand 1 is divided by operand 2 to produce a quotient stored in
     operand 0 and a remainder stored in operand 3.

     For machines with an instruction that produces both a quotient and
     a remainder, provide a pattern for `divmodM4' but do not provide
     patterns for `divM3' and `modM3'.  This allows optimization in the
     relatively common case when both the quotient and remainder are
     computed.

     If an instruction that just produces a quotient or just a remainder
     exists and is more efficient than the instruction that produces
     both, write the output routine of `divmodM4' to call
     `find_reg_note' and look for a `REG_UNUSED' note on the quotient
     or remainder and generate the appropriate instruction.

`udivmodM4'
     Similar, but does unsigned division.

`ashlM3'
     Arithmetic-shift operand 1 left by a number of bits specified by
     operand 2, and store the result in operand 0.  Here M is the mode
     of operand 0 and operand 1; operand 2's mode is specified by the
     instruction pattern, and the compiler will convert the operand to
     that mode before generating the instruction.

`ashrM3', `lshrM3', `rotlM3', `rotrM3'
     Other shift and rotate instructions, analogous to the `ashlM3'
     instructions.

`negM2'
     Negate operand 1 and store the result in operand 0.

`absM2'
     Store the absolute value of operand 1 into operand 0.

`sqrtM2'
     Store the square root of operand 1 into operand 0.

     The `sqrt' built-in function of C always uses the mode which
     corresponds to the C data type `double'.

`ffsM2'
     Store into operand 0 one plus the index of the least significant
     1-bit of operand 1.  If operand 1 is zero, store zero.  M is the
     mode of operand 0; operand 1's mode is specified by the instruction
     pattern, and the compiler will convert the operand to that mode
     before generating the instruction.

     The `ffs' built-in function of C always uses the mode which
     corresponds to the C data type `int'.

`one_cmplM2'
     Store the bitwise-complement of operand 1 into operand 0.

`cmpM'
     Compare operand 0 and operand 1, and set the condition codes.  The
     RTL pattern should look like this:

          (set (cc0) (compare (match_operand:M 0 ...)
                              (match_operand:M 1 ...)))

`tstM'
     Compare operand 0 against zero, and set the condition codes.  The
     RTL pattern should look like this:

          (set (cc0) (match_operand:M 0 ...))

     `tstM' patterns should not be defined for machines that do not use
     `(cc0)'.  Doing so would confuse the optimizer since it would no
     longer be clear which `set' operations were comparisons.  The
     `cmpM' patterns should be used instead.

`movstrM'
     Block move instruction.  The addresses of the destination and
     source strings are the first two operands, and both are in mode
     `Pmode'.

     The number of bytes to move is the third operand, in mode M.
     Usually, you specify `word_mode' for M.  However, if you can
     generate better code knowing the range of valid lengths is smaller
     than those representable in a full word, you should provide a
     pattern with a mode corresponding to the range of values you can
     handle efficiently (e.g., `QImode' for values in the range 0-127;
     note we avoid numbers that appear negative) and also a pattern
     with `word_mode'.

     The fourth operand is the known shared alignment of the source and
     destination, in the form of a `const_int' rtx.  Thus, if the
     compiler knows that both source and destination are word-aligned,
     it may provide the value 4 for this operand.

     Descriptions of multiple `movstrM' patterns can only be beneficial
     if the patterns for smaller modes have fewer restrictions on their
     first, second and fourth operands.  Note that the mode M in
     `movstrM' does not impose any restriction on the mode of
     individually moved data units in the block.

     These patterns need not give special consideration to the
     possibility that the source and destination strings might overlap.

`clrstrM'
     Block clear instruction.  The addresses of the destination string
     is the first operand, in mode `Pmode'.  The number of bytes to
     clear is the second operand, in mode M.  See `movstrM' for a
     discussion of the choice of mode.

     The third operand is the known alignment of the destination, in
     the form of a `const_int' rtx.  Thus, if the compiler knows that
     the destination is word-aligned, it may provide the value 4 for
     this operand.

     The use for multiple `clrstrM' is as for `movstrM'.

`cmpstrM'
     Block compare instruction, with five operands.  Operand 0 is the
     output; it has mode M.  The remaining four operands are like the
     operands of `movstrM'.  The two memory blocks specified are
     compared byte by byte in lexicographic order.  The effect of the
     instruction is to store a value in operand 0 whose sign indicates
     the result of the comparison.

`strlenM'
     Compute the length of a string, with three operands.  Operand 0 is
     the result (of mode M), operand 1 is a `mem' referring to the
     first character of the string, operand 2 is the character to
     search for (normally zero), and operand 3 is a constant describing
     the known alignment of the beginning of the string.

`floatMN2'
     Convert signed integer operand 1 (valid for fixed point mode M) to
     floating point mode N and store in operand 0 (which has mode N).

`floatunsMN2'
     Convert unsigned integer operand 1 (valid for fixed point mode M)
     to floating point mode N and store in operand 0 (which has mode N).

`fixMN2'
     Convert operand 1 (valid for floating point mode M) to fixed point
     mode N as a signed number and store in operand 0 (which has mode
     N).  This instruction's result is defined only when the value of
     operand 1 is an integer.

`fixunsMN2'
     Convert operand 1 (valid for floating point mode M) to fixed point
     mode N as an unsigned number and store in operand 0 (which has
     mode N).  This instruction's result is defined only when the value
     of operand 1 is an integer.

`ftruncM2'
     Convert operand 1 (valid for floating point mode M) to an integer
     value, still represented in floating point mode M, and store it in
     operand 0 (valid for floating point mode M).

`fix_truncMN2'
     Like `fixMN2' but works for any floating point value of mode M by
     converting the value to an integer.

`fixuns_truncMN2'
     Like `fixunsMN2' but works for any floating point value of mode M
     by converting the value to an integer.

`truncMN2'
     Truncate operand 1 (valid for mode M) to mode N and store in
     operand 0 (which has mode N).  Both modes must be fixed point or
     both floating point.

`extendMN2'
     Sign-extend operand 1 (valid for mode M) to mode N and store in
     operand 0 (which has mode N).  Both modes must be fixed point or
     both floating point.

`zero_extendMN2'
     Zero-extend operand 1 (valid for mode M) to mode N and store in
     operand 0 (which has mode N).  Both modes must be fixed point.

`extv'
     Extract a bit-field from operand 1 (a register or memory operand),
     where operand 2 specifies the width in bits and operand 3 the
     starting bit, and store it in operand 0.  Operand 0 must have mode
     `word_mode'.  Operand 1 may have mode `byte_mode' or `word_mode';
     often `word_mode' is allowed only for registers.  Operands 2 and 3
     must be valid for `word_mode'.

     The RTL generation pass generates this instruction only with
     constants for operands 2 and 3.

     The bit-field value is sign-extended to a full word integer before
     it is stored in operand 0.

`extzv'
     Like `extv' except that the bit-field value is zero-extended.

`insv'
     Store operand 3 (which must be valid for `word_mode') into a
     bit-field in operand 0, where operand 1 specifies the width in
     bits and operand 2 the starting bit.  Operand 0 may have mode
     `byte_mode' or `word_mode'; often `word_mode' is allowed only for
     registers.  Operands 1 and 2 must be valid for `word_mode'.

     The RTL generation pass generates this instruction only with
     constants for operands 1 and 2.

`movMODEcc'
     Conditionally move operand 2 or operand 3 into operand 0 according
     to the comparison in operand 1.  If the comparison is true,
     operand 2 is moved into operand 0, otherwise operand 3 is moved.

     The mode of the operands being compared need not be the same as
     the operands being moved.  Some machines, sparc64 for example,
     have instructions that conditionally move an integer value based
     on the floating point condition codes and vice versa.

     If the machine does not have conditional move instructions, do not
     define these patterns.

`sCOND'
     Store zero or nonzero in the operand according to the condition
     codes.  Value stored is nonzero iff the condition COND is true.
     COND is the name of a comparison operation expression code, such
     as `eq', `lt' or `leu'.

     You specify the mode that the operand must have when you write the
     `match_operand' expression.  The compiler automatically sees which
     mode you have used and supplies an operand of that mode.

     The value stored for a true condition must have 1 as its low bit,
     or else must be negative.  Otherwise the instruction is not
     suitable and you should omit it from the machine description.  You
     describe to the compiler exactly which value is stored by defining
     the macro `STORE_FLAG_VALUE' (*note Misc::).  If a description
     cannot be found that can be used for all the `sCOND' patterns, you
     should omit those operations from the machine description.

     These operations may fail, but should do so only in relatively
     uncommon cases; if they would fail for common cases involving
     integer comparisons, it is best to omit these patterns.

     If these operations are omitted, the compiler will usually
     generate code that copies the constant one to the target and
     branches around an assignment of zero to the target.  If this code
     is more efficient than the potential instructions used for the
     `sCOND' pattern followed by those required to convert the result
     into a 1 or a zero in `SImode', you should omit the `sCOND'
     operations from the machine description.

`bCOND'
     Conditional branch instruction.  Operand 0 is a `label_ref' that
     refers to the label to jump to.  Jump if the condition codes meet
     condition COND.

     Some machines do not follow the model assumed here where a
     comparison instruction is followed by a conditional branch
     instruction.  In that case, the `cmpM' (and `tstM') patterns should
     simply store the operands away and generate all the required insns
     in a `define_expand' (*note Expander Definitions::) for the
     conditional branch operations.  All calls to expand `bCOND'
     patterns are immediately preceded by calls to expand either a
     `cmpM' pattern or a `tstM' pattern.

     Machines that use a pseudo register for the condition code value,
     or where the mode used for the comparison depends on the condition
     being tested, should also use the above mechanism.  *Note Jump
     Patterns::.

     The above discussion also applies to the `movMODEcc' and `sCOND'
     patterns.

`jump'
     A jump inside a function; an unconditional branch.  Operand 0 is
     the `label_ref' of the label to jump to.  This pattern name is
     mandatory on all machines.

`call'
     Subroutine call instruction returning no value.  Operand 0 is the
     function to call; operand 1 is the number of bytes of arguments
     pushed as a `const_int'; operand 2 is the number of registers used
     as operands.

     On most machines, operand 2 is not actually stored into the RTL
     pattern.  It is supplied for the sake of some RISC machines which
     need to put this information into the assembler code; they can put
     it in the RTL instead of operand 1.

     Operand 0 should be a `mem' RTX whose address is the address of the
     function.  Note, however, that this address can be a `symbol_ref'
     expression even if it would not be a legitimate memory address on
     the target machine.  If it is also not a valid argument for a call
     instruction, the pattern for this operation should be a
     `define_expand' (*note Expander Definitions::) that places the
     address into a register and uses that register in the call
     instruction.

`call_value'
     Subroutine call instruction returning a value.  Operand 0 is the
     hard register in which the value is returned.  There are three more
     operands, the same as the three operands of the `call' instruction
     (but with numbers increased by one).

     Subroutines that return `BLKmode' objects use the `call' insn.

`call_pop', `call_value_pop'
     Similar to `call' and `call_value', except used if defined and if
     `RETURN_POPS_ARGS' is nonzero.  They should emit a `parallel' that
     contains both the function call and a `set' to indicate the
     adjustment made to the frame pointer.

     For machines where `RETURN_POPS_ARGS' can be nonzero, the use of
     these patterns increases the number of functions for which the
     frame pointer can be eliminated, if desired.

`untyped_call'
     Subroutine call instruction returning a value of any type.
     Operand 0 is the function to call; operand 1 is a memory location
     where the result of calling the function is to be stored; operand
     2 is a `parallel' expression where each element is a `set'
     expression that indicates the saving of a function return value
     into the result block.

     This instruction pattern should be defined to support
     `__builtin_apply' on machines where special instructions are needed
     to call a subroutine with arbitrary arguments or to save the value
     returned.  This instruction pattern is required on machines that
     have multiple registers that can hold a return value (i.e.
     `FUNCTION_VALUE_REGNO_P' is true for more than one register).

`return'
     Subroutine return instruction.  This instruction pattern name
     should be defined only if a single instruction can do all the work
     of returning from a function.

     Like the `movM' patterns, this pattern is also used after the RTL
     generation phase.  In this case it is to support machines where
     multiple instructions are usually needed to return from a
     function, but some class of functions only requires one
     instruction to implement a return.  Normally, the applicable
     functions are those which do not need to save any registers or
     allocate stack space.

     For such machines, the condition specified in this pattern should
     only be true when `reload_completed' is nonzero and the function's
     epilogue would only be a single instruction.  For machines with
     register windows, the routine `leaf_function_p' may be used to
     determine if a register window push is required.

     Machines that have conditional return instructions should define
     patterns such as

          (define_insn ""
            [(set (pc)
                  (if_then_else (match_operator
                                   0 "comparison_operator"
                                   [(cc0) (const_int 0)])
                                (return)
                                (pc)))]
            "CONDITION"
            "...")

     where CONDITION would normally be the same condition specified on
     the named `return' pattern.

`untyped_return'
     Untyped subroutine return instruction.  This instruction pattern
     should be defined to support `__builtin_return' on machines where
     special instructions are needed to return a value of any type.

     Operand 0 is a memory location where the result of calling a
     function with `__builtin_apply' is stored; operand 1 is a
     `parallel' expression where each element is a `set' expression
     that indicates the restoring of a function return value from the
     result block.

`nop'
     No-op instruction.  This instruction pattern name should always be
     defined to output a no-op in assembler code.  `(const_int 0)' will
     do as an RTL pattern.

`indirect_jump'
     An instruction to jump to an address which is operand zero.  This
     pattern name is mandatory on all machines.

`casesi'
     Instruction to jump through a dispatch table, including bounds
     checking.  This instruction takes five operands:

       1. The index to dispatch on, which has mode `SImode'.

       2. The lower bound for indices in the table, an integer constant.

       3. The total range of indices in the table--the largest index
          minus the smallest one (both inclusive).

       4. A label that precedes the table itself.

       5. A label to jump to if the index has a value outside the
          bounds.  (If the machine-description macro
          `CASE_DROPS_THROUGH' is defined, then an out-of-bounds index
          drops through to the code following the jump table instead of
          jumping to this label.  In that case, this label is not
          actually used by the `casesi' instruction, but it is always
          provided as an operand.)

     The table is a `addr_vec' or `addr_diff_vec' inside of a
     `jump_insn'.  The number of elements in the table is one plus the
     difference between the upper bound and the lower bound.

`tablejump'
     Instruction to jump to a variable address.  This is a low-level
     capability which can be used to implement a dispatch table when
     there is no `casesi' pattern.

     This pattern requires two operands: the address or offset, and a
     label which should immediately precede the jump table.  If the
     macro `CASE_VECTOR_PC_RELATIVE' evaluates to a nonzero value then
     the first operand is an offset which counts from the address of
     the table; otherwise, it is an absolute address to jump to.  In
     either case, the first operand has mode `Pmode'.

     The `tablejump' insn is always the last insn before the jump table
     it uses.  Its assembler code normally has no need to use the
     second operand, but you should incorporate it in the RTL pattern so
     that the jump optimizer will not delete the table as unreachable
     code.

`decrement_and_branch_until_zero'
     Conditional branch instruction that decrements a register and
     jumps if the register is nonzero.  Operand 0 is the register to
     decrement and test; operand 1 is the label to jump to if the
     register is nonzero.  *Note Looping Patterns::.

     This optional instruction pattern is only used by the combiner,
     typically for loops reversed by the loop optimizer when strength
     reduction is enabled.

`doloop_end'
     Conditional branch instruction that decrements a register and
     jumps if the register is nonzero.  This instruction takes five
     operands: Operand 0 is the register to decrement and test; operand
     1 is the number of loop iterations as a `const_int' or
     `const0_rtx' if this cannot be determined until run-time; operand
     2 is the actual or estimated maximum number of iterations as a
     `const_int'; operand 3 is the number of enclosed loops as a
     `const_int' (an innermost loop has a value of 1); operand 4 is the
     label to jump to if the register is nonzero.  *Note Looping
     Patterns::.

     This optional instruction pattern should be defined for machines
     with low-overhead looping instructions as the loop optimizer will
     try to modify suitable loops to utilize it.  If nested
     low-overhead looping is not supported, use a `define_expand'
     (*note Expander Definitions::) and make the pattern fail if
     operand 3 is not `const1_rtx'.  Similarly, if the actual or
     estimated maximum number of iterations is too large for this
     instruction, make it fail.

`doloop_begin'
     Companion instruction to `doloop_end' required for machines that
     need to perform some initialization, such as loading special
     registers used by a low-overhead looping instruction.  If
     initialization insns do not always need to be emitted, use a
     `define_expand' (*note Expander Definitions::) and make it fail.

`canonicalize_funcptr_for_compare'
     Canonicalize the function pointer in operand 1 and store the result
     into operand 0.

     Operand 0 is always a `reg' and has mode `Pmode'; operand 1 may be
     a `reg', `mem', `symbol_ref', `const_int', etc and also has mode
     `Pmode'.

     Canonicalization of a function pointer usually involves computing
     the address of the function which would be called if the function
     pointer were used in an indirect call.

     Only define this pattern if function pointers on the target machine
     can have different values but still call the same function when
     used in an indirect call.

`save_stack_block'
`save_stack_function'
`save_stack_nonlocal'
`restore_stack_block'
`restore_stack_function'
`restore_stack_nonlocal'
     Most machines save and restore the stack pointer by copying it to
     or from an object of mode `Pmode'.  Do not define these patterns on
     such machines.

     Some machines require special handling for stack pointer saves and
     restores.  On those machines, define the patterns corresponding to
     the non-standard cases by using a `define_expand' (*note Expander
     Definitions::) that produces the required insns.  The three types
     of saves and restores are:

       1. `save_stack_block' saves the stack pointer at the start of a
          block that allocates a variable-sized object, and
          `restore_stack_block' restores the stack pointer when the
          block is exited.

       2. `save_stack_function' and `restore_stack_function' do a
          similar job for the outermost block of a function and are
          used when the function allocates variable-sized objects or
          calls `alloca'.  Only the epilogue uses the restored stack
          pointer, allowing a simpler save or restore sequence on some
          machines.

       3. `save_stack_nonlocal' is used in functions that contain labels
          branched to by nested functions.  It saves the stack pointer
          in such a way that the inner function can use
          `restore_stack_nonlocal' to restore the stack pointer.  The
          compiler generates code to restore the frame and argument
          pointer registers, but some machines require saving and
          restoring additional data such as register window information
          or stack backchains.  Place insns in these patterns to save
          and restore any such required data.

     When saving the stack pointer, operand 0 is the save area and
     operand 1 is the stack pointer.  The mode used to allocate the
     save area defaults to `Pmode' but you can override that choice by
     defining the `STACK_SAVEAREA_MODE' macro (*note Storage Layout::).
     You must specify an integral mode, or `VOIDmode' if no save area
     is needed for a particular type of save (either because no save is
     needed or because a machine-specific save area can be used).
     Operand 0 is the stack pointer and operand 1 is the save area for
     restore operations.  If `save_stack_block' is defined, operand 0
     must not be `VOIDmode' since these saves can be arbitrarily nested.

     A save area is a `mem' that is at a constant offset from
     `virtual_stack_vars_rtx' when the stack pointer is saved for use by
     nonlocal gotos and a `reg' in the other two cases.

`allocate_stack'
     Subtract (or add if `STACK_GROWS_DOWNWARD' is undefined) operand 1
     from the stack pointer to create space for dynamically allocated
     data.

     Store the resultant pointer to this space into operand 0.  If you
     are allocating space from the main stack, do this by emitting a
     move insn to copy `virtual_stack_dynamic_rtx' to operand 0.  If
     you are allocating the space elsewhere, generate code to copy the
     location of the space to operand 0.  In the latter case, you must
     ensure this space gets freed when the corresponding space on the
     main stack is free.

     Do not define this pattern if all that must be done is the
     subtraction.  Some machines require other operations such as stack
     probes or maintaining the back chain.  Define this pattern to emit
     those operations in addition to updating the stack pointer.

`probe'
     Some machines require instructions to be executed after space is
     allocated from the stack, for example to generate a reference at
     the bottom of the stack.

     If you need to emit instructions before the stack has been
     adjusted, put them into the `allocate_stack' pattern.  Otherwise,
     define this pattern to emit the required instructions.

     No operands are provided.

`check_stack'
     If stack checking cannot be done on your system by probing the
     stack with a load or store instruction (*note Stack Checking::),
     define this pattern to perform the needed check and signaling an
     error if the stack has overflowed.  The single operand is the
     location in the stack furthest from the current stack pointer that
     you need to validate.  Normally, on machines where this pattern is
     needed, you would obtain the stack limit from a global or
     thread-specific variable or register.

`nonlocal_goto'
     Emit code to generate a non-local goto, e.g., a jump from one
     function to a label in an outer function.  This pattern has four
     arguments, each representing a value to be used in the jump.  The
     first argument is to be loaded into the frame pointer, the second
     is the address to branch to (code to dispatch to the actual label),
     the third is the address of a location where the stack is saved,
     and the last is the address of the label, to be placed in the
     location for the incoming static chain.

     On most machines you need not define this pattern, since GCC will
     already generate the correct code, which is to load the frame
     pointer and static chain, restore the stack (using the
     `restore_stack_nonlocal' pattern, if defined), and jump indirectly
     to the dispatcher.  You need only define this pattern if this code
     will not work on your machine.

`nonlocal_goto_receiver'
     This pattern, if defined, contains code needed at the target of a
     nonlocal goto after the code already generated by GCC.  You will
     not normally need to define this pattern.  A typical reason why
     you might need this pattern is if some value, such as a pointer to
     a global table, must be restored when the frame pointer is
     restored.  Note that a nonlocal goto only occurs within a
     unit-of-translation, so a global table pointer that is shared by
     all functions of a given module need not be restored.  There are
     no arguments.

`exception_receiver'
     This pattern, if defined, contains code needed at the site of an
     exception handler that isn't needed at the site of a nonlocal
     goto.  You will not normally need to define this pattern.  A
     typical reason why you might need this pattern is if some value,
     such as a pointer to a global table, must be restored after
     control flow is branched to the handler of an exception.  There
     are no arguments.

`builtin_setjmp_setup'
     This pattern, if defined, contains additional code needed to
     initialize the `jmp_buf'.  You will not normally need to define
     this pattern.  A typical reason why you might need this pattern is
     if some value, such as a pointer to a global table, must be
     restored.  Though it is preferred that the pointer value be
     recalculated if possible (given the address of a label for
     instance).  The single argument is a pointer to the `jmp_buf'.
     Note that the buffer is five words long and that the first three
     are normally used by the generic mechanism.

`builtin_setjmp_receiver'
     This pattern, if defined, contains code needed at the site of an
     built-in setjmp that isn't needed at the site of a nonlocal goto.
     You will not normally need to define this pattern.  A typical
     reason why you might need this pattern is if some value, such as a
     pointer to a global table, must be restored.  It takes one
     argument, which is the label to which builtin_longjmp transfered
     control; this pattern may be emitted at a small offset from that
     label.

`builtin_longjmp'
     This pattern, if defined, performs the entire action of the
     longjmp.  You will not normally need to define this pattern unless
     you also define `builtin_setjmp_setup'.  The single argument is a
     pointer to the `jmp_buf'.

`eh_return'
     This pattern, if defined, affects the way `__builtin_eh_return',
     and thence the call frame exception handling library routines, are
     built.  It is intended to handle non-trivial actions needed along
     the abnormal return path.

     The pattern takes two arguments.  The first is an offset to be
     applied to the stack pointer.  It will have been copied to some
     appropriate location (typically `EH_RETURN_STACKADJ_RTX') which
     will survive until after reload to when the normal epilogue is
     generated.  The second argument is the address of the exception
     handler to which the function should return.  This will normally
     need to copied by the pattern to some special register or memory
     location.

     This pattern only needs to be defined if call frame exception
     handling is to be used, and simple moves involving
     `EH_RETURN_STACKADJ_RTX' and `EH_RETURN_HANDLER_RTX' are not
     sufficient.

`prologue'
     This pattern, if defined, emits RTL for entry to a function.  The
     function entry is responsible for setting up the stack frame,
     initializing the frame pointer register, saving callee saved
     registers, etc.

     Using a prologue pattern is generally preferred over defining
     `TARGET_ASM_FUNCTION_PROLOGUE' to emit assembly code for the
     prologue.

     The `prologue' pattern is particularly useful for targets which
     perform instruction scheduling.

`epilogue'
     This pattern emits RTL for exit from a function.  The function
     exit is responsible for deallocating the stack frame, restoring
     callee saved registers and emitting the return instruction.

     Using an epilogue pattern is generally preferred over defining
     `TARGET_ASM_FUNCTION_EPILOGUE' to emit assembly code for the
     epilogue.

     The `epilogue' pattern is particularly useful for targets which
     perform instruction scheduling or which have delay slots for their
     return instruction.

`sibcall_epilogue'
     This pattern, if defined, emits RTL for exit from a function
     without the final branch back to the calling function.  This
     pattern will be emitted before any sibling call (aka tail call)
     sites.

     The `sibcall_epilogue' pattern must not clobber any arguments used
     for parameter passing or any stack slots for arguments passed to
     the current function.

`trap'
     This pattern, if defined, signals an error, typically by causing
     some kind of signal to be raised.  Among other places, it is used
     by the Java front end to signal `invalid array index' exceptions.

`conditional_trap'
     Conditional trap instruction.  Operand 0 is a piece of RTL which
     performs a comparison.  Operand 1 is the trap code, an integer.

     A typical `conditional_trap' pattern looks like

          (define_insn "conditional_trap"
            [(trap_if (match_operator 0 "trap_operator"
                       [(cc0) (const_int 0)])
                      (match_operand 1 "const_int_operand" "i"))]
            ""
            "...")

`prefetch'
     This pattern, if defined, emits code for a non-faulting data
     prefetch instruction.  Operand 0 is the address of the memory to
     prefetch.  Operand 1 is a constant 1 if the prefetch is preparing
     for a write to the memory address, or a constant 0 otherwise.
     Operand 2 is the expected degree of temporal locality of the data
     and is a value between 0 and 3, inclusive; 0 means that the data
     has no temporal locality, so it need not be left in the cache
     after the access; 3 means that the data has a high degree of
     temporal locality and should be left in all levels of cache
     possible;  1 and 2 mean, respectively, a low or moderate degree of
     temporal locality.

     Targets that do not support write prefetches or locality hints can
     ignore the values of operands 1 and 2.

`cycle_display'
     This pattern, if present, will be emitted by the instruction
     scheduler at the beginning of each new clock cycle.  This can be
     used for annotating the assembler output with cycle counts.
     Operand 0 is a `const_int' that holds the clock cycle.



File: gccint.info,  Node: Pattern Ordering,  Next: Dependent Patterns,  Prev: Standard Names,  Up: Machine Desc

9.9 When the Order of Patterns Matters
======================================

Sometimes an insn can match more than one instruction pattern.  Then the
pattern that appears first in the machine description is the one used.
Therefore, more specific patterns (patterns that will match fewer
things) and faster instructions (those that will produce better code
when they do match) should usually go first in the description.

   In some cases the effect of ordering the patterns can be used to hide
a pattern when it is not valid.  For example, the 68000 has an
instruction for converting a fullword to floating point and another for
converting a byte to floating point.  An instruction converting an
integer to floating point could match either one.  We put the pattern
to convert the fullword first to make sure that one will be used rather
than the other.  (Otherwise a large integer might be generated as a
single-byte immediate quantity, which would not work.)  Instead of
using this pattern ordering it would be possible to make the pattern
for convert-a-byte smart enough to deal properly with any constant
value.


File: gccint.info,  Node: Dependent Patterns,  Next: Jump Patterns,  Prev: Pattern Ordering,  Up: Machine Desc

9.10 Interdependence of Patterns
================================

Every machine description must have a named pattern for each of the
conditional branch names `bCOND'.  The recognition template must always
have the form

     (set (pc)
          (if_then_else (COND (cc0) (const_int 0))
                        (label_ref (match_operand 0 "" ""))
                        (pc)))

In addition, every machine description must have an anonymous pattern
for each of the possible reverse-conditional branches.  Their templates
look like

     (set (pc)
          (if_then_else (COND (cc0) (const_int 0))
                        (pc)
                        (label_ref (match_operand 0 "" ""))))

They are necessary because jump optimization can turn direct-conditional
branches into reverse-conditional branches.

   It is often convenient to use the `match_operator' construct to
reduce the number of patterns that must be specified for branches.  For
example,

     (define_insn ""
       [(set (pc)
             (if_then_else (match_operator 0 "comparison_operator"
                                           [(cc0) (const_int 0)])
                           (pc)
                           (label_ref (match_operand 1 "" ""))))]
       "CONDITION"
       "...")

   In some cases machines support instructions identical except for the
machine mode of one or more operands.  For example, there may be
"sign-extend halfword" and "sign-extend byte" instructions whose
patterns are

     (set (match_operand:SI 0 ...)
          (extend:SI (match_operand:HI 1 ...)))

     (set (match_operand:SI 0 ...)
          (extend:SI (match_operand:QI 1 ...)))

Constant integers do not specify a machine mode, so an instruction to
extend a constant value could match either pattern.  The pattern it
actually will match is the one that appears first in the file.  For
correct results, this must be the one for the widest possible mode
(`HImode', here).  If the pattern matches the `QImode' instruction, the
results will be incorrect if the constant value does not actually fit
that mode.

   Such instructions to extend constants are rarely generated because
they are optimized away, but they do occasionally happen in nonoptimized
compilations.

   If a constraint in a pattern allows a constant, the reload pass may
replace a register with a constant permitted by the constraint in some
cases.  Similarly for memory references.  Because of this substitution,
you should not provide separate patterns for increment and decrement
instructions.  Instead, they should be generated from the same pattern
that supports register-register add insns by examining the operands and
generating the appropriate machine instruction.


File: gccint.info,  Node: Jump Patterns,  Next: Looping Patterns,  Prev: Dependent Patterns,  Up: Machine Desc

9.11 Defining Jump Instruction Patterns
=======================================

For most machines, GCC assumes that the machine has a condition code.
A comparison insn sets the condition code, recording the results of both
signed and unsigned comparison of the given operands.  A separate branch
insn tests the condition code and branches or not according its value.
The branch insns come in distinct signed and unsigned flavors.  Many
common machines, such as the VAX, the 68000 and the 32000, work this
way.

   Some machines have distinct signed and unsigned compare
instructions, and only one set of conditional branch instructions.  The
easiest way to handle these machines is to treat them just like the
others until the final stage where assembly code is written.  At this
time, when outputting code for the compare instruction, peek ahead at
the following branch using `next_cc0_user (insn)'.  (The variable
`insn' refers to the insn being output, in the output-writing code in
an instruction pattern.)  If the RTL says that is an unsigned branch,
output an unsigned compare; otherwise output a signed compare.  When
the branch itself is output, you can treat signed and unsigned branches
identically.

   The reason you can do this is that GCC always generates a pair of
consecutive RTL insns, possibly separated by `note' insns, one to set
the condition code and one to test it, and keeps the pair inviolate
until the end.

   To go with this technique, you must define the machine-description
macro `NOTICE_UPDATE_CC' to do `CC_STATUS_INIT'; in other words, no
compare instruction is superfluous.

   Some machines have compare-and-branch instructions and no condition
code.  A similar technique works for them.  When it is time to "output"
a compare instruction, record its operands in two static variables.
When outputting the branch-on-condition-code instruction that follows,
actually output a compare-and-branch instruction that uses the
remembered operands.

   It also works to define patterns for compare-and-branch instructions.
In optimizing compilation, the pair of compare and branch instructions
will be combined according to these patterns.  But this does not happen
if optimization is not requested.  So you must use one of the solutions
above in addition to any special patterns you define.

   In many RISC machines, most instructions do not affect the condition
code and there may not even be a separate condition code register.  On
these machines, the restriction that the definition and use of the
condition code be adjacent insns is not necessary and can prevent
important optimizations.  For example, on the IBM RS/6000, there is a
delay for taken branches unless the condition code register is set three
instructions earlier than the conditional branch.  The instruction
scheduler cannot perform this optimization if it is not permitted to
separate the definition and use of the condition code register.

   On these machines, do not use `(cc0)', but instead use a register to
represent the condition code.  If there is a specific condition code
register in the machine, use a hard register.  If the condition code or
comparison result can be placed in any general register, or if there are
multiple condition registers, use a pseudo register.

   On some machines, the type of branch instruction generated may
depend on the way the condition code was produced; for example, on the
68k and Sparc, setting the condition code directly from an add or
subtract instruction does not clear the overflow bit the way that a test
instruction does, so a different branch instruction must be used for
some conditional branches.  For machines that use `(cc0)', the set and
use of the condition code must be adjacent (separated only by `note'
insns) allowing flags in `cc_status' to be used.  (*Note Condition
Code::.)  Also, the comparison and branch insns can be located from
each other by using the functions `prev_cc0_setter' and `next_cc0_user'.

   However, this is not true on machines that do not use `(cc0)'.  On
those machines, no assumptions can be made about the adjacency of the
compare and branch insns and the above methods cannot be used.  Instead,
we use the machine mode of the condition code register to record
different formats of the condition code register.

   Registers used to store the condition code value should have a mode
that is in class `MODE_CC'.  Normally, it will be `CCmode'.  If
additional modes are required (as for the add example mentioned above in
the Sparc), define the macro `EXTRA_CC_MODES' to list the additional
modes required (*note Condition Code::).  Also define `SELECT_CC_MODE'
to choose a mode given an operand of a compare.

   If it is known during RTL generation that a different mode will be
required (for example, if the machine has separate compare instructions
for signed and unsigned quantities, like most IBM processors), they can
be specified at that time.

   If the cases that require different modes would be made by
instruction combination, the macro `SELECT_CC_MODE' determines which
machine mode should be used for the comparison result.  The patterns
should be written using that mode.  To support the case of the add on
the Sparc discussed above, we have the pattern

     (define_insn ""
       [(set (reg:CC_NOOV 0)
             (compare:CC_NOOV
               (plus:SI (match_operand:SI 0 "register_operand" "%r")
                        (match_operand:SI 1 "arith_operand" "rI"))
               (const_int 0)))]
       ""
       "...")

   The `SELECT_CC_MODE' macro on the Sparc returns `CC_NOOVmode' for
comparisons whose argument is a `plus'.


File: gccint.info,  Node: Looping Patterns,  Next: Insn Canonicalizations,  Prev: Jump Patterns,  Up: Machine Desc

9.12 Defining Looping Instruction Patterns
==========================================

Some machines have special jump instructions that can be utilised to
make loops more efficient.  A common example is the 68000 `dbra'
instruction which performs a decrement of a register and a branch if the
result was greater than zero.  Other machines, in particular digital
signal processors (DSPs), have special block repeat instructions to
provide low-overhead loop support.  For example, the TI TMS320C3x/C4x
DSPs have a block repeat instruction that loads special registers to
mark the top and end of a loop and to count the number of loop
iterations.  This avoids the need for fetching and executing a
`dbra'-like instruction and avoids pipeline stalls associated with the
jump.

   GCC has three special named patterns to support low overhead looping.
They are `decrement_and_branch_until_zero', `doloop_begin', and
`doloop_end'.  The first pattern, `decrement_and_branch_until_zero', is
not emitted during RTL generation but may be emitted during the
instruction combination phase.  This requires the assistance of the
loop optimizer, using information collected during strength reduction,
to reverse a loop to count down to zero.  Some targets also require the
loop optimizer to add a `REG_NONNEG' note to indicate that the
iteration count is always positive.  This is needed if the target
performs a signed loop termination test.  For example, the 68000 uses a
pattern similar to the following for its `dbra' instruction:

     (define_insn "decrement_and_branch_until_zero"
       [(set (pc)
     	(if_then_else
     	  (ge (plus:SI (match_operand:SI 0 "general_operand" "+d*am")
     		       (const_int -1))
     	      (const_int 0))
     	  (label_ref (match_operand 1 "" ""))
     	  (pc)))
        (set (match_dup 0)
     	(plus:SI (match_dup 0)
     		 (const_int -1)))]
       "find_reg_note (insn, REG_NONNEG, 0)"
       "...")

   Note that since the insn is both a jump insn and has an output, it
must deal with its own reloads, hence the `m' constraints.  Also note
that since this insn is generated by the instruction combination phase
combining two sequential insns together into an implicit parallel insn,
the iteration counter needs to be biased by the same amount as the
decrement operation, in this case -1.  Note that the following similar
pattern will not be matched by the combiner.

     (define_insn "decrement_and_branch_until_zero"
       [(set (pc)
     	(if_then_else
     	  (ge (match_operand:SI 0 "general_operand" "+d*am")
     	      (const_int 1))
     	  (label_ref (match_operand 1 "" ""))
     	  (pc)))
        (set (match_dup 0)
     	(plus:SI (match_dup 0)
     		 (const_int -1)))]
       "find_reg_note (insn, REG_NONNEG, 0)"
       "...")

   The other two special looping patterns, `doloop_begin' and
`doloop_end', are emitted by the loop optimizer for certain
well-behaved loops with a finite number of loop iterations using
information collected during strength reduction.

   The `doloop_end' pattern describes the actual looping instruction
(or the implicit looping operation) and the `doloop_begin' pattern is
an optional companion pattern that can be used for initialization
needed for some low-overhead looping instructions.

   Note that some machines require the actual looping instruction to be
emitted at the top of the loop (e.g., the TMS320C3x/C4x DSPs).  Emitting
the true RTL for a looping instruction at the top of the loop can cause
problems with flow analysis.  So instead, a dummy `doloop' insn is
emitted at the end of the loop.  The machine dependent reorg pass checks
for the presence of this `doloop' insn and then searches back to the
top of the loop, where it inserts the true looping insn (provided there
are no instructions in the loop which would cause problems).  Any
additional labels can be emitted at this point.  In addition, if the
desired special iteration counter register was not allocated, this
machine dependent reorg pass could emit a traditional compare and jump
instruction pair.

   The essential difference between the
`decrement_and_branch_until_zero' and the `doloop_end' patterns is that
the loop optimizer allocates an additional pseudo register for the
latter as an iteration counter.  This pseudo register cannot be used
within the loop (i.e., general induction variables cannot be derived
from it), however, in many cases the loop induction variable may become
redundant and removed by the flow pass.


File: gccint.info,  Node: Insn Canonicalizations,  Next: Expander Definitions,  Prev: Looping Patterns,  Up: Machine Desc

9.13 Canonicalization of Instructions
=====================================

There are often cases where multiple RTL expressions could represent an
operation performed by a single machine instruction.  This situation is
most commonly encountered with logical, branch, and multiply-accumulate
instructions.  In such cases, the compiler attempts to convert these
multiple RTL expressions into a single canonical form to reduce the
number of insn patterns required.

   In addition to algebraic simplifications, following canonicalizations
are performed:

   * For commutative and comparison operators, a constant is always
     made the second operand.  If a machine only supports a constant as
     the second operand, only patterns that match a constant in the
     second operand need be supplied.

     For these operators, if only one operand is a `neg', `not',
     `mult', `plus', or `minus' expression, it will be the first
     operand.

   * For the `compare' operator, a constant is always the second operand
     on machines where `cc0' is used (*note Jump Patterns::).  On other
     machines, there are rare cases where the compiler might want to
     construct a `compare' with a constant as the first operand.
     However, these cases are not common enough for it to be worthwhile
     to provide a pattern matching a constant as the first operand
     unless the machine actually has such an instruction.

     An operand of `neg', `not', `mult', `plus', or `minus' is made the
     first operand under the same conditions as above.

   * `(minus X (const_int N))' is converted to `(plus X (const_int
     -N))'.

   * Within address computations (i.e., inside `mem'), a left shift is
     converted into the appropriate multiplication by a power of two.

   * De`Morgan's Law is used to move bitwise negation inside a bitwise
     logical-and or logical-or operation.  If this results in only one
     operand being a `not' expression, it will be the first one.

     A machine that has an instruction that performs a bitwise
     logical-and of one operand with the bitwise negation of the other
     should specify the pattern for that instruction as

          (define_insn ""
            [(set (match_operand:M 0 ...)
                  (and:M (not:M (match_operand:M 1 ...))
                               (match_operand:M 2 ...)))]
            "..."
            "...")

     Similarly, a pattern for a "NAND" instruction should be written

          (define_insn ""
            [(set (match_operand:M 0 ...)
                  (ior:M (not:M (match_operand:M 1 ...))
                               (not:M (match_operand:M 2 ...))))]
            "..."
            "...")

     In both cases, it is not necessary to include patterns for the many
     logically equivalent RTL expressions.

   * The only possible RTL expressions involving both bitwise
     exclusive-or and bitwise negation are `(xor:M X Y)' and `(not:M
     (xor:M X Y))'.

   * The sum of three items, one of which is a constant, will only
     appear in the form

          (plus:M (plus:M X Y) CONSTANT)

   * On machines that do not use `cc0', `(compare X (const_int 0))'
     will be converted to X.

   * Equality comparisons of a group of bits (usually a single bit)
     with zero will be written using `zero_extract' rather than the
     equivalent `and' or `sign_extract' operations.



File: gccint.info,  Node: Expander Definitions,  Next: Insn Splitting,  Prev: Insn Canonicalizations,  Up: Machine Desc

9.14 Defining RTL Sequences for Code Generation
===============================================

On some target machines, some standard pattern names for RTL generation
cannot be handled with single insn, but a sequence of RTL insns can
represent them.  For these target machines, you can write a
`define_expand' to specify how to generate the sequence of RTL.

   A `define_expand' is an RTL expression that looks almost like a
`define_insn'; but, unlike the latter, a `define_expand' is used only
for RTL generation and it can produce more than one RTL insn.

   A `define_expand' RTX has four operands:

   * The name.  Each `define_expand' must have a name, since the only
     use for it is to refer to it by name.

   * The RTL template.  This is a vector of RTL expressions representing
     a sequence of separate instructions.  Unlike `define_insn', there
     is no implicit surrounding `PARALLEL'.

   * The condition, a string containing a C expression.  This
     expression is used to express how the availability of this pattern
     depends on subclasses of target machine, selected by command-line
     options when GCC is run.  This is just like the condition of a
     `define_insn' that has a standard name.  Therefore, the condition
     (if present) may not depend on the data in the insn being matched,
     but only the target-machine-type flags.  The compiler needs to
     test these conditions during initialization in order to learn
     exactly which named instructions are available in a particular run.

   * The preparation statements, a string containing zero or more C
     statements which are to be executed before RTL code is generated
     from the RTL template.

     Usually these statements prepare temporary registers for use as
     internal operands in the RTL template, but they can also generate
     RTL insns directly by calling routines such as `emit_insn', etc.
     Any such insns precede the ones that come from the RTL template.

   Every RTL insn emitted by a `define_expand' must match some
`define_insn' in the machine description.  Otherwise, the compiler will
crash when trying to generate code for the insn or trying to optimize
it.

   The RTL template, in addition to controlling generation of RTL insns,
also describes the operands that need to be specified when this pattern
is used.  In particular, it gives a predicate for each operand.

   A true operand, which needs to be specified in order to generate RTL
from the pattern, should be described with a `match_operand' in its
first occurrence in the RTL template.  This enters information on the
operand's predicate into the tables that record such things.  GCC uses
the information to preload the operand into a register if that is
required for valid RTL code.  If the operand is referred to more than
once, subsequent references should use `match_dup'.

   The RTL template may also refer to internal "operands" which are
temporary registers or labels used only within the sequence made by the
`define_expand'.  Internal operands are substituted into the RTL
template with `match_dup', never with `match_operand'.  The values of
the internal operands are not passed in as arguments by the compiler
when it requests use of this pattern.  Instead, they are computed
within the pattern, in the preparation statements.  These statements
compute the values and store them into the appropriate elements of
`operands' so that `match_dup' can find them.

   There are two special macros defined for use in the preparation
statements: `DONE' and `FAIL'.  Use them with a following semicolon, as
a statement.

`DONE'
     Use the `DONE' macro to end RTL generation for the pattern.  The
     only RTL insns resulting from the pattern on this occasion will be
     those already emitted by explicit calls to `emit_insn' within the
     preparation statements; the RTL template will not be generated.

`FAIL'
     Make the pattern fail on this occasion.  When a pattern fails, it
     means that the pattern was not truly available.  The calling
     routines in the compiler will try other strategies for code
     generation using other patterns.

     Failure is currently supported only for binary (addition,
     multiplication, shifting, etc.) and bit-field (`extv', `extzv',
     and `insv') operations.

   If the preparation falls through (invokes neither `DONE' nor
`FAIL'), then the `define_expand' acts like a `define_insn' in that the
RTL template is used to generate the insn.

   The RTL template is not used for matching, only for generating the
initial insn list.  If the preparation statement always invokes `DONE'
or `FAIL', the RTL template may be reduced to a simple list of
operands, such as this example:

     (define_expand "addsi3"
       [(match_operand:SI 0 "register_operand" "")
        (match_operand:SI 1 "register_operand" "")
        (match_operand:SI 2 "register_operand" "")]
       ""
       "
     {
       handle_add (operands[0], operands[1], operands[2]);
       DONE;
     }")

   Here is an example, the definition of left-shift for the SPUR chip:

     (define_expand "ashlsi3"
       [(set (match_operand:SI 0 "register_operand" "")
             (ashift:SI
               (match_operand:SI 1 "register_operand" "")
               (match_operand:SI 2 "nonmemory_operand" "")))]
       ""
       "

     {
       if (GET_CODE (operands[2]) != CONST_INT
           || (unsigned) INTVAL (operands[2]) > 3)
         FAIL;
     }")

This example uses `define_expand' so that it can generate an RTL insn
for shifting when the shift-count is in the supported range of 0 to 3
but fail in other cases where machine insns aren't available.  When it
fails, the compiler tries another strategy using different patterns
(such as, a library call).

   If the compiler were able to handle nontrivial condition-strings in
patterns with names, then it would be possible to use a `define_insn'
in that case.  Here is another case (zero-extension on the 68000) which
makes more use of the power of `define_expand':

     (define_expand "zero_extendhisi2"
       [(set (match_operand:SI 0 "general_operand" "")
             (const_int 0))
        (set (strict_low_part
               (subreg:HI
                 (match_dup 0)
                 0))
             (match_operand:HI 1 "general_operand" ""))]
       ""
       "operands[1] = make_safe_from (operands[1], operands[0]);")

Here two RTL insns are generated, one to clear the entire output operand
and the other to copy the input operand into its low half.  This
sequence is incorrect if the input operand refers to [the old value of]
the output operand, so the preparation statement makes sure this isn't
so.  The function `make_safe_from' copies the `operands[1]' into a
temporary register if it refers to `operands[0]'.  It does this by
emitting another RTL insn.

   Finally, a third example shows the use of an internal operand.
Zero-extension on the SPUR chip is done by `and'-ing the result against
a halfword mask.  But this mask cannot be represented by a `const_int'
because the constant value is too large to be legitimate on this
machine.  So it must be copied into a register with `force_reg' and
then the register used in the `and'.

     (define_expand "zero_extendhisi2"
       [(set (match_operand:SI 0 "register_operand" "")
             (and:SI (subreg:SI
                       (match_operand:HI 1 "register_operand" "")
                       0)
                     (match_dup 2)))]
       ""
       "operands[2]
          = force_reg (SImode, GEN_INT (65535)); ")

   *Note_* If the `define_expand' is used to serve a standard binary or
unary arithmetic operation or a bit-field operation, then the last insn
it generates must not be a `code_label', `barrier' or `note'.  It must
be an `insn', `jump_insn' or `call_insn'.  If you don't need a real insn
at the end, emit an insn to copy the result of the operation into
itself.  Such an insn will generate no code, but it can avoid problems
in the compiler.


File: gccint.info,  Node: Insn Splitting,  Next: Including Patterns,  Prev: Expander Definitions,  Up: Machine Desc

9.15 Defining How to Split Instructions
=======================================

There are two cases where you should specify how to split a pattern into
multiple insns.  On machines that have instructions requiring delay
slots (*note Delay Slots::) or that have instructions whose output is
not available for multiple cycles (*note Function Units::), the compiler
phases that optimize these cases need to be able to move insns into
one-instruction delay slots.  However, some insns may generate more
than one machine instruction.  These insns cannot be placed into a
delay slot.

   Often you can rewrite the single insn as a list of individual insns,
each corresponding to one machine instruction.  The disadvantage of
doing so is that it will cause the compilation to be slower and require
more space.  If the resulting insns are too complex, it may also
suppress some optimizations.  The compiler splits the insn if there is a
reason to believe that it might improve instruction or delay slot
scheduling.

   The insn combiner phase also splits putative insns.  If three insns
are merged into one insn with a complex expression that cannot be
matched by some `define_insn' pattern, the combiner phase attempts to
split the complex pattern into two insns that are recognized.  Usually
it can break the complex pattern into two patterns by splitting out some
subexpression.  However, in some other cases, such as performing an
addition of a large constant in two insns on a RISC machine, the way to
split the addition into two insns is machine-dependent.

   The `define_split' definition tells the compiler how to split a
complex insn into several simpler insns.  It looks like this:

     (define_split
       [INSN-PATTERN]
       "CONDITION"
       [NEW-INSN-PATTERN-1
        NEW-INSN-PATTERN-2
        ...]
       "PREPARATION-STATEMENTS")

   INSN-PATTERN is a pattern that needs to be split and CONDITION is
the final condition to be tested, as in a `define_insn'.  When an insn
matching INSN-PATTERN and satisfying CONDITION is found, it is replaced
in the insn list with the insns given by NEW-INSN-PATTERN-1,
NEW-INSN-PATTERN-2, etc.

   The PREPARATION-STATEMENTS are similar to those statements that are
specified for `define_expand' (*note Expander Definitions::) and are
executed before the new RTL is generated to prepare for the generated
code or emit some insns whose pattern is not fixed.  Unlike those in
`define_expand', however, these statements must not generate any new
pseudo-registers.  Once reload has completed, they also must not
allocate any space in the stack frame.

   Patterns are matched against INSN-PATTERN in two different
circumstances.  If an insn needs to be split for delay slot scheduling
or insn scheduling, the insn is already known to be valid, which means
that it must have been matched by some `define_insn' and, if
`reload_completed' is nonzero, is known to satisfy the constraints of
that `define_insn'.  In that case, the new insn patterns must also be
insns that are matched by some `define_insn' and, if `reload_completed'
is nonzero, must also satisfy the constraints of those definitions.

   As an example of this usage of `define_split', consider the following
example from `a29k.md', which splits a `sign_extend' from `HImode' to
`SImode' into a pair of shift insns:

     (define_split
       [(set (match_operand:SI 0 "gen_reg_operand" "")
             (sign_extend:SI (match_operand:HI 1 "gen_reg_operand" "")))]
       ""
       [(set (match_dup 0)
             (ashift:SI (match_dup 1)
                        (const_int 16)))
        (set (match_dup 0)
             (ashiftrt:SI (match_dup 0)
                          (const_int 16)))]
       "
     { operands[1] = gen_lowpart (SImode, operands[1]); }")

   When the combiner phase tries to split an insn pattern, it is always
the case that the pattern is _not_ matched by any `define_insn'.  The
combiner pass first tries to split a single `set' expression and then
the same `set' expression inside a `parallel', but followed by a
`clobber' of a pseudo-reg to use as a scratch register.  In these
cases, the combiner expects exactly two new insn patterns to be
generated.  It will verify that these patterns match some `define_insn'
definitions, so you need not do this test in the `define_split' (of
course, there is no point in writing a `define_split' that will never
produce insns that match).

   Here is an example of this use of `define_split', taken from
`rs6000.md':

     (define_split
       [(set (match_operand:SI 0 "gen_reg_operand" "")
             (plus:SI (match_operand:SI 1 "gen_reg_operand" "")
                      (match_operand:SI 2 "non_add_cint_operand" "")))]
       ""
       [(set (match_dup 0) (plus:SI (match_dup 1) (match_dup 3)))
        (set (match_dup 0) (plus:SI (match_dup 0) (match_dup 4)))]
     "
     {
       int low = INTVAL (operands[2]) & 0xffff;
       int high = (unsigned) INTVAL (operands[2]) >> 16;

       if (low & 0x8000)
         high++, low |= 0xffff0000;

       operands[3] = GEN_INT (high << 16);
       operands[4] = GEN_INT (low);
     }")

   Here the predicate `non_add_cint_operand' matches any `const_int'
that is _not_ a valid operand of a single add insn.  The add with the
smaller displacement is written so that it can be substituted into the
address of a subsequent operation.

   An example that uses a scratch register, from the same file,
generates an equality comparison of a register and a large constant:

     (define_split
       [(set (match_operand:CC 0 "cc_reg_operand" "")
             (compare:CC (match_operand:SI 1 "gen_reg_operand" "")
                         (match_operand:SI 2 "non_short_cint_operand" "")))
        (clobber (match_operand:SI 3 "gen_reg_operand" ""))]
       "find_single_use (operands[0], insn, 0)
        && (GET_CODE (*find_single_use (operands[0], insn, 0)) == EQ
            || GET_CODE (*find_single_use (operands[0], insn, 0)) == NE)"
       [(set (match_dup 3) (xor:SI (match_dup 1) (match_dup 4)))
        (set (match_dup 0) (compare:CC (match_dup 3) (match_dup 5)))]
       "
     {
       /* Get the constant we are comparing against, C, and see what it
          looks like sign-extended to 16 bits.  Then see what constant
          could be XOR'ed with C to get the sign-extended value.  */

       int c = INTVAL (operands[2]);
       int sextc = (c << 16) >> 16;
       int xorv = c ^ sextc;

       operands[4] = GEN_INT (xorv);
       operands[5] = GEN_INT (sextc);
     }")

   To avoid confusion, don't write a single `define_split' that accepts
some insns that match some `define_insn' as well as some insns that
don't.  Instead, write two separate `define_split' definitions, one for
the insns that are valid and one for the insns that are not valid.

   The splitter is allowed to split jump instructions into sequence of
jumps or create new jumps in while splitting non-jump instructions.  As
the central flowgraph and branch prediction information needs to be
updated, several restriction apply.

   Splitting of jump instruction into sequence that over by another jump
instruction is always valid, as compiler expect identical behavior of
new jump.  When new sequence contains multiple jump instructions or new
labels, more assistance is needed.  Splitter is required to create only
unconditional jumps, or simple conditional jump instructions.
Additionally it must attach a `REG_BR_PROB' note to each conditional
jump. An global variable `split_branch_probability' hold the
probability of original branch in case it was an simple conditional
jump, -1 otherwise.  To simplify recomputing of edge frequencies, new
sequence is required to have only forward jumps to the newly created
labels.

   For the common case where the pattern of a define_split exactly
matches the pattern of a define_insn, use `define_insn_and_split'.  It
looks like this:

     (define_insn_and_split
       [INSN-PATTERN]
       "CONDITION"
       "OUTPUT-TEMPLATE"
       "SPLIT-CONDITION"
       [NEW-INSN-PATTERN-1
        NEW-INSN-PATTERN-2
        ...]
       "PREPARATION-STATEMENTS"
       [INSN-ATTRIBUTES])

   INSN-PATTERN, CONDITION, OUTPUT-TEMPLATE, and INSN-ATTRIBUTES are
used as in `define_insn'.  The NEW-INSN-PATTERN vector and the
PREPARATION-STATEMENTS are used as in a `define_split'.  The
SPLIT-CONDITION is also used as in `define_split', with the additional
behavior that if the condition starts with `&&', the condition used for
the split will be the constructed as a logical "and" of the split
condition with the insn condition.  For example, from i386.md:

     (define_insn_and_split "zero_extendhisi2_and"
       [(set (match_operand:SI 0 "register_operand" "=r")
          (zero_extend:SI (match_operand:HI 1 "register_operand" "0")))
        (clobber (reg:CC 17))]
       "TARGET_ZERO_EXTEND_WITH_AND && !optimize_size"
       "#"
       "&& reload_completed"
       [(parallel [(set (match_dup 0)
                        (and:SI (match_dup 0) (const_int 65535)))
     	      (clobber (reg:CC 17))])]
       ""
       [(set_attr "type" "alu1")])

   In this case, the actual split condition will be
`TARGET_ZERO_EXTEND_WITH_AND && !optimize_size && reload_completed'.

   The `define_insn_and_split' construction provides exactly the same
functionality as two separate `define_insn' and `define_split'
patterns.  It exists for compactness, and as a maintenance tool to
prevent having to ensure the two patterns' templates match.


File: gccint.info,  Node: Including Patterns,  Next: Peephole Definitions,  Prev: Insn Splitting,  Up: Machine Desc

9.16 Including Patterns in Machine Descriptions.
================================================

The `include' pattern tells the compiler tools where to look for
patterns that are in files other than in the file `.md'. This is used
only at build time and there is no preprocessing allowed.

   It looks like:


     (include
       PATHNAME)

   For example:


     (include "filestuff")

   Where PATHNAME is a string that specifies the the location of the
file, specifies the include file to be in
`gcc/config/target/filestuff'. The directory `gcc/config/target' is
regarded as the default directory.

   Machine descriptions may be split up into smaller more manageable
subsections and placed into subdirectories.

   By specifying:


     (include "BOGUS/filestuff")

   the include file is specified to be in
`gcc/config/TARGET/BOGUS/filestuff'.

   Specifying an absolute path for the include file such as;

     (include "/u2/BOGUS/filestuff")
   is permitted but is not encouraged.

9.16.1 RTL Generation Tool Options for Directory Search
-------------------------------------------------------

The `-IDIR' option specifies directories to search for machine
descriptions.  For example:


     genrecog -I/p1/abc/proc1 -I/p2/abcd/pro2 target.md

   Add the directory DIR to the head of the list of directories to be
searched for header files.  This can be used to override a system
machine definition file, substituting your own version, since these
directories are searched before the default machine description file
directories.  If you use more than one `-I' option, the directories are
scanned in left-to-right order; the standard default directory come
after.


File: gccint.info,  Node: Peephole Definitions,  Next: Insn Attributes,  Prev: Including Patterns,  Up: Machine Desc

9.17 Machine-Specific Peephole Optimizers
=========================================

In addition to instruction patterns the `md' file may contain
definitions of machine-specific peephole optimizations.

   The combiner does not notice certain peephole optimizations when the
data flow in the program does not suggest that it should try them.  For
example, sometimes two consecutive insns related in purpose can be
combined even though the second one does not appear to use a register
computed in the first one.  A machine-specific peephole optimizer can
detect such opportunities.

   There are two forms of peephole definitions that may be used.  The
original `define_peephole' is run at assembly output time to match
insns and substitute assembly text.  Use of `define_peephole' is
deprecated.

   A newer `define_peephole2' matches insns and substitutes new insns.
The `peephole2' pass is run after register allocation but before
scheduling, which may result in much better code for targets that do
scheduling.

* Menu:

* define_peephole::     RTL to Text Peephole Optimizers
* define_peephole2::    RTL to RTL Peephole Optimizers


File: gccint.info,  Node: define_peephole,  Next: define_peephole2,  Up: Peephole Definitions

9.17.1 RTL to Text Peephole Optimizers
--------------------------------------

A definition looks like this:

     (define_peephole
       [INSN-PATTERN-1
        INSN-PATTERN-2
        ...]
       "CONDITION"
       "TEMPLATE"
       "OPTIONAL-INSN-ATTRIBUTES")

The last string operand may be omitted if you are not using any
machine-specific information in this machine description.  If present,
it must obey the same rules as in a `define_insn'.

   In this skeleton, INSN-PATTERN-1 and so on are patterns to match
consecutive insns.  The optimization applies to a sequence of insns when
INSN-PATTERN-1 matches the first one, INSN-PATTERN-2 matches the next,
and so on.

   Each of the insns matched by a peephole must also match a
`define_insn'.  Peepholes are checked only at the last stage just
before code generation, and only optionally.  Therefore, any insn which
would match a peephole but no `define_insn' will cause a crash in code
generation in an unoptimized compilation, or at various optimization
stages.

   The operands of the insns are matched with `match_operands',
`match_operator', and `match_dup', as usual.  What is not usual is that
the operand numbers apply to all the insn patterns in the definition.
So, you can check for identical operands in two insns by using
`match_operand' in one insn and `match_dup' in the other.

   The operand constraints used in `match_operand' patterns do not have
any direct effect on the applicability of the peephole, but they will
be validated afterward, so make sure your constraints are general enough
to apply whenever the peephole matches.  If the peephole matches but
the constraints are not satisfied, the compiler will crash.

   It is safe to omit constraints in all the operands of the peephole;
or you can write constraints which serve as a double-check on the
criteria previously tested.

   Once a sequence of insns matches the patterns, the CONDITION is
checked.  This is a C expression which makes the final decision whether
to perform the optimization (we do so if the expression is nonzero).  If
CONDITION is omitted (in other words, the string is empty) then the
optimization is applied to every sequence of insns that matches the
patterns.

   The defined peephole optimizations are applied after register
allocation is complete.  Therefore, the peephole definition can check
which operands have ended up in which kinds of registers, just by
looking at the operands.

   The way to refer to the operands in CONDITION is to write
`operands[I]' for operand number I (as matched by `(match_operand I
...)').  Use the variable `insn' to refer to the last of the insns
being matched; use `prev_active_insn' to find the preceding insns.

   When optimizing computations with intermediate results, you can use
CONDITION to match only when the intermediate results are not used
elsewhere.  Use the C expression `dead_or_set_p (INSN, OP)', where INSN
is the insn in which you expect the value to be used for the last time
(from the value of `insn', together with use of `prev_nonnote_insn'),
and OP is the intermediate value (from `operands[I]').

   Applying the optimization means replacing the sequence of insns with
one new insn.  The TEMPLATE controls ultimate output of assembler code
for this combined insn.  It works exactly like the template of a
`define_insn'.  Operand numbers in this template are the same ones used
in matching the original sequence of insns.

   The result of a defined peephole optimizer does not need to match
any of the insn patterns in the machine description; it does not even
have an opportunity to match them.  The peephole optimizer definition
itself serves as the insn pattern to control how the insn is output.

   Defined peephole optimizers are run as assembler code is being
output, so the insns they produce are never combined or rearranged in
any way.

   Here is an example, taken from the 68000 machine description:

     (define_peephole
       [(set (reg:SI 15) (plus:SI (reg:SI 15) (const_int 4)))
        (set (match_operand:DF 0 "register_operand" "=f")
             (match_operand:DF 1 "register_operand" "ad"))]
       "FP_REG_P (operands[0]) && ! FP_REG_P (operands[1])"
     {
       rtx xoperands[2];
       xoperands[1] = gen_rtx (REG, SImode, REGNO (operands[1]) + 1);
     #ifdef MOTOROLA
       output_asm_insn ("move.l %1,(sp)", xoperands);
       output_asm_insn ("move.l %1,-(sp)", operands);
       return "fmove.d (sp)+,%0";
     #else
       output_asm_insn ("movel %1,sp@", xoperands);
       output_asm_insn ("movel %1,sp@-", operands);
       return "fmoved sp@+,%0";
     #endif
     })

   The effect of this optimization is to change

     jbsr _foobar
     addql #4,sp
     movel d1,sp@-
     movel d0,sp@-
     fmoved sp@+,fp0

into

     jbsr _foobar
     movel d1,sp@
     movel d0,sp@-
     fmoved sp@+,fp0

   INSN-PATTERN-1 and so on look _almost_ like the second operand of
`define_insn'.  There is one important difference: the second operand
of `define_insn' consists of one or more RTX's enclosed in square
brackets.  Usually, there is only one: then the same action can be
written as an element of a `define_peephole'.  But when there are
multiple actions in a `define_insn', they are implicitly enclosed in a
`parallel'.  Then you must explicitly write the `parallel', and the
square brackets within it, in the `define_peephole'.  Thus, if an insn
pattern looks like this,

     (define_insn "divmodsi4"
       [(set (match_operand:SI 0 "general_operand" "=d")
             (div:SI (match_operand:SI 1 "general_operand" "0")
                     (match_operand:SI 2 "general_operand" "dmsK")))
        (set (match_operand:SI 3 "general_operand" "=d")
             (mod:SI (match_dup 1) (match_dup 2)))]
       "TARGET_68020"
       "divsl%.l %2,%3:%0")

then the way to mention this insn in a peephole is as follows:

     (define_peephole
       [...
        (parallel
         [(set (match_operand:SI 0 "general_operand" "=d")
               (div:SI (match_operand:SI 1 "general_operand" "0")
                       (match_operand:SI 2 "general_operand" "dmsK")))
          (set (match_operand:SI 3 "general_operand" "=d")
               (mod:SI (match_dup 1) (match_dup 2)))])
        ...]
       ...)


File: gccint.info,  Node: define_peephole2,  Prev: define_peephole,  Up: Peephole Definitions

9.17.2 RTL to RTL Peephole Optimizers
-------------------------------------

The `define_peephole2' definition tells the compiler how to substitute
one sequence of instructions for another sequence, what additional
scratch registers may be needed and what their lifetimes must be.

     (define_peephole2
       [INSN-PATTERN-1
        INSN-PATTERN-2
        ...]
       "CONDITION"
       [NEW-INSN-PATTERN-1
        NEW-INSN-PATTERN-2
        ...]
       "PREPARATION-STATEMENTS")

   The definition is almost identical to `define_split' (*note Insn
Splitting::) except that the pattern to match is not a single
instruction, but a sequence of instructions.

   It is possible to request additional scratch registers for use in the
output template.  If appropriate registers are not free, the pattern
will simply not match.

   Scratch registers are requested with a `match_scratch' pattern at
the top level of the input pattern.  The allocated register (initially)
will be dead at the point requested within the original sequence.  If
the scratch is used at more than a single point, a `match_dup' pattern
at the top level of the input pattern marks the last position in the
input sequence at which the register must be available.

   Here is an example from the IA-32 machine description:

     (define_peephole2
       [(match_scratch:SI 2 "r")
        (parallel [(set (match_operand:SI 0 "register_operand" "")
                        (match_operator:SI 3 "arith_or_logical_operator"
                          [(match_dup 0)
                           (match_operand:SI 1 "memory_operand" "")]))
                   (clobber (reg:CC 17))])]
       "! optimize_size && ! TARGET_READ_MODIFY"
       [(set (match_dup 2) (match_dup 1))
        (parallel [(set (match_dup 0)
                        (match_op_dup 3 [(match_dup 0) (match_dup 2)]))
                   (clobber (reg:CC 17))])]
       "")

This pattern tries to split a load from its use in the hopes that we'll
be able to schedule around the memory load latency.  It allocates a
single `SImode' register of class `GENERAL_REGS' (`"r"') that needs to
be live only at the point just before the arithmetic.

   A real example requiring extended scratch lifetimes is harder to
come by, so here's a silly made-up example:

     (define_peephole2
       [(match_scratch:SI 4 "r")
        (set (match_operand:SI 0 "" "") (match_operand:SI 1 "" ""))
        (set (match_operand:SI 2 "" "") (match_dup 1))
        (match_dup 4)
        (set (match_operand:SI 3 "" "") (match_dup 1))]
       "/* determine 1 does not overlap 0 and 2 */"
       [(set (match_dup 4) (match_dup 1))
        (set (match_dup 0) (match_dup 4))
        (set (match_dup 2) (match_dup 4))]
        (set (match_dup 3) (match_dup 4))]
       "")

If we had not added the `(match_dup 4)' in the middle of the input
sequence, it might have been the case that the register we chose at the
beginning of the sequence is killed by the first or second `set'.


File: gccint.info,  Node: Insn Attributes,  Next: Conditional Execution,  Prev: Peephole Definitions,  Up: Machine Desc

9.18 Instruction Attributes
===========================

In addition to describing the instruction supported by the target
machine, the `md' file also defines a group of "attributes" and a set of
values for each.  Every generated insn is assigned a value for each
attribute.  One possible attribute would be the effect that the insn
has on the machine's condition code.  This attribute can then be used
by `NOTICE_UPDATE_CC' to track the condition codes.

* Menu:

* Defining Attributes:: Specifying attributes and their values.
* Expressions::         Valid expressions for attribute values.
* Tagging Insns::       Assigning attribute values to insns.
* Attr Example::        An example of assigning attributes.
* Insn Lengths::        Computing the length of insns.
* Constant Attributes:: Defining attributes that are constant.
* Delay Slots::         Defining delay slots required for a machine.
* Function Units::      Specifying information for insn scheduling.


File: gccint.info,  Node: Defining Attributes,  Next: Expressions,  Up: Insn Attributes

9.18.1 Defining Attributes and their Values
-------------------------------------------

The `define_attr' expression is used to define each attribute required
by the target machine.  It looks like:

     (define_attr NAME LIST-OF-VALUES DEFAULT)

   NAME is a string specifying the name of the attribute being defined.

   LIST-OF-VALUES is either a string that specifies a comma-separated
list of values that can be assigned to the attribute, or a null string
to indicate that the attribute takes numeric values.

   DEFAULT is an attribute expression that gives the value of this
attribute for insns that match patterns whose definition does not
include an explicit value for this attribute.  *Note Attr Example::,
for more information on the handling of defaults.  *Note Constant
Attributes::, for information on attributes that do not depend on any
particular insn.

   For each defined attribute, a number of definitions are written to
the `insn-attr.h' file.  For cases where an explicit set of values is
specified for an attribute, the following are defined:

   * A `#define' is written for the symbol `HAVE_ATTR_NAME'.

   * An enumeral class is defined for `attr_NAME' with elements of the
     form `UPPER-NAME_UPPER-VALUE' where the attribute name and value
     are first converted to upper case.

   * A function `get_attr_NAME' is defined that is passed an insn and
     returns the attribute value for that insn.

   For example, if the following is present in the `md' file:

     (define_attr "type" "branch,fp,load,store,arith" ...)

the following lines will be written to the file `insn-attr.h'.

     #define HAVE_ATTR_type
     enum attr_type {TYPE_BRANCH, TYPE_FP, TYPE_LOAD,
                      TYPE_STORE, TYPE_ARITH};
     extern enum attr_type get_attr_type ();

   If the attribute takes numeric values, no `enum' type will be
defined and the function to obtain the attribute's value will return
`int'.


File: gccint.info,  Node: Expressions,  Next: Tagging Insns,  Prev: Defining Attributes,  Up: Insn Attributes

9.18.2 Attribute Expressions
----------------------------

RTL expressions used to define attributes use the codes described above
plus a few specific to attribute definitions, to be discussed below.
Attribute value expressions must have one of the following forms:

`(const_int I)'
     The integer I specifies the value of a numeric attribute.  I must
     be non-negative.

     The value of a numeric attribute can be specified either with a
     `const_int', or as an integer represented as a string in
     `const_string', `eq_attr' (see below), `attr', `symbol_ref',
     simple arithmetic expressions, and `set_attr' overrides on
     specific instructions (*note Tagging Insns::).

`(const_string VALUE)'
     The string VALUE specifies a constant attribute value.  If VALUE
     is specified as `"*"', it means that the default value of the
     attribute is to be used for the insn containing this expression.
     `"*"' obviously cannot be used in the DEFAULT expression of a
     `define_attr'.

     If the attribute whose value is being specified is numeric, VALUE
     must be a string containing a non-negative integer (normally
     `const_int' would be used in this case).  Otherwise, it must
     contain one of the valid values for the attribute.

`(if_then_else TEST TRUE-VALUE FALSE-VALUE)'
     TEST specifies an attribute test, whose format is defined below.
     The value of this expression is TRUE-VALUE if TEST is true,
     otherwise it is FALSE-VALUE.

`(cond [TEST1 VALUE1 ...] DEFAULT)'
     The first operand of this expression is a vector containing an even
     number of expressions and consisting of pairs of TEST and VALUE
     expressions.  The value of the `cond' expression is that of the
     VALUE corresponding to the first true TEST expression.  If none of
     the TEST expressions are true, the value of the `cond' expression
     is that of the DEFAULT expression.

   TEST expressions can have one of the following forms:

`(const_int I)'
     This test is true if I is nonzero and false otherwise.

`(not TEST)'
`(ior TEST1 TEST2)'
`(and TEST1 TEST2)'
     These tests are true if the indicated logical function is true.

`(match_operand:M N PRED CONSTRAINTS)'
     This test is true if operand N of the insn whose attribute value
     is being determined has mode M (this part of the test is ignored
     if M is `VOIDmode') and the function specified by the string PRED
     returns a nonzero value when passed operand N and mode M (this
     part of the test is ignored if PRED is the null string).

     The CONSTRAINTS operand is ignored and should be the null string.

`(le ARITH1 ARITH2)'
`(leu ARITH1 ARITH2)'
`(lt ARITH1 ARITH2)'
`(ltu ARITH1 ARITH2)'
`(gt ARITH1 ARITH2)'
`(gtu ARITH1 ARITH2)'
`(ge ARITH1 ARITH2)'
`(geu ARITH1 ARITH2)'
`(ne ARITH1 ARITH2)'
`(eq ARITH1 ARITH2)'
     These tests are true if the indicated comparison of the two
     arithmetic expressions is true.  Arithmetic expressions are formed
     with `plus', `minus', `mult', `div', `mod', `abs', `neg', `and',
     `ior', `xor', `not', `ashift', `lshiftrt', and `ashiftrt'
     expressions.

     `const_int' and `symbol_ref' are always valid terms (*note Insn
     Lengths::,for additional forms).  `symbol_ref' is a string
     denoting a C expression that yields an `int' when evaluated by the
     `get_attr_...' routine.  It should normally be a global variable.

`(eq_attr NAME VALUE)'
     NAME is a string specifying the name of an attribute.

     VALUE is a string that is either a valid value for attribute NAME,
     a comma-separated list of values, or `!' followed by a value or
     list.  If VALUE does not begin with a `!', this test is true if
     the value of the NAME attribute of the current insn is in the list
     specified by VALUE.  If VALUE begins with a `!', this test is true
     if the attribute's value is _not_ in the specified list.

     For example,

          (eq_attr "type" "load,store")

     is equivalent to

          (ior (eq_attr "type" "load") (eq_attr "type" "store"))

     If NAME specifies an attribute of `alternative', it refers to the
     value of the compiler variable `which_alternative' (*note Output
     Statement::) and the values must be small integers.  For example,

          (eq_attr "alternative" "2,3")

     is equivalent to

          (ior (eq (symbol_ref "which_alternative") (const_int 2))
               (eq (symbol_ref "which_alternative") (const_int 3)))

     Note that, for most attributes, an `eq_attr' test is simplified in
     cases where the value of the attribute being tested is known for
     all insns matching a particular pattern.  This is by far the most
     common case.

`(attr_flag NAME)'
     The value of an `attr_flag' expression is true if the flag
     specified by NAME is true for the `insn' currently being scheduled.

     NAME is a string specifying one of a fixed set of flags to test.
     Test the flags `forward' and `backward' to determine the direction
     of a conditional branch.  Test the flags `very_likely', `likely',
     `very_unlikely', and `unlikely' to determine if a conditional
     branch is expected to be taken.

     If the `very_likely' flag is true, then the `likely' flag is also
     true.  Likewise for the `very_unlikely' and `unlikely' flags.

     This example describes a conditional branch delay slot which can
     be nullified for forward branches that are taken (annul-true) or
     for backward branches which are not taken (annul-false).

          (define_delay (eq_attr "type" "cbranch")
            [(eq_attr "in_branch_delay" "true")
             (and (eq_attr "in_branch_delay" "true")
                  (attr_flag "forward"))
             (and (eq_attr "in_branch_delay" "true")
                  (attr_flag "backward"))])

     The `forward' and `backward' flags are false if the current `insn'
     being scheduled is not a conditional branch.

     The `very_likely' and `likely' flags are true if the `insn' being
     scheduled is not a conditional branch.  The `very_unlikely' and
     `unlikely' flags are false if the `insn' being scheduled is not a
     conditional branch.

     `attr_flag' is only used during delay slot scheduling and has no
     meaning to other passes of the compiler.

`(attr NAME)'
     The value of another attribute is returned.  This is most useful
     for numeric attributes, as `eq_attr' and `attr_flag' produce more
     efficient code for non-numeric attributes.


File: gccint.info,  Node: Tagging Insns,  Next: Attr Example,  Prev: Expressions,  Up: Insn Attributes

9.18.3 Assigning Attribute Values to Insns
------------------------------------------

The value assigned to an attribute of an insn is primarily determined by
which pattern is matched by that insn (or which `define_peephole'
generated it).  Every `define_insn' and `define_peephole' can have an
optional last argument to specify the values of attributes for matching
insns.  The value of any attribute not specified in a particular insn
is set to the default value for that attribute, as specified in its
`define_attr'.  Extensive use of default values for attributes permits
the specification of the values for only one or two attributes in the
definition of most insn patterns, as seen in the example in the next
section.

   The optional last argument of `define_insn' and `define_peephole' is
a vector of expressions, each of which defines the value for a single
attribute.  The most general way of assigning an attribute's value is
to use a `set' expression whose first operand is an `attr' expression
giving the name of the attribute being set.  The second operand of the
`set' is an attribute expression (*note Expressions::) giving the value
of the attribute.

   When the attribute value depends on the `alternative' attribute
(i.e., which is the applicable alternative in the constraint of the
insn), the `set_attr_alternative' expression can be used.  It allows
the specification of a vector of attribute expressions, one for each
alternative.

   When the generality of arbitrary attribute expressions is not
required, the simpler `set_attr' expression can be used, which allows
specifying a string giving either a single attribute value or a list of
attribute values, one for each alternative.

   The form of each of the above specifications is shown below.  In
each case, NAME is a string specifying the attribute to be set.

`(set_attr NAME VALUE-STRING)'
     VALUE-STRING is either a string giving the desired attribute value,
     or a string containing a comma-separated list giving the values for
     succeeding alternatives.  The number of elements must match the
     number of alternatives in the constraint of the insn pattern.

     Note that it may be useful to specify `*' for some alternative, in
     which case the attribute will assume its default value for insns
     matching that alternative.

`(set_attr_alternative NAME [VALUE1 VALUE2 ...])'
     Depending on the alternative of the insn, the value will be one of
     the specified values.  This is a shorthand for using a `cond' with
     tests on the `alternative' attribute.

`(set (attr NAME) VALUE)'
     The first operand of this `set' must be the special RTL expression
     `attr', whose sole operand is a string giving the name of the
     attribute being set.  VALUE is the value of the attribute.

   The following shows three different ways of representing the same
attribute value specification:

     (set_attr "type" "load,store,arith")

     (set_attr_alternative "type"
                           [(const_string "load") (const_string "store")
                            (const_string "arith")])

     (set (attr "type")
          (cond [(eq_attr "alternative" "1") (const_string "load")
                 (eq_attr "alternative" "2") (const_string "store")]
                (const_string "arith")))

   The `define_asm_attributes' expression provides a mechanism to
specify the attributes assigned to insns produced from an `asm'
statement.  It has the form:

     (define_asm_attributes [ATTR-SETS])

where ATTR-SETS is specified the same as for both the `define_insn' and
the `define_peephole' expressions.

   These values will typically be the "worst case" attribute values.
For example, they might indicate that the condition code will be
clobbered.

   A specification for a `length' attribute is handled specially.  The
way to compute the length of an `asm' insn is to multiply the length
specified in the expression `define_asm_attributes' by the number of
machine instructions specified in the `asm' statement, determined by
counting the number of semicolons and newlines in the string.
Therefore, the value of the `length' attribute specified in a
`define_asm_attributes' should be the maximum possible length of a
single machine instruction.


File: gccint.info,  Node: Attr Example,  Next: Insn Lengths,  Prev: Tagging Insns,  Up: Insn Attributes

9.18.4 Example of Attribute Specifications
------------------------------------------

The judicious use of defaulting is important in the efficient use of
insn attributes.  Typically, insns are divided into "types" and an
attribute, customarily called `type', is used to represent this value.
This attribute is normally used only to define the default value for
other attributes.  An example will clarify this usage.

   Assume we have a RISC machine with a condition code and in which only
full-word operations are performed in registers.  Let us assume that we
can divide all insns into loads, stores, (integer) arithmetic
operations, floating point operations, and branches.

   Here we will concern ourselves with determining the effect of an
insn on the condition code and will limit ourselves to the following
possible effects:  The condition code can be set unpredictably
(clobbered), not be changed, be set to agree with the results of the
operation, or only changed if the item previously set into the
condition code has been modified.

   Here is part of a sample `md' file for such a machine:

     (define_attr "type" "load,store,arith,fp,branch" (const_string "arith"))

     (define_attr "cc" "clobber,unchanged,set,change0"
                  (cond [(eq_attr "type" "load")
                             (const_string "change0")
                         (eq_attr "type" "store,branch")
                             (const_string "unchanged")
                         (eq_attr "type" "arith")
                             (if_then_else (match_operand:SI 0 "" "")
                                           (const_string "set")
                                           (const_string "clobber"))]
                        (const_string "clobber")))

     (define_insn ""
       [(set (match_operand:SI 0 "general_operand" "=r,r,m")
             (match_operand:SI 1 "general_operand" "r,m,r"))]
       ""
       "@
        move %0,%1
        load %0,%1
        store %0,%1"
       [(set_attr "type" "arith,load,store")])

   Note that we assume in the above example that arithmetic operations
performed on quantities smaller than a machine word clobber the
condition code since they will set the condition code to a value
corresponding to the full-word result.


File: gccint.info,  Node: Insn Lengths,  Next: Constant Attributes,  Prev: Attr Example,  Up: Insn Attributes

9.18.5 Computing the Length of an Insn
--------------------------------------

For many machines, multiple types of branch instructions are provided,
each for different length branch displacements.  In most cases, the
assembler will choose the correct instruction to use.  However, when
the assembler cannot do so, GCC can when a special attribute, the
`length' attribute, is defined.  This attribute must be defined to have
numeric values by specifying a null string in its `define_attr'.

   In the case of the `length' attribute, two additional forms of
arithmetic terms are allowed in test expressions:

`(match_dup N)'
     This refers to the address of operand N of the current insn, which
     must be a `label_ref'.

`(pc)'
     This refers to the address of the _current_ insn.  It might have
     been more consistent with other usage to make this the address of
     the _next_ insn but this would be confusing because the length of
     the current insn is to be computed.

   For normal insns, the length will be determined by value of the
`length' attribute.  In the case of `addr_vec' and `addr_diff_vec' insn
patterns, the length is computed as the number of vectors multiplied by
the size of each vector.

   Lengths are measured in addressable storage units (bytes).

   The following macros can be used to refine the length computation:

`FIRST_INSN_ADDRESS'
     When the `length' insn attribute is used, this macro specifies the
     value to be assigned to the address of the first insn in a
     function.  If not specified, 0 is used.

`ADJUST_INSN_LENGTH (INSN, LENGTH)'
     If defined, modifies the length assigned to instruction INSN as a
     function of the context in which it is used.  LENGTH is an lvalue
     that contains the initially computed length of the insn and should
     be updated with the correct length of the insn.

     This macro will normally not be required.  A case in which it is
     required is the ROMP.  On this machine, the size of an `addr_vec'
     insn must be increased by two to compensate for the fact that
     alignment may be required.

   The routine that returns `get_attr_length' (the value of the
`length' attribute) can be used by the output routine to determine the
form of the branch instruction to be written, as the example below
illustrates.

   As an example of the specification of variable-length branches,
consider the IBM 360.  If we adopt the convention that a register will
be set to the starting address of a function, we can jump to labels
within 4k of the start using a four-byte instruction.  Otherwise, we
need a six-byte sequence to load the address from memory and then
branch to it.

   On such a machine, a pattern for a branch instruction might be
specified as follows:

     (define_insn "jump"
       [(set (pc)
             (label_ref (match_operand 0 "" "")))]
       ""
     {
        return (get_attr_length (insn) == 4
                ? "b %l0" : "l r15,=a(%l0); br r15");
     }
       [(set (attr "length")
             (if_then_else (lt (match_dup 0) (const_int 4096))
                           (const_int 4)
                           (const_int 6)))])


File: gccint.info,  Node: Constant Attributes,  Next: Delay Slots,  Prev: Insn Lengths,  Up: Insn Attributes

9.18.6 Constant Attributes
--------------------------

A special form of `define_attr', where the expression for the default
value is a `const' expression, indicates an attribute that is constant
for a given run of the compiler.  Constant attributes may be used to
specify which variety of processor is used.  For example,

     (define_attr "cpu" "m88100,m88110,m88000"
      (const
       (cond [(symbol_ref "TARGET_88100") (const_string "m88100")
              (symbol_ref "TARGET_88110") (const_string "m88110")]
             (const_string "m88000"))))

     (define_attr "memory" "fast,slow"
      (const
       (if_then_else (symbol_ref "TARGET_FAST_MEM")
                     (const_string "fast")
                     (const_string "slow"))))

   The routine generated for constant attributes has no parameters as it
does not depend on any particular insn.  RTL expressions used to define
the value of a constant attribute may use the `symbol_ref' form, but
may not use either the `match_operand' form or `eq_attr' forms
involving insn attributes.


File: gccint.info,  Node: Delay Slots,  Next: Function Units,  Prev: Constant Attributes,  Up: Insn Attributes

9.18.7 Delay Slot Scheduling
----------------------------

The insn attribute mechanism can be used to specify the requirements for
delay slots, if any, on a target machine.  An instruction is said to
require a "delay slot" if some instructions that are physically after
the instruction are executed as if they were located before it.
Classic examples are branch and call instructions, which often execute
the following instruction before the branch or call is performed.

   On some machines, conditional branch instructions can optionally
"annul" instructions in the delay slot.  This means that the
instruction will not be executed for certain branch outcomes.  Both
instructions that annul if the branch is true and instructions that
annul if the branch is false are supported.

   Delay slot scheduling differs from instruction scheduling in that
determining whether an instruction needs a delay slot is dependent only
on the type of instruction being generated, not on data flow between the
instructions.  See the next section for a discussion of data-dependent
instruction scheduling.

   The requirement of an insn needing one or more delay slots is
indicated via the `define_delay' expression.  It has the following form:

     (define_delay TEST
                   [DELAY-1 ANNUL-TRUE-1 ANNUL-FALSE-1
                    DELAY-2 ANNUL-TRUE-2 ANNUL-FALSE-2
                    ...])

   TEST is an attribute test that indicates whether this `define_delay'
applies to a particular insn.  If so, the number of required delay
slots is determined by the length of the vector specified as the second
argument.  An insn placed in delay slot N must satisfy attribute test
DELAY-N.  ANNUL-TRUE-N is an attribute test that specifies which insns
may be annulled if the branch is true.  Similarly, ANNUL-FALSE-N
specifies which insns in the delay slot may be annulled if the branch
is false.  If annulling is not supported for that delay slot, `(nil)'
should be coded.

   For example, in the common case where branch and call insns require
a single delay slot, which may contain any insn other than a branch or
call, the following would be placed in the `md' file:

     (define_delay (eq_attr "type" "branch,call")
                   [(eq_attr "type" "!branch,call") (nil) (nil)])

   Multiple `define_delay' expressions may be specified.  In this case,
each such expression specifies different delay slot requirements and
there must be no insn for which tests in two `define_delay' expressions
are both true.

   For example, if we have a machine that requires one delay slot for
branches but two for calls,  no delay slot can contain a branch or call
insn, and any valid insn in the delay slot for the branch can be
annulled if the branch is true, we might represent this as follows:

     (define_delay (eq_attr "type" "branch")
        [(eq_attr "type" "!branch,call")
         (eq_attr "type" "!branch,call")
         (nil)])

     (define_delay (eq_attr "type" "call")
                   [(eq_attr "type" "!branch,call") (nil) (nil)
                    (eq_attr "type" "!branch,call") (nil) (nil)])


File: gccint.info,  Node: Function Units,  Prev: Delay Slots,  Up: Insn Attributes

9.18.8 Specifying Function Units
--------------------------------

On most RISC machines, there are instructions whose results are not
available for a specific number of cycles.  Common cases are
instructions that load data from memory.  On many machines, a pipeline
stall will result if the data is referenced too soon after the load
instruction.

   In addition, many newer microprocessors have multiple function
units, usually one for integer and one for floating point, and often
will incur pipeline stalls when a result that is needed is not yet
ready.

   The descriptions in this section allow the specification of how much
time must elapse between the execution of an instruction and the time
when its result is used.  It also allows specification of when the
execution of an instruction will delay execution of similar instructions
due to function unit conflicts.

   For the purposes of the specifications in this section, a machine is
divided into "function units", each of which execute a specific class
of instructions in first-in-first-out order.  Function units that
accept one instruction each cycle and allow a result to be used in the
succeeding instruction (usually via forwarding) need not be specified.
Classic RISC microprocessors will normally have a single function unit,
which we can call `memory'.  The newer "superscalar" processors will
often have function units for floating point operations, usually at
least a floating point adder and multiplier.

   Each usage of a function units by a class of insns is specified with
a `define_function_unit' expression, which looks like this:

     (define_function_unit NAME MULTIPLICITY SIMULTANEITY
                           TEST READY-DELAY ISSUE-DELAY
                          [CONFLICT-LIST])

   NAME is a string giving the name of the function unit.

   MULTIPLICITY is an integer specifying the number of identical units
in the processor.  If more than one unit is specified, they will be
scheduled independently.  Only truly independent units should be
counted; a pipelined unit should be specified as a single unit.  (The
only common example of a machine that has multiple function units for a
single instruction class that are truly independent and not pipelined
are the two multiply and two increment units of the CDC 6600.)

   SIMULTANEITY specifies the maximum number of insns that can be
executing in each instance of the function unit simultaneously or zero
if the unit is pipelined and has no limit.

   All `define_function_unit' definitions referring to function unit
NAME must have the same name and values for MULTIPLICITY and
SIMULTANEITY.

   TEST is an attribute test that selects the insns we are describing
in this definition.  Note that an insn may use more than one function
unit and a function unit may be specified in more than one
`define_function_unit'.

   READY-DELAY is an integer that specifies the number of cycles after
which the result of the instruction can be used without introducing any
stalls.

   ISSUE-DELAY is an integer that specifies the number of cycles after
the instruction matching the TEST expression begins using this unit
until a subsequent instruction can begin.  A cost of N indicates an N-1
cycle delay.  A subsequent instruction may also be delayed if an
earlier instruction has a longer READY-DELAY value.  This blocking
effect is computed using the SIMULTANEITY, READY-DELAY, ISSUE-DELAY,
and CONFLICT-LIST terms.  For a normal non-pipelined function unit,
SIMULTANEITY is one, the unit is taken to block for the READY-DELAY
cycles of the executing insn, and smaller values of ISSUE-DELAY are
ignored.

   CONFLICT-LIST is an optional list giving detailed conflict costs for
this unit.  If specified, it is a list of condition test expressions to
be applied to insns chosen to execute in NAME following the particular
insn matching TEST that is already executing in NAME.  For each insn in
the list, ISSUE-DELAY specifies the conflict cost; for insns not in the
list, the cost is zero.  If not specified, CONFLICT-LIST defaults to
all instructions that use the function unit.

   Typical uses of this vector are where a floating point function unit
can pipeline either single- or double-precision operations, but not
both, or where a memory unit can pipeline loads, but not stores, etc.

   As an example, consider a classic RISC machine where the result of a
load instruction is not available for two cycles (a single "delay"
instruction is required) and where only one load instruction can be
executed simultaneously.  This would be specified as:

     (define_function_unit "memory" 1 1 (eq_attr "type" "load") 2 0)

   For the case of a floating point function unit that can pipeline
either single or double precision, but not both, the following could be
specified:

     (define_function_unit
        "fp" 1 0 (eq_attr "type" "sp_fp") 4 4 [(eq_attr "type" "dp_fp")])
     (define_function_unit
        "fp" 1 0 (eq_attr "type" "dp_fp") 4 4 [(eq_attr "type" "sp_fp")])

   *Note_* The scheduler attempts to avoid function unit conflicts and
uses all the specifications in the `define_function_unit' expression.
It has recently come to our attention that these specifications may not
allow modeling of some of the newer "superscalar" processors that have
insns using multiple pipelined units.  These insns will cause a
potential conflict for the second unit used during their execution and
there is no way of representing that conflict.  We welcome any examples
of how function unit conflicts work in such processors and suggestions
for their representation.


File: gccint.info,  Node: Conditional Execution,  Next: Constant Definitions,  Prev: Insn Attributes,  Up: Machine Desc

9.19 Conditional Execution
==========================

A number of architectures provide for some form of conditional
execution, or predication.  The hallmark of this feature is the ability
to nullify most of the instructions in the instruction set.  When the
instruction set is large and not entirely symmetric, it can be quite
tedious to describe these forms directly in the `.md' file.  An
alternative is the `define_cond_exec' template.

     (define_cond_exec
       [PREDICATE-PATTERN]
       "CONDITION"
       "OUTPUT-TEMPLATE")

   PREDICATE-PATTERN is the condition that must be true for the insn to
be executed at runtime and should match a relational operator.  One can
use `match_operator' to match several relational operators at once.
Any `match_operand' operands must have no more than one alternative.

   CONDITION is a C expression that must be true for the generated
pattern to match.

   OUTPUT-TEMPLATE is a string similar to the `define_insn' output
template (*note Output Template::), except that the `*' and `@' special
cases do not apply.  This is only useful if the assembly text for the
predicate is a simple prefix to the main insn.  In order to handle the
general case, there is a global variable `current_insn_predicate' that
will contain the entire predicate if the current insn is predicated,
and will otherwise be `NULL'.

   When `define_cond_exec' is used, an implicit reference to the
`predicable' instruction attribute is made.  *Note Insn Attributes::.
This attribute must be boolean (i.e. have exactly two elements in its
LIST-OF-VALUES).  Further, it must not be used with complex
expressions.  That is, the default and all uses in the insns must be a
simple constant, not dependent on the alternative or anything else.

   For each `define_insn' for which the `predicable' attribute is true,
a new `define_insn' pattern will be generated that matches a predicated
version of the instruction.  For example,

     (define_insn "addsi"
       [(set (match_operand:SI 0 "register_operand" "r")
             (plus:SI (match_operand:SI 1 "register_operand" "r")
                      (match_operand:SI 2 "register_operand" "r")))]
       "TEST1"
       "add %2,%1,%0")

     (define_cond_exec
       [(ne (match_operand:CC 0 "register_operand" "c")
            (const_int 0))]
       "TEST2"
       "(%0)")

generates a new pattern

     (define_insn ""
       [(cond_exec
          (ne (match_operand:CC 3 "register_operand" "c") (const_int 0))
          (set (match_operand:SI 0 "register_operand" "r")
               (plus:SI (match_operand:SI 1 "register_operand" "r")
                        (match_operand:SI 2 "register_operand" "r"))))]
       "(TEST2) && (TEST1)"
       "(%3) add %2,%1,%0")


File: gccint.info,  Node: Constant Definitions,  Prev: Conditional Execution,  Up: Machine Desc

9.20 Constant Definitions
=========================

Using literal constants inside instruction patterns reduces legibility
and can be a maintenance problem.

   To overcome this problem, you may use the `define_constants'
expression.  It contains a vector of name-value pairs.  From that point
on, wherever any of the names appears in the MD file, it is as if the
corresponding value had been written instead.  You may use
`define_constants' multiple times; each appearance adds more constants
to the table.  It is an error to redefine a constant with a different
value.

   To come back to the a29k load multiple example, instead of

     (define_insn ""
       [(match_parallel 0 "load_multiple_operation"
          [(set (match_operand:SI 1 "gpc_reg_operand" "=r")
                (match_operand:SI 2 "memory_operand" "m"))
           (use (reg:SI 179))
           (clobber (reg:SI 179))])]
       ""
       "loadm 0,0,%1,%2")

   You could write:

     (define_constants [
         (R_BP 177)
         (R_FC 178)
         (R_CR 179)
         (R_Q  180)
     ])

     (define_insn ""
       [(match_parallel 0 "load_multiple_operation"
          [(set (match_operand:SI 1 "gpc_reg_operand" "=r")
                (match_operand:SI 2 "memory_operand" "m"))
           (use (reg:SI R_CR))
           (clobber (reg:SI R_CR))])]
       ""
       "loadm 0,0,%1,%2")

   The constants that are defined with a define_constant are also output
in the insn-codes.h header file as #defines.


File: gccint.info,  Node: Target Macros,  Next: Host Config,  Prev: Machine Desc,  Up: Top

10 Target Description Macros and Functions
******************************************

In addition to the file `MACHINE.md', a machine description includes a
C header file conventionally given the name `MACHINE.h' and a C source
file named `MACHINE.c'.  The header file defines numerous macros that
convey the information about the target machine that does not fit into
the scheme of the `.md' file.  The file `tm.h' should be a link to
`MACHINE.h'.  The header file `config.h' includes `tm.h' and most
compiler source files include `config.h'.  The source file defines a
variable `targetm', which is a structure containing pointers to
functions and data relating to the target machine.  `MACHINE.c' should
also contain their definitions, if they are not defined elsewhere in
GCC, and other functions called through the macros defined in the `.h'
file.

* Menu:

* Target Structure::    The `targetm' variable.
* Driver::              Controlling how the driver runs the compilation passes.
* Run-time Target::     Defining `-m' options like `-m68000' and `-m68020'.
* Per-Function Data::   Defining data structures for per-function information.
* Storage Layout::      Defining sizes and alignments of data.
* Type Layout::         Defining sizes and properties of basic user data types.
* Escape Sequences::    Defining the value of target character escape sequences
* Registers::           Naming and describing the hardware registers.
* Register Classes::    Defining the classes of hardware registers.
* Stack and Calling::   Defining which way the stack grows and by how much.
* Varargs::		Defining the varargs macros.
* Trampolines::         Code set up at run time to enter a nested function.
* Library Calls::       Controlling how library routines are implicitly called.
* Addressing Modes::    Defining addressing modes valid for memory operands.
* Condition Code::      Defining how insns update the condition code.
* Costs::               Defining relative costs of different operations.
* Scheduling::          Adjusting the behavior of the instruction scheduler.
* Sections::            Dividing storage into text, data, and other sections.
* PIC::			Macros for position independent code.
* Assembler Format::    Defining how to write insns and pseudo-ops to output.
* Debugging Info::      Defining the format of debugging output.
* Cross-compilation::   Handling floating point for cross-compilers.
* Mode Switching::      Insertion of mode-switching instructions.
* Target Attributes::   Defining target-specific uses of `__attribute__'.
* Misc::                Everything else.


File: gccint.info,  Node: Target Structure,  Next: Driver,  Up: Target Macros

10.1 The Global `targetm' Variable
==================================

 -- Variable: struct gcc_target targetm
     The target `.c' file must define the global `targetm' variable
     which contains pointers to functions and data relating to the
     target machine.  The variable is declared in `target.h';
     `target-def.h' defines the macro `TARGET_INITIALIZER' which is
     used to initialize the variable, and macros for the default
     initializers for elements of the structure.  The `.c' file should
     override those macros for which the default definition is
     inappropriate.  For example:
          #include "target.h"
          #include "target-def.h"

          /* Initialize the GCC target structure.  */

          #undef TARGET_COMP_TYPE_ATTRIBUTES
          #define TARGET_COMP_TYPE_ATTRIBUTES MACHINE_comp_type_attributes

          struct gcc_target targetm = TARGET_INITIALIZER;

Where a macro should be defined in the `.c' file in this manner to form
part of the `targetm' structure, it is documented below as a "Target
Hook" with a prototype.  Many macros will change in future from being
defined in the `.h' file to being part of the `targetm' structure.


File: gccint.info,  Node: Driver,  Next: Run-time Target,  Prev: Target Structure,  Up: Target Macros

10.2 Controlling the Compilation Driver, `gcc'
==============================================

You can control the compilation driver.

`SWITCH_TAKES_ARG (CHAR)'
     A C expression which determines whether the option `-CHAR' takes
     arguments.  The value should be the number of arguments that
     option takes-zero, for many options.

     By default, this macro is defined as `DEFAULT_SWITCH_TAKES_ARG',
     which handles the standard options properly.  You need not define
     `SWITCH_TAKES_ARG' unless you wish to add additional options which
     take arguments.  Any redefinition should call
     `DEFAULT_SWITCH_TAKES_ARG' and then check for additional options.

`WORD_SWITCH_TAKES_ARG (NAME)'
     A C expression which determines whether the option `-NAME' takes
     arguments.  The value should be the number of arguments that
     option takes-zero, for many options.  This macro rather than
     `SWITCH_TAKES_ARG' is used for multi-character option names.

     By default, this macro is defined as
     `DEFAULT_WORD_SWITCH_TAKES_ARG', which handles the standard options
     properly.  You need not define `WORD_SWITCH_TAKES_ARG' unless you
     wish to add additional options which take arguments.  Any
     redefinition should call `DEFAULT_WORD_SWITCH_TAKES_ARG' and then
     check for additional options.

`SWITCH_CURTAILS_COMPILATION (CHAR)'
     A C expression which determines whether the option `-CHAR' stops
     compilation before the generation of an executable.  The value is
     boolean, nonzero if the option does stop an executable from being
     generated, zero otherwise.

     By default, this macro is defined as
     `DEFAULT_SWITCH_CURTAILS_COMPILATION', which handles the standard
     options properly.  You need not define
     `SWITCH_CURTAILS_COMPILATION' unless you wish to add additional
     options which affect the generation of an executable.  Any
     redefinition should call `DEFAULT_SWITCH_CURTAILS_COMPILATION' and
     then check for additional options.

`SWITCHES_NEED_SPACES'
     A string-valued C expression which enumerates the options for which
     the linker needs a space between the option and its argument.

     If this macro is not defined, the default value is `""'.

`TARGET_OPTION_TRANSLATE_TABLE'
     If defined, a list of pairs of strings, the first of which is a
     potential command line target to the `gcc' driver program, and the
     second of which is a space-separated (tabs and other whitespace
     are not supported) list of options with which to replace the first
     option.  The target defining this list is responsible for assuring
     that the results are valid.  Replacement options may not be the
     `--opt' style, they must be the `-opt' style.  It is the intention
     of this macro to provide a mechanism for substitution that affects
     the multilibs chosen, such as one option that enables many
     options, some of which select multilibs.  Example nonsensical
     definition, where `-malt-abi', `-EB', and `-mspoo' cause different
     multilibs to be chosen:

          #define TARGET_OPTION_TRANSLATE_TABLE \
          { "-fast",   "-march=fast-foo -malt-abi -I/usr/fast-foo" }, \
          { "-compat", "-EB -malign=4 -mspoo" }

`CPP_SPEC'
     A C string constant that tells the GCC driver program options to
     pass to CPP.  It can also specify how to translate options you
     give to GCC into options for GCC to pass to the CPP.

     Do not define this macro if it does not need to do anything.

`CPLUSPLUS_CPP_SPEC'
     This macro is just like `CPP_SPEC', but is used for C++, rather
     than C.  If you do not define this macro, then the value of
     `CPP_SPEC' (if any) will be used instead.

`NO_BUILTIN_SIZE_TYPE'
     If this macro is defined, the preprocessor will not define the
     built-in macro `__SIZE_TYPE__'.  The macro `__SIZE_TYPE__' must
     then be defined by `CPP_SPEC' instead.

     This should be defined if `SIZE_TYPE' depends on target dependent
     flags which are not accessible to the preprocessor.  Otherwise, it
     should not be defined.

`NO_BUILTIN_PTRDIFF_TYPE'
     If this macro is defined, the preprocessor will not define the
     built-in macro `__PTRDIFF_TYPE__'.  The macro `__PTRDIFF_TYPE__'
     must then be defined by `CPP_SPEC' instead.

     This should be defined if `PTRDIFF_TYPE' depends on target
     dependent flags which are not accessible to the preprocessor.
     Otherwise, it should not be defined.

`NO_BUILTIN_WCHAR_TYPE'
     If this macro is defined, the preprocessor will not define the
     built-in macro `__WCHAR_TYPE__'.  The macro `__WCHAR_TYPE__' must
     then be defined by `CPP_SPEC' instead.

     This should be defined if `WCHAR_TYPE' depends on target dependent
     flags which are not accessible to the preprocessor.  Otherwise, it
     should not be defined.

`NO_BUILTIN_WINT_TYPE'
     If this macro is defined, the preprocessor will not define the
     built-in macro `__WINT_TYPE__'.  The macro `__WINT_TYPE__' must
     then be defined by `CPP_SPEC' instead.

     This should be defined if `WINT_TYPE' depends on target dependent
     flags which are not accessible to the preprocessor.  Otherwise, it
     should not be defined.

`CC1_SPEC'
     A C string constant that tells the GCC driver program options to
     pass to `cc1', `cc1plus', `f771', and the other language front
     ends.  It can also specify how to translate options you give to
     GCC into options for GCC to pass to front ends.

     Do not define this macro if it does not need to do anything.

`CC1PLUS_SPEC'
     A C string constant that tells the GCC driver program options to
     pass to `cc1plus'.  It can also specify how to translate options
     you give to GCC into options for GCC to pass to the `cc1plus'.

     Do not define this macro if it does not need to do anything.  Note
     that everything defined in CC1_SPEC is already passed to `cc1plus'
     so there is no need to duplicate the contents of CC1_SPEC in
     CC1PLUS_SPEC.

`ASM_SPEC'
     A C string constant that tells the GCC driver program options to
     pass to the assembler.  It can also specify how to translate
     options you give to GCC into options for GCC to pass to the
     assembler.  See the file `sun3.h' for an example of this.

     Do not define this macro if it does not need to do anything.

`ASM_FINAL_SPEC'
     A C string constant that tells the GCC driver program how to run
     any programs which cleanup after the normal assembler.  Normally,
     this is not needed.  See the file `mips.h' for an example of this.

     Do not define this macro if it does not need to do anything.

`LINK_SPEC'
     A C string constant that tells the GCC driver program options to
     pass to the linker.  It can also specify how to translate options
     you give to GCC into options for GCC to pass to the linker.

     Do not define this macro if it does not need to do anything.

`LIB_SPEC'
     Another C string constant used much like `LINK_SPEC'.  The
     difference between the two is that `LIB_SPEC' is used at the end
     of the command given to the linker.

     If this macro is not defined, a default is provided that loads the
     standard C library from the usual place.  See `gcc.c'.

`LIBGCC_SPEC'
     Another C string constant that tells the GCC driver program how
     and when to place a reference to `libgcc.a' into the linker
     command line.  This constant is placed both before and after the
     value of `LIB_SPEC'.

     If this macro is not defined, the GCC driver provides a default
     that passes the string `-lgcc' to the linker.

`STARTFILE_SPEC'
     Another C string constant used much like `LINK_SPEC'.  The
     difference between the two is that `STARTFILE_SPEC' is used at the
     very beginning of the command given to the linker.

     If this macro is not defined, a default is provided that loads the
     standard C startup file from the usual place.  See `gcc.c'.

`ENDFILE_SPEC'
     Another C string constant used much like `LINK_SPEC'.  The
     difference between the two is that `ENDFILE_SPEC' is used at the
     very end of the command given to the linker.

     Do not define this macro if it does not need to do anything.

`THREAD_MODEL_SPEC'
     GCC `-v' will print the thread model GCC was configured to use.
     However, this doesn't work on platforms that are multilibbed on
     thread models, such as AIX 4.3.  On such platforms, define
     `THREAD_MODEL_SPEC' such that it evaluates to a string without
     blanks that names one of the recognized thread models.  `%*', the
     default value of this macro, will expand to the value of
     `thread_file' set in `config.gcc'.

`EXTRA_SPECS'
     Define this macro to provide additional specifications to put in
     the `specs' file that can be used in various specifications like
     `CC1_SPEC'.

     The definition should be an initializer for an array of structures,
     containing a string constant, that defines the specification name,
     and a string constant that provides the specification.

     Do not define this macro if it does not need to do anything.

     `EXTRA_SPECS' is useful when an architecture contains several
     related targets, which have various `..._SPECS' which are similar
     to each other, and the maintainer would like one central place to
     keep these definitions.

     For example, the PowerPC System V.4 targets use `EXTRA_SPECS' to
     define either `_CALL_SYSV' when the System V calling sequence is
     used or `_CALL_AIX' when the older AIX-based calling sequence is
     used.

     The `config/rs6000/rs6000.h' target file defines:

          #define EXTRA_SPECS \
            { "cpp_sysv_default", CPP_SYSV_DEFAULT },

          #define CPP_SYS_DEFAULT ""

     The `config/rs6000/sysv.h' target file defines:
          #undef CPP_SPEC
          #define CPP_SPEC \
          "%{posix: -D_POSIX_SOURCE } \
          %{mcall-sysv: -D_CALL_SYSV } %{mcall-aix: -D_CALL_AIX } \
          %{!mcall-sysv: %{!mcall-aix: %(cpp_sysv_default) }} \
          %{msoft-float: -D_SOFT_FLOAT} %{mcpu=403: -D_SOFT_FLOAT}"

          #undef CPP_SYSV_DEFAULT
          #define CPP_SYSV_DEFAULT "-D_CALL_SYSV"

     while the `config/rs6000/eabiaix.h' target file defines
     `CPP_SYSV_DEFAULT' as:

          #undef CPP_SYSV_DEFAULT
          #define CPP_SYSV_DEFAULT "-D_CALL_AIX"

`LINK_LIBGCC_SPECIAL'
     Define this macro if the driver program should find the library
     `libgcc.a' itself and should not pass `-L' options to the linker.
     If you do not define this macro, the driver program will pass the
     argument `-lgcc' to tell the linker to do the search and will pass
     `-L' options to it.

`LINK_LIBGCC_SPECIAL_1'
     Define this macro if the driver program should find the library
     `libgcc.a'.  If you do not define this macro, the driver program
     will pass the argument `-lgcc' to tell the linker to do the search.
     This macro is similar to `LINK_LIBGCC_SPECIAL', except that it does
     not affect `-L' options.

`LINK_GCC_C_SEQUENCE_SPEC'
     The sequence in which libgcc and libc are specified to the linker.
     By default this is `%G %L %G'.

`LINK_COMMAND_SPEC'
     A C string constant giving the complete command line need to
     execute the linker.  When you do this, you will need to update
     your port each time a change is made to the link command line
     within `gcc.c'.  Therefore, define this macro only if you need to
     completely redefine the command line for invoking the linker and
     there is no other way to accomplish the effect you need.
     Overriding this macro may be avoidable by overriding
     `LINK_GCC_C_SEQUENCE_SPEC' instead.

`LINK_ELIMINATE_DUPLICATE_LDIRECTORIES'
     A nonzero value causes `collect2' to remove duplicate
     `-LDIRECTORY' search directories from linking commands.  Do not
     give it a nonzero value if removing duplicate search directories
     changes the linker's semantics.

`MULTILIB_DEFAULTS'
     Define this macro as a C expression for the initializer of an
     array of string to tell the driver program which options are
     defaults for this target and thus do not need to be handled
     specially when using `MULTILIB_OPTIONS'.

     Do not define this macro if `MULTILIB_OPTIONS' is not defined in
     the target makefile fragment or if none of the options listed in
     `MULTILIB_OPTIONS' are set by default.  *Note Target Fragment::.

`RELATIVE_PREFIX_NOT_LINKDIR'
     Define this macro to tell `gcc' that it should only translate a
     `-B' prefix into a `-L' linker option if the prefix indicates an
     absolute file name.

`STANDARD_EXEC_PREFIX'
     Define this macro as a C string constant if you wish to override
     the standard choice of `/usr/local/lib/gcc-lib/' as the default
     prefix to try when searching for the executable files of the
     compiler.

`MD_EXEC_PREFIX'
     If defined, this macro is an additional prefix to try after
     `STANDARD_EXEC_PREFIX'.  `MD_EXEC_PREFIX' is not searched when the
     `-b' option is used, or the compiler is built as a cross compiler.
     If you define `MD_EXEC_PREFIX', then be sure to add it to the
     list of directories used to find the assembler in `configure.in'.

`STANDARD_STARTFILE_PREFIX'
     Define this macro as a C string constant if you wish to override
     the standard choice of `/usr/local/lib/' as the default prefix to
     try when searching for startup files such as `crt0.o'.

`MD_STARTFILE_PREFIX'
     If defined, this macro supplies an additional prefix to try after
     the standard prefixes.  `MD_EXEC_PREFIX' is not searched when the
     `-b' option is used, or when the compiler is built as a cross
     compiler.

`MD_STARTFILE_PREFIX_1'
     If defined, this macro supplies yet another prefix to try after the
     standard prefixes.  It is not searched when the `-b' option is
     used, or when the compiler is built as a cross compiler.

`INIT_ENVIRONMENT'
     Define this macro as a C string constant if you wish to set
     environment variables for programs called by the driver, such as
     the assembler and loader.  The driver passes the value of this
     macro to `putenv' to initialize the necessary environment
     variables.

`LOCAL_INCLUDE_DIR'
     Define this macro as a C string constant if you wish to override
     the standard choice of `/usr/local/include' as the default prefix
     to try when searching for local header files.  `LOCAL_INCLUDE_DIR'
     comes before `SYSTEM_INCLUDE_DIR' in the search order.

     Cross compilers do not search either `/usr/local/include' or its
     replacement.

`MODIFY_TARGET_NAME'
     Define this macro if you with to define command-line switches that
     modify the default target name

     For each switch, you can include a string to be appended to the
     first part of the configuration name or a string to be deleted
     from the configuration name, if present.  The definition should be
     an initializer for an array of structures.  Each array element
     should have three elements: the switch name (a string constant,
     including the initial dash), one of the enumeration codes `ADD' or
     `DELETE' to indicate whether the string should be inserted or
     deleted, and the string to be inserted or deleted (a string
     constant).

     For example, on a machine where `64' at the end of the
     configuration name denotes a 64-bit target and you want the `-32'
     and `-64' switches to select between 32- and 64-bit targets, you
     would code

          #define MODIFY_TARGET_NAME \
            { { "-32", DELETE, "64"}, \
               {"-64", ADD, "64"}}

`SYSTEM_INCLUDE_DIR'
     Define this macro as a C string constant if you wish to specify a
     system-specific directory to search for header files before the
     standard directory.  `SYSTEM_INCLUDE_DIR' comes before
     `STANDARD_INCLUDE_DIR' in the search order.

     Cross compilers do not use this macro and do not search the
     directory specified.

`STANDARD_INCLUDE_DIR'
     Define this macro as a C string constant if you wish to override
     the standard choice of `/usr/include' as the default prefix to try
     when searching for header files.

     Cross compilers do not use this macro and do not search either
     `/usr/include' or its replacement.

`STANDARD_INCLUDE_COMPONENT'
     The "component" corresponding to `STANDARD_INCLUDE_DIR'.  See
     `INCLUDE_DEFAULTS', below, for the description of components.  If
     you do not define this macro, no component is used.

`INCLUDE_DEFAULTS'
     Define this macro if you wish to override the entire default
     search path for include files.  For a native compiler, the default
     search path usually consists of `GCC_INCLUDE_DIR',
     `LOCAL_INCLUDE_DIR', `SYSTEM_INCLUDE_DIR',
     `GPLUSPLUS_INCLUDE_DIR', and `STANDARD_INCLUDE_DIR'.  In addition,
     `GPLUSPLUS_INCLUDE_DIR' and `GCC_INCLUDE_DIR' are defined
     automatically by `Makefile', and specify private search areas for
     GCC.  The directory `GPLUSPLUS_INCLUDE_DIR' is used only for C++
     programs.

     The definition should be an initializer for an array of structures.
     Each array element should have four elements: the directory name (a
     string constant), the component name (also a string constant), a
     flag for C++-only directories, and a flag showing that the
     includes in the directory don't need to be wrapped in `extern `C''
     when compiling C++.  Mark the end of the array with a null element.

     The component name denotes what GNU package the include file is
     part of, if any, in all upper-case letters.  For example, it might
     be `GCC' or `BINUTILS'.  If the package is part of a
     vendor-supplied operating system, code the component name as `0'.

     For example, here is the definition used for VAX/VMS:

          #define INCLUDE_DEFAULTS \
          {                                       \
            { "GNU_GXX_INCLUDE:", "G++", 1, 1},   \
            { "GNU_CC_INCLUDE:", "GCC", 0, 0},    \
            { "SYS$SYSROOT:[SYSLIB.]", 0, 0, 0},  \
            { ".", 0, 0, 0},                      \
            { 0, 0, 0, 0}                         \
          }

   Here is the order of prefixes tried for exec files:

  1. Any prefixes specified by the user with `-B'.

  2. The environment variable `GCC_EXEC_PREFIX', if any.

  3. The directories specified by the environment variable
     `COMPILER_PATH'.

  4. The macro `STANDARD_EXEC_PREFIX'.

  5. `/usr/lib/gcc/'.

  6. The macro `MD_EXEC_PREFIX', if any.

   Here is the order of prefixes tried for startfiles:

  1. Any prefixes specified by the user with `-B'.

  2. The environment variable `GCC_EXEC_PREFIX', if any.

  3. The directories specified by the environment variable
     `LIBRARY_PATH' (or port-specific name; native only, cross
     compilers do not use this).

  4. The macro `STANDARD_EXEC_PREFIX'.

  5. `/usr/lib/gcc/'.

  6. The macro `MD_EXEC_PREFIX', if any.

  7. The macro `MD_STARTFILE_PREFIX', if any.

  8. The macro `STANDARD_STARTFILE_PREFIX'.

  9. `/lib/'.

 10. `/usr/lib/'.


File: gccint.info,  Node: Run-time Target,  Next: Per-Function Data,  Prev: Driver,  Up: Target Macros

10.3 Run-time Target Specification
==================================

Here are run-time target specifications.

`CPP_PREDEFINES'
     Define this to be a string constant containing `-D' options to
     define the predefined macros that identify this machine and system.
     These macros will be predefined unless the `-ansi' option (or a
     `-std' option for strict ISO C conformance) is specified.

     In addition, a parallel set of macros are predefined, whose names
     are made by appending `__' at the beginning and at the end.  These
     `__' macros are permitted by the ISO standard, so they are
     predefined regardless of whether `-ansi' or a `-std' option is
     specified.

     For example, on the Sun, one can use the following value:

          "-Dmc68000 -Dsun -Dunix"

     The result is to define the macros `__mc68000__', `__sun__' and
     `__unix__' unconditionally, and the macros `mc68000', `sun' and
     `unix' provided `-ansi' is not specified.

`extern int target_flags;'
     This declaration should be present.

`TARGET_...'
     This series of macros is to allow compiler command arguments to
     enable or disable the use of optional features of the target
     machine.  For example, one machine description serves both the
     68000 and the 68020; a command argument tells the compiler whether
     it should use 68020-only instructions or not.  This command
     argument works by means of a macro `TARGET_68020' that tests a bit
     in `target_flags'.

     Define a macro `TARGET_FEATURENAME' for each such option.  Its
     definition should test a bit in `target_flags'.  It is recommended
     that a helper macro `TARGET_MASK_FEATURENAME' is defined for each
     bit-value to test, and used in `TARGET_FEATURENAME' and
     `TARGET_SWITCHES'.  For example:

          #define TARGET_MASK_68020 1
          #define TARGET_68020 (target_flags & TARGET_MASK_68020)

     One place where these macros are used is in the
     condition-expressions of instruction patterns.  Note how
     `TARGET_68020' appears frequently in the 68000 machine description
     file, `m68k.md'.  Another place they are used is in the
     definitions of the other macros in the `MACHINE.h' file.

`TARGET_SWITCHES'
     This macro defines names of command options to set and clear bits
     in `target_flags'.  Its definition is an initializer with a
     subgrouping for each command option.

     Each subgrouping contains a string constant, that defines the
     option name, a number, which contains the bits to set in
     `target_flags', and a second string which is the description
     displayed by `--help'.  If the number is negative then the bits
     specified by the number are cleared instead of being set.  If the
     description string is present but empty, then no help information
     will be displayed for that option, but it will not count as an
     undocumented option.  The actual option name is made by appending
     `-m' to the specified name.  Non-empty description strings should
     be marked with `N_(...)' for `xgettext'.  Please do not mark empty
     strings because the empty string is reserved by GNU gettext.
     `gettext("")' returns the header entry of the message catalog with
     meta information, not the empty string.

     In addition to the description for `--help', more detailed
     documentation for each option should be added to `invoke.texi'.

     One of the subgroupings should have a null string.  The number in
     this grouping is the default value for `target_flags'.  Any target
     options act starting with that value.

     Here is an example which defines `-m68000' and `-m68020' with
     opposite meanings, and picks the latter as the default:

          #define TARGET_SWITCHES \
            { { "68020", TARGET_MASK_68020, "" },      \
              { "68000", -TARGET_MASK_68020, \
                N_("Compile for the 68000") }, \
              { "", TARGET_MASK_68020, "" }}

`TARGET_OPTIONS'
     This macro is similar to `TARGET_SWITCHES' but defines names of
     command options that have values.  Its definition is an
     initializer with a subgrouping for each command option.

     Each subgrouping contains a string constant, that defines the
     fixed part of the option name, the address of a variable, and a
     description string.  Non-empty description strings should be
     marked with `N_(...)' for `xgettext'.  Please do not mark empty
     strings because the empty string is reserved by GNU gettext.
     `gettext("")' returns the header entry of the message catalog with
     meta information, not the empty string.

     The variable, type `char *', is set to the variable part of the
     given option if the fixed part matches.  The actual option name is
     made by appending `-m' to the specified name.  Again, each option
     should also be documented in `invoke.texi'.

     Here is an example which defines `-mshort-data-NUMBER'.  If the
     given option is `-mshort-data-512', the variable `m88k_short_data'
     will be set to the string `"512"'.

          extern char *m88k_short_data;
          #define TARGET_OPTIONS \
           { { "short-data-", &m88k_short_data, \
               N_("Specify the size of the short data section") } }

`TARGET_VERSION'
     This macro is a C statement to print on `stderr' a string
     describing the particular machine description choice.  Every
     machine description should define `TARGET_VERSION'.  For example:

          #ifdef MOTOROLA
          #define TARGET_VERSION \
            fprintf (stderr, " (68k, Motorola syntax)");
          #else
          #define TARGET_VERSION \
            fprintf (stderr, " (68k, MIT syntax)");
          #endif

`OVERRIDE_OPTIONS'
     Sometimes certain combinations of command options do not make
     sense on a particular target machine.  You can define a macro
     `OVERRIDE_OPTIONS' to take account of this.  This macro, if
     defined, is executed once just after all the command options have
     been parsed.

     Don't use this macro to turn on various extra optimizations for
     `-O'.  That is what `OPTIMIZATION_OPTIONS' is for.

`OPTIMIZATION_OPTIONS (LEVEL, SIZE)'
     Some machines may desire to change what optimizations are
     performed for various optimization levels.   This macro, if
     defined, is executed once just after the optimization level is
     determined and before the remainder of the command options have
     been parsed.  Values set in this macro are used as the default
     values for the other command line options.

     LEVEL is the optimization level specified; 2 if `-O2' is
     specified, 1 if `-O' is specified, and 0 if neither is specified.

     SIZE is nonzero if `-Os' is specified and zero otherwise.

     You should not use this macro to change options that are not
     machine-specific.  These should uniformly selected by the same
     optimization level on all supported machines.  Use this macro to
     enable machine-specific optimizations.

     *Do not examine `write_symbols' in this macro!* The debugging
     options are not supposed to alter the generated code.

`CAN_DEBUG_WITHOUT_FP'
     Define this macro if debugging can be performed even without a
     frame pointer.  If this macro is defined, GCC will turn on the
     `-fomit-frame-pointer' option whenever `-O' is specified.


File: gccint.info,  Node: Per-Function Data,  Next: Storage Layout,  Prev: Run-time Target,  Up: Target Macros

10.4 Defining data structures for per-function information.
===========================================================

If the target needs to store information on a per-function basis, GCC
provides a macro and a couple of variables to allow this.  Note, just
using statics to store the information is a bad idea, since GCC supports
nested functions, so you can be halfway through encoding one function
when another one comes along.

   GCC defines a data structure called `struct function' which contains
all of the data specific to an individual function.  This structure
contains a field called `machine' whose type is `struct
machine_function *', which can be used by targets to point to their own
specific data.

   If a target needs per-function specific data it should define the
type `struct machine_function' and also the macro `INIT_EXPANDERS'.
This macro should be used to initialize some or all of the function
pointers `init_machine_status', `free_machine_status' and
`mark_machine_status'.  These pointers are explained below.

   One typical use of per-function, target specific data is to create an
RTX to hold the register containing the function's return address.  This
RTX can then be used to implement the `__builtin_return_address'
function, for level 0.

   Note--earlier implementations of GCC used a single data area to hold
all of the per-function information.  Thus when processing of a nested
function began the old per-function data had to be pushed onto a stack,
and when the processing was finished, it had to be popped off the
stack.  GCC used to provide function pointers called
`save_machine_status' and `restore_machine_status' to handle the saving
and restoring of the target specific information.  Since the single
data area approach is no longer used, these pointers are no longer
supported.

   The macro and function pointers are described below.

`INIT_EXPANDERS'
     Macro called to initialize any target specific information.  This
     macro is called once per function, before generation of any RTL
     has begun.  The intention of this macro is to allow the
     initialization of the function pointers below.

`init_machine_status'
     This is a `void (*)(struct function *)' function pointer.  If this
     pointer is non-`NULL' it will be called once per function, before
     function compilation starts, in order to allow the target to
     perform any target specific initialization of the `struct
     function' structure.  It is intended that this would be used to
     initialize the `machine' of that structure.

`free_machine_status'
     This is a `void (*)(struct function *)' function pointer.  If this
     pointer is non-`NULL' it will be called once per function, after
     the function has been compiled, in order to allow any memory
     allocated during the `init_machine_status' function call to be
     freed.

`mark_machine_status'
     This is a `void (*)(struct function *)' function pointer.  If this
     pointer is non-`NULL' it will be called once per function in order
     to mark any data items in the `struct machine_function' structure
     which need garbage collection.



File: gccint.info,  Node: Storage Layout,  Next: Type Layout,  Prev: Per-Function Data,  Up: Target Macros

10.5 Storage Layout
===================

Note that the definitions of the macros in this table which are sizes or
alignments measured in bits do not need to be constant.  They can be C
expressions that refer to static variables, such as the `target_flags'.
*Note Run-time Target::.

`BITS_BIG_ENDIAN'
     Define this macro to have the value 1 if the most significant bit
     in a byte has the lowest number; otherwise define it to have the
     value zero.  This means that bit-field instructions count from the
     most significant bit.  If the machine has no bit-field
     instructions, then this must still be defined, but it doesn't
     matter which value it is defined to.  This macro need not be a
     constant.

     This macro does not affect the way structure fields are packed into
     bytes or words; that is controlled by `BYTES_BIG_ENDIAN'.

`BYTES_BIG_ENDIAN'
     Define this macro to have the value 1 if the most significant byte
     in a word has the lowest number.  This macro need not be a
     constant.

`WORDS_BIG_ENDIAN'
     Define this macro to have the value 1 if, in a multiword object,
     the most significant word has the lowest number.  This applies to
     both memory locations and registers; GCC fundamentally assumes
     that the order of words in memory is the same as the order in
     registers.  This macro need not be a constant.

`LIBGCC2_WORDS_BIG_ENDIAN'
     Define this macro if `WORDS_BIG_ENDIAN' is not constant.  This
     must be a constant value with the same meaning as
     `WORDS_BIG_ENDIAN', which will be used only when compiling
     `libgcc2.c'.  Typically the value will be set based on
     preprocessor defines.

`FLOAT_WORDS_BIG_ENDIAN'
     Define this macro to have the value 1 if `DFmode', `XFmode' or
     `TFmode' floating point numbers are stored in memory with the word
     containing the sign bit at the lowest address; otherwise define it
     to have the value 0.  This macro need not be a constant.

     You need not define this macro if the ordering is the same as for
     multi-word integers.

`BITS_PER_UNIT'
     Define this macro to be the number of bits in an addressable
     storage unit (byte); normally 8.

`BITS_PER_WORD'
     Number of bits in a word; normally 32.

`MAX_BITS_PER_WORD'
     Maximum number of bits in a word.  If this is undefined, the
     default is `BITS_PER_WORD'.  Otherwise, it is the constant value
     that is the largest value that `BITS_PER_WORD' can have at
     run-time.

`UNITS_PER_WORD'
     Number of storage units in a word; normally 4.

`MIN_UNITS_PER_WORD'
     Minimum number of units in a word.  If this is undefined, the
     default is `UNITS_PER_WORD'.  Otherwise, it is the constant value
     that is the smallest value that `UNITS_PER_WORD' can have at
     run-time.

`POINTER_SIZE'
     Width of a pointer, in bits.  You must specify a value no wider
     than the width of `Pmode'.  If it is not equal to the width of
     `Pmode', you must define `POINTERS_EXTEND_UNSIGNED'.

`POINTERS_EXTEND_UNSIGNED'
     A C expression whose value is greater than zero if pointers that
     need to be extended from being `POINTER_SIZE' bits wide to `Pmode'
     are to be zero-extended and zero if they are to be sign-extended.
     If the value is less then zero then there must be an "ptr_extend"
     instruction that extends a pointer from `POINTER_SIZE' to `Pmode'.

     You need not define this macro if the `POINTER_SIZE' is equal to
     the width of `Pmode'.

`PROMOTE_MODE (M, UNSIGNEDP, TYPE)'
     A macro to update M and UNSIGNEDP when an object whose type is
     TYPE and which has the specified mode and signedness is to be
     stored in a register.  This macro is only called when TYPE is a
     scalar type.

     On most RISC machines, which only have operations that operate on
     a full register, define this macro to set M to `word_mode' if M is
     an integer mode narrower than `BITS_PER_WORD'.  In most cases,
     only integer modes should be widened because wider-precision
     floating-point operations are usually more expensive than their
     narrower counterparts.

     For most machines, the macro definition does not change UNSIGNEDP.
     However, some machines, have instructions that preferentially
     handle either signed or unsigned quantities of certain modes.  For
     example, on the DEC Alpha, 32-bit loads from memory and 32-bit add
     instructions sign-extend the result to 64 bits.  On such machines,
     set UNSIGNEDP according to which kind of extension is more
     efficient.

     Do not define this macro if it would never modify M.

`PROMOTE_FUNCTION_ARGS'
     Define this macro if the promotion described by `PROMOTE_MODE'
     should also be done for outgoing function arguments.

`PROMOTE_FUNCTION_RETURN'
     Define this macro if the promotion described by `PROMOTE_MODE'
     should also be done for the return value of functions.

     If this macro is defined, `FUNCTION_VALUE' must perform the same
     promotions done by `PROMOTE_MODE'.

`PROMOTE_FOR_CALL_ONLY'
     Define this macro if the promotion described by `PROMOTE_MODE'
     should _only_ be performed for outgoing function arguments or
     function return values, as specified by `PROMOTE_FUNCTION_ARGS'
     and `PROMOTE_FUNCTION_RETURN', respectively.

`PARM_BOUNDARY'
     Normal alignment required for function parameters on the stack, in
     bits.  All stack parameters receive at least this much alignment
     regardless of data type.  On most machines, this is the same as the
     size of an integer.

`STACK_BOUNDARY'
     Define this macro to the minimum alignment enforced by hardware
     for the stack pointer on this machine.  The definition is a C
     expression for the desired alignment (measured in bits).  This
     value is used as a default if `PREFERRED_STACK_BOUNDARY' is not
     defined.  On most machines, this should be the same as
     `PARM_BOUNDARY'.

`PREFERRED_STACK_BOUNDARY'
     Define this macro if you wish to preserve a certain alignment for
     the stack pointer, greater than what the hardware enforces.  The
     definition is a C expression for the desired alignment (measured
     in bits).  This macro must evaluate to a value equal to or larger
     than `STACK_BOUNDARY'.

`FORCE_PREFERRED_STACK_BOUNDARY_IN_MAIN'
     A C expression that evaluates true if `PREFERRED_STACK_BOUNDARY' is
     not guaranteed by the runtime and we should emit code to align the
     stack at the beginning of `main'.

     If `PUSH_ROUNDING' is not defined, the stack will always be aligned
     to the specified boundary.  If `PUSH_ROUNDING' is defined and
     specifies a less strict alignment than `PREFERRED_STACK_BOUNDARY',
     the stack may be momentarily unaligned while pushing arguments.

`FUNCTION_BOUNDARY'
     Alignment required for a function entry point, in bits.

`BIGGEST_ALIGNMENT'
     Biggest alignment that any data type can require on this machine,
     in bits.

`MINIMUM_ATOMIC_ALIGNMENT'
     If defined, the smallest alignment, in bits, that can be given to
     an object that can be referenced in one operation, without
     disturbing any nearby object.  Normally, this is `BITS_PER_UNIT',
     but may be larger on machines that don't have byte or half-word
     store operations.

`BIGGEST_FIELD_ALIGNMENT'
     Biggest alignment that any structure or union field can require on
     this machine, in bits.  If defined, this overrides
     `BIGGEST_ALIGNMENT' for structure and union fields only, unless
     the field alignment has been set by the `__attribute__ ((aligned
     (N)))' construct.

`ADJUST_FIELD_ALIGN (FIELD, COMPUTED)'
     An expression for the alignment of a structure field FIELD if the
     alignment computed in the usual way (including applying of
     `BIGGEST_ALIGNMENT' and `BIGGEST_FIELD_ALIGNMENT' to the
     alignment) is COMPUTED.  It overrides alignment only if the field
     alignment has not been set by the `__attribute__ ((aligned (N)))'
     construct.

`MAX_OFILE_ALIGNMENT'
     Biggest alignment supported by the object file format of this
     machine.  Use this macro to limit the alignment which can be
     specified using the `__attribute__ ((aligned (N)))' construct.  If
     not defined, the default value is `BIGGEST_ALIGNMENT'.

`DATA_ALIGNMENT (TYPE, BASIC-ALIGN)'
     If defined, a C expression to compute the alignment for a variable
     in the static store.  TYPE is the data type, and BASIC-ALIGN is
     the alignment that the object would ordinarily have.  The value of
     this macro is used instead of that alignment to align the object.

     If this macro is not defined, then BASIC-ALIGN is used.

     One use of this macro is to increase alignment of medium-size data
     to make it all fit in fewer cache lines.  Another is to cause
     character arrays to be word-aligned so that `strcpy' calls that
     copy constants to character arrays can be done inline.

`CONSTANT_ALIGNMENT (CONSTANT, BASIC-ALIGN)'
     If defined, a C expression to compute the alignment given to a
     constant that is being placed in memory.  CONSTANT is the constant
     and BASIC-ALIGN is the alignment that the object would ordinarily
     have.  The value of this macro is used instead of that alignment to
     align the object.

     If this macro is not defined, then BASIC-ALIGN is used.

     The typical use of this macro is to increase alignment for string
     constants to be word aligned so that `strcpy' calls that copy
     constants can be done inline.

`LOCAL_ALIGNMENT (TYPE, BASIC-ALIGN)'
     If defined, a C expression to compute the alignment for a variable
     in the local store.  TYPE is the data type, and BASIC-ALIGN is the
     alignment that the object would ordinarily have.  The value of this
     macro is used instead of that alignment to align the object.

     If this macro is not defined, then BASIC-ALIGN is used.

     One use of this macro is to increase alignment of medium-size data
     to make it all fit in fewer cache lines.

`EMPTY_FIELD_BOUNDARY'
     Alignment in bits to be given to a structure bit-field that
     follows an empty field such as `int : 0;'.

     Note that `PCC_BITFIELD_TYPE_MATTERS' also affects the alignment
     that results from an empty field.

`STRUCTURE_SIZE_BOUNDARY'
     Number of bits which any structure or union's size must be a
     multiple of.  Each structure or union's size is rounded up to a
     multiple of this.

     If you do not define this macro, the default is the same as
     `BITS_PER_UNIT'.

`STRICT_ALIGNMENT'
     Define this macro to be the value 1 if instructions will fail to
     work if given data not on the nominal alignment.  If instructions
     will merely go slower in that case, define this macro as 0.

`PCC_BITFIELD_TYPE_MATTERS'
     Define this if you wish to imitate the way many other C compilers
     handle alignment of bit-fields and the structures that contain
     them.

     The behavior is that the type written for a bit-field (`int',
     `short', or other integer type) imposes an alignment for the
     entire structure, as if the structure really did contain an
     ordinary field of that type.  In addition, the bit-field is placed
     within the structure so that it would fit within such a field, not
     crossing a boundary for it.

     Thus, on most machines, a bit-field whose type is written as `int'
     would not cross a four-byte boundary, and would force four-byte
     alignment for the whole structure.  (The alignment used may not be
     four bytes; it is controlled by the other alignment parameters.)

     If the macro is defined, its definition should be a C expression;
     a nonzero value for the expression enables this behavior.

     Note that if this macro is not defined, or its value is zero, some
     bit-fields may cross more than one alignment boundary.  The
     compiler can support such references if there are `insv', `extv',
     and `extzv' insns that can directly reference memory.

     The other known way of making bit-fields work is to define
     `STRUCTURE_SIZE_BOUNDARY' as large as `BIGGEST_ALIGNMENT'.  Then
     every structure can be accessed with fullwords.

     Unless the machine has bit-field instructions or you define
     `STRUCTURE_SIZE_BOUNDARY' that way, you must define
     `PCC_BITFIELD_TYPE_MATTERS' to have a nonzero value.

     If your aim is to make GCC use the same conventions for laying out
     bit-fields as are used by another compiler, here is how to
     investigate what the other compiler does.  Compile and run this
     program:

          struct foo1
          {
            char x;
            char :0;
            char y;
          };

          struct foo2
          {
            char x;
            int :0;
            char y;
          };

          main ()
          {
            printf ("Size of foo1 is %d\n",
                    sizeof (struct foo1));
            printf ("Size of foo2 is %d\n",
                    sizeof (struct foo2));
            exit (0);
          }

     If this prints 2 and 5, then the compiler's behavior is what you
     would get from `PCC_BITFIELD_TYPE_MATTERS'.

`BITFIELD_NBYTES_LIMITED'
     Like `PCC_BITFIELD_TYPE_MATTERS' except that its effect is limited
     to aligning a bit-field within the structure.

`MEMBER_TYPE_FORCES_BLK (FIELD)'
     Return 1 if a structure or array containing FIELD should be
     accessed using `BLKMODE'.

     Normally, this is not needed.  See the file `c4x.h' for an example
     of how to use this macro to prevent a structure having a floating
     point field from being accessed in an integer mode.

`ROUND_TYPE_SIZE (TYPE, COMPUTED, SPECIFIED)'
     Define this macro as an expression for the overall size of a type
     (given by TYPE as a tree node) when the size computed in the usual
     way is COMPUTED and the alignment is SPECIFIED.

     The default is to round COMPUTED up to a multiple of SPECIFIED.

`ROUND_TYPE_SIZE_UNIT (TYPE, COMPUTED, SPECIFIED)'
     Similar to `ROUND_TYPE_SIZE', but sizes and alignments are
     specified in units (bytes).  If you define `ROUND_TYPE_SIZE', you
     must also define this macro and they must be defined consistently
     with each other.

`ROUND_TYPE_ALIGN (TYPE, COMPUTED, SPECIFIED)'
     Define this macro as an expression for the alignment of a type
     (given by TYPE as a tree node) if the alignment computed in the
     usual way is COMPUTED and the alignment explicitly specified was
     SPECIFIED.

     The default is to use SPECIFIED if it is larger; otherwise, use
     the smaller of COMPUTED and `BIGGEST_ALIGNMENT'

`MAX_FIXED_MODE_SIZE'
     An integer expression for the size in bits of the largest integer
     machine mode that should actually be used.  All integer machine
     modes of this size or smaller can be used for structures and
     unions with the appropriate sizes.  If this macro is undefined,
     `GET_MODE_BITSIZE (DImode)' is assumed.

`VECTOR_MODE_SUPPORTED_P(MODE)'
     Define this macro to be nonzero if the port is prepared to handle
     insns involving vector mode MODE.  At the very least, it must have
     move patterns for this mode.

`STACK_SAVEAREA_MODE (SAVE_LEVEL)'
     If defined, an expression of type `enum machine_mode' that
     specifies the mode of the save area operand of a
     `save_stack_LEVEL' named pattern (*note Standard Names::).
     SAVE_LEVEL is one of `SAVE_BLOCK', `SAVE_FUNCTION', or
     `SAVE_NONLOCAL' and selects which of the three named patterns is
     having its mode specified.

     You need not define this macro if it always returns `Pmode'.  You
     would most commonly define this macro if the `save_stack_LEVEL'
     patterns need to support both a 32- and a 64-bit mode.

`STACK_SIZE_MODE'
     If defined, an expression of type `enum machine_mode' that
     specifies the mode of the size increment operand of an
     `allocate_stack' named pattern (*note Standard Names::).

     You need not define this macro if it always returns `word_mode'.
     You would most commonly define this macro if the `allocate_stack'
     pattern needs to support both a 32- and a 64-bit mode.

`CHECK_FLOAT_VALUE (MODE, VALUE, OVERFLOW)'
     A C statement to validate the value VALUE (of type `double') for
     mode MODE.  This means that you check whether VALUE fits within
     the possible range of values for mode MODE on this target machine.
     The mode MODE is always a mode of class `MODE_FLOAT'.  OVERFLOW
     is nonzero if the value is already known to be out of range.

     If VALUE is not valid or if OVERFLOW is nonzero, you should set
     OVERFLOW to 1 and then assign some valid value to VALUE.  Allowing
     an invalid value to go through the compiler can produce incorrect
     assembler code which may even cause Unix assemblers to crash.

     This macro need not be defined if there is no work for it to do.

`TARGET_FLOAT_FORMAT'
     A code distinguishing the floating point format of the target
     machine.  There are five defined values:

    `IEEE_FLOAT_FORMAT'
          This code indicates IEEE floating point.  It is the default;
          there is no need to define this macro when the format is IEEE.

    `VAX_FLOAT_FORMAT'
          This code indicates the "D float" format used on the VAX.

    `IBM_FLOAT_FORMAT'
          This code indicates the format used on the IBM System/370.

    `C4X_FLOAT_FORMAT'
          This code indicates the format used on the TMS320C3x/C4x.

    `UNKNOWN_FLOAT_FORMAT'
          This code indicates any other format.

     The value of this macro is compared with `HOST_FLOAT_FORMAT', which
     is defined by the `configure' script, to determine whether the
     target machine has the same format as the host machine.  If any
     other formats are actually in use on supported machines, new codes
     should be defined for them.

     The ordering of the component words of floating point values
     stored in memory is controlled by `FLOAT_WORDS_BIG_ENDIAN'.


 -- Target Hook: bool TARGET_MS_BITFIELD_LAYOUT_P (tree RECORD_TYPE)
     This target hook returns `true' if bit-fields in the given
     RECORD_TYPE are to be laid out following the rules of Microsoft
     Visual C/C++, namely: (i) a bit-field won't share the same storage
     unit with the previous bit-field if their underlying types have
     different sizes, and the bit-field will be aligned to the highest
     alignment of the underlying types of itself and of the previous
     bit-field; (ii) a zero-sized bit-field will affect the alignment of
     the whole enclosing structure, even if it is unnamed; except that
     (iii) a zero-sized bit-field will be disregarded unless it follows
     another bit-field of non-zero size.  If this hook returns `true',
     other macros that control bit-field layout are ignored.


File: gccint.info,  Node: Type Layout,  Next: Escape Sequences,  Prev: Storage Layout,  Up: Target Macros

10.6 Layout of Source Language Data Types
=========================================

These macros define the sizes and other characteristics of the standard
basic data types used in programs being compiled.  Unlike the macros in
the previous section, these apply to specific features of C and related
languages, rather than to fundamental aspects of storage layout.

`INT_TYPE_SIZE'
     A C expression for the size in bits of the type `int' on the
     target machine.  If you don't define this, the default is one word.

`SHORT_TYPE_SIZE'
     A C expression for the size in bits of the type `short' on the
     target machine.  If you don't define this, the default is half a
     word.  (If this would be less than one storage unit, it is rounded
     up to one unit.)

`LONG_TYPE_SIZE'
     A C expression for the size in bits of the type `long' on the
     target machine.  If you don't define this, the default is one word.

`ADA_LONG_TYPE_SIZE'
     On some machines, the size used for the Ada equivalent of the type
     `long' by a native Ada compiler differs from that used by C.  In
     that situation, define this macro to be a C expression to be used
     for the size of that type.  If you don't define this, the default
     is the value of `LONG_TYPE_SIZE'.

`MAX_LONG_TYPE_SIZE'
     Maximum number for the size in bits of the type `long' on the
     target machine.  If this is undefined, the default is
     `LONG_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `LONG_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`LONG_LONG_TYPE_SIZE'
     A C expression for the size in bits of the type `long long' on the
     target machine.  If you don't define this, the default is two
     words.  If you want to support GNU Ada on your machine, the value
     of this macro must be at least 64.

`CHAR_TYPE_SIZE'
     A C expression for the size in bits of the type `char' on the
     target machine.  If you don't define this, the default is
     `BITS_PER_UNIT'.

`MAX_CHAR_TYPE_SIZE'
     Maximum number for the size in bits of the type `char' on the
     target machine.  If this is undefined, the default is
     `CHAR_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `CHAR_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`BOOL_TYPE_SIZE'
     A C expression for the size in bits of the C++ type `bool' and C99
     type `_Bool' on the target machine.  If you don't define this, and
     you probably shouldn't, the default is `CHAR_TYPE_SIZE'.

`FLOAT_TYPE_SIZE'
     A C expression for the size in bits of the type `float' on the
     target machine.  If you don't define this, the default is one word.

`DOUBLE_TYPE_SIZE'
     A C expression for the size in bits of the type `double' on the
     target machine.  If you don't define this, the default is two
     words.

`LONG_DOUBLE_TYPE_SIZE'
     A C expression for the size in bits of the type `long double' on
     the target machine.  If you don't define this, the default is two
     words.

     Maximum number for the size in bits of the type `long double' on
     the target machine.  If this is undefined, the default is
     `LONG_DOUBLE_TYPE_SIZE'.  Otherwise, it is the constant value that
     is the largest value that `LONG_DOUBLE_TYPE_SIZE' can have at
     run-time.  This is used in `cpp'.

     Define this macro to be 1 if the target machine uses 80-bit
     floating-point values with 128-bit size and alignment.  This is
     used in `real.c'.

`WIDEST_HARDWARE_FP_SIZE'
     A C expression for the size in bits of the widest floating-point
     format supported by the hardware.  If you define this macro, you
     must specify a value less than or equal to the value of
     `LONG_DOUBLE_TYPE_SIZE'.  If you do not define this macro, the
     value of `LONG_DOUBLE_TYPE_SIZE' is the default.

`DEFAULT_SIGNED_CHAR'
     An expression whose value is 1 or 0, according to whether the type
     `char' should be signed or unsigned by default.  The user can
     always override this default with the options `-fsigned-char' and
     `-funsigned-char'.

`DEFAULT_SHORT_ENUMS'
     A C expression to determine whether to give an `enum' type only as
     many bytes as it takes to represent the range of possible values
     of that type.  A nonzero value means to do that; a zero value
     means all `enum' types should be allocated like `int'.

     If you don't define the macro, the default is 0.

`SIZE_TYPE'
     A C expression for a string describing the name of the data type
     to use for size values.  The typedef name `size_t' is defined
     using the contents of the string.

     The string can contain more than one keyword.  If so, separate
     them with spaces, and write first any length keyword, then
     `unsigned' if appropriate, and finally `int'.  The string must
     exactly match one of the data type names defined in the function
     `init_decl_processing' in the file `c-decl.c'.  You may not omit
     `int' or change the order--that would cause the compiler to crash
     on startup.

     If you don't define this macro, the default is `"long unsigned
     int"'.

`PTRDIFF_TYPE'
     A C expression for a string describing the name of the data type
     to use for the result of subtracting two pointers.  The typedef
     name `ptrdiff_t' is defined using the contents of the string.  See
     `SIZE_TYPE' above for more information.

     If you don't define this macro, the default is `"long int"'.

`WCHAR_TYPE'
     A C expression for a string describing the name of the data type
     to use for wide characters.  The typedef name `wchar_t' is defined
     using the contents of the string.  See `SIZE_TYPE' above for more
     information.

     If you don't define this macro, the default is `"int"'.

`WCHAR_TYPE_SIZE'
     A C expression for the size in bits of the data type for wide
     characters.  This is used in `cpp', which cannot make use of
     `WCHAR_TYPE'.

`MAX_WCHAR_TYPE_SIZE'
     Maximum number for the size in bits of the data type for wide
     characters.  If this is undefined, the default is
     `WCHAR_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `WCHAR_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`GCOV_TYPE_SIZE'
     A C expression for the size in bits of the type used for gcov
     counters on the target machine.  If you don't define this, the
     default is one `LONG_TYPE_SIZE' in case it is greater or equal to
     64-bit and `LONG_LONG_TYPE_SIZE' otherwise.  You may want to
     re-define the type to ensure atomicity for counters in
     multithreaded programs.

`WINT_TYPE'
     A C expression for a string describing the name of the data type to
     use for wide characters passed to `printf' and returned from
     `getwc'.  The typedef name `wint_t' is defined using the contents
     of the string.  See `SIZE_TYPE' above for more information.

     If you don't define this macro, the default is `"unsigned int"'.

`INTMAX_TYPE'
     A C expression for a string describing the name of the data type
     that can represent any value of any standard or extended signed
     integer type.  The typedef name `intmax_t' is defined using the
     contents of the string.  See `SIZE_TYPE' above for more
     information.

     If you don't define this macro, the default is the first of
     `"int"', `"long int"', or `"long long int"' that has as much
     precision as `long long int'.

`UINTMAX_TYPE'
     A C expression for a string describing the name of the data type
     that can represent any value of any standard or extended unsigned
     integer type.  The typedef name `uintmax_t' is defined using the
     contents of the string.  See `SIZE_TYPE' above for more
     information.

     If you don't define this macro, the default is the first of
     `"unsigned int"', `"long unsigned int"', or `"long long unsigned
     int"' that has as much precision as `long long unsigned int'.

`TARGET_PTRMEMFUNC_VBIT_LOCATION'
     The C++ compiler represents a pointer-to-member-function with a
     struct that looks like:

            struct {
              union {
                void (*fn)();
                ptrdiff_t vtable_index;
              };
              ptrdiff_t delta;
            };

     The C++ compiler must use one bit to indicate whether the function
     that will be called through a pointer-to-member-function is
     virtual.  Normally, we assume that the low-order bit of a function
     pointer must always be zero.  Then, by ensuring that the
     vtable_index is odd, we can distinguish which variant of the union
     is in use.  But, on some platforms function pointers can be odd,
     and so this doesn't work.  In that case, we use the low-order bit
     of the `delta' field, and shift the remainder of the `delta' field
     to the left.

     GCC will automatically make the right selection about where to
     store this bit using the `FUNCTION_BOUNDARY' setting for your
     platform.  However, some platforms such as ARM/Thumb have
     `FUNCTION_BOUNDARY' set such that functions always start at even
     addresses, but the lowest bit of pointers to functions indicate
     whether the function at that address is in ARM or Thumb mode.  If
     this is the case of your architecture, you should define this
     macro to `ptrmemfunc_vbit_in_delta'.

     In general, you should not have to define this macro.  On
     architectures in which function addresses are always even,
     according to `FUNCTION_BOUNDARY', GCC will automatically define
     this macro to `ptrmemfunc_vbit_in_pfn'.

`TARGET_VTABLE_USES_DESCRIPTORS'
     Normally, the C++ compiler uses function pointers in vtables.  This
     macro allows the target to change to use "function descriptors"
     instead.  Function descriptors are found on targets for whom a
     function pointer is actually a small data structure.  Normally the
     data structure consists of the actual code address plus a data
     pointer to which the function's data is relative.

     If vtables are used, the value of this macro should be the number
     of words that the function descriptor occupies.


File: gccint.info,  Node: Escape Sequences,  Next: Registers,  Prev: Type Layout,  Up: Target Macros

10.7 Target Character Escape Sequences
======================================

By default, GCC assumes that the C character escape sequences take on
their ASCII values for the target.  If this is not correct, you must
explicitly define all of the macros below.

`TARGET_BELL'
     A C constant expression for the integer value for escape sequence
     `\a'.

`TARGET_ESC'
     A C constant expression for the integer value of the target escape
     character.  As an extension, GCC evaluates the escape sequences
     `\e' and `\E' to this.

`TARGET_BS'
`TARGET_TAB'
`TARGET_NEWLINE'
     C constant expressions for the integer values for escape sequences
     `\b', `\t' and `\n'.

`TARGET_VT'
`TARGET_FF'
`TARGET_CR'
     C constant expressions for the integer values for escape sequences
     `\v', `\f' and `\r'.


File: gccint.info,  Node: Registers,  Next: Register Classes,  Prev: Escape Sequences,  Up: Target Macros

10.8 Register Usage
===================

This section explains how to describe what registers the target machine
has, and how (in general) they can be used.

   The description of which registers a specific instruction can use is
done with register classes; see *Note Register Classes::.  For
information on using registers to access a stack frame, see *Note Frame
Registers::.  For passing values in registers, see *Note Register
Arguments::.  For returning values in registers, see *Note Scalar
Return::.

* Menu:

* Register Basics::		Number and kinds of registers.
* Allocation Order::		Order in which registers are allocated.
* Values in Registers::		What kinds of values each reg can hold.
* Leaf Functions::		Renumbering registers for leaf functions.
* Stack Registers::		Handling a register stack such as 80387.


File: gccint.info,  Node: Register Basics,  Next: Allocation Order,  Up: Registers

10.8.1 Basic Characteristics of Registers
-----------------------------------------

Registers have various characteristics.

`FIRST_PSEUDO_REGISTER'
     Number of hardware registers known to the compiler.  They receive
     numbers 0 through `FIRST_PSEUDO_REGISTER-1'; thus, the first
     pseudo register's number really is assigned the number
     `FIRST_PSEUDO_REGISTER'.

`FIXED_REGISTERS'
     An initializer that says which registers are used for fixed
     purposes all throughout the compiled code and are therefore not
     available for general allocation.  These would include the stack
     pointer, the frame pointer (except on machines where that can be
     used as a general register when no frame pointer is needed), the
     program counter on machines where that is considered one of the
     addressable registers, and any other numbered register with a
     standard use.

     This information is expressed as a sequence of numbers, separated
     by commas and surrounded by braces.  The Nth number is 1 if
     register N is fixed, 0 otherwise.

     The table initialized from this macro, and the table initialized by
     the following one, may be overridden at run time either
     automatically, by the actions of the macro
     `CONDITIONAL_REGISTER_USAGE', or by the user with the command
     options `-ffixed-REG', `-fcall-used-REG' and `-fcall-saved-REG'.

`CALL_USED_REGISTERS'
     Like `FIXED_REGISTERS' but has 1 for each register that is
     clobbered (in general) by function calls as well as for fixed
     registers.  This macro therefore identifies the registers that are
     not available for general allocation of values that must live
     across function calls.

     If a register has 0 in `CALL_USED_REGISTERS', the compiler
     automatically saves it on function entry and restores it on
     function exit, if the register is used within the function.

`CALL_REALLY_USED_REGISTERS'
     Like `CALL_USED_REGISTERS' except this macro doesn't require that
     the entire set of `FIXED_REGISTERS' be included.
     (`CALL_USED_REGISTERS' must be a superset of `FIXED_REGISTERS').
     This macro is optional.  If not specified, it defaults to the value
     of `CALL_USED_REGISTERS'.

`HARD_REGNO_CALL_PART_CLOBBERED (REGNO, MODE)'
     A C expression that is nonzero if it is not permissible to store a
     value of mode MODE in hard register number REGNO across a call
     without some part of it being clobbered.  For most machines this
     macro need not be defined.  It is only required for machines that
     do not preserve the entire contents of a register across a call.

`CONDITIONAL_REGISTER_USAGE'
     Zero or more C statements that may conditionally modify five
     variables `fixed_regs', `call_used_regs', `global_regs',
     `reg_names', and `reg_class_contents', to take into account any
     dependence of these register sets on target flags.  The first three
     of these are of type `char []' (interpreted as Boolean vectors).
     `global_regs' is a `const char *[]', and `reg_class_contents' is a
     `HARD_REG_SET'.  Before the macro is called, `fixed_regs',
     `call_used_regs', `reg_class_contents', and `reg_names' have been
     initialized from `FIXED_REGISTERS', `CALL_USED_REGISTERS',
     `REG_CLASS_CONTENTS', and `REGISTER_NAMES', respectively.
     `global_regs' has been cleared, and any `-ffixed-REG',
     `-fcall-used-REG' and `-fcall-saved-REG' command options have been
     applied.

     You need not define this macro if it has no work to do.

     If the usage of an entire class of registers depends on the target
     flags, you may indicate this to GCC by using this macro to modify
     `fixed_regs' and `call_used_regs' to 1 for each of the registers
     in the classes which should not be used by GCC.  Also define the
     macro `REG_CLASS_FROM_LETTER' to return `NO_REGS' if it is called
     with a letter for a class that shouldn't be used.

     (However, if this class is not included in `GENERAL_REGS' and all
     of the insn patterns whose constraints permit this class are
     controlled by target switches, then GCC will automatically avoid
     using these registers when the target switches are opposed to
     them.)

`NON_SAVING_SETJMP'
     If this macro is defined and has a nonzero value, it means that
     `setjmp' and related functions fail to save the registers, or that
     `longjmp' fails to restore them.  To compensate, the compiler
     avoids putting variables in registers in functions that use
     `setjmp'.

`INCOMING_REGNO (OUT)'
     Define this macro if the target machine has register windows.
     This C expression returns the register number as seen by the
     called function corresponding to the register number OUT as seen
     by the calling function.  Return OUT if register number OUT is not
     an outbound register.

`OUTGOING_REGNO (IN)'
     Define this macro if the target machine has register windows.
     This C expression returns the register number as seen by the
     calling function corresponding to the register number IN as seen
     by the called function.  Return IN if register number IN is not an
     inbound register.

`LOCAL_REGNO (REGNO)'
     Define this macro if the target machine has register windows.
     This C expression returns true if the register is call-saved but
     is in the register window.  Unlike most call-saved registers, such
     registers need not be explicitly restored on function exit or
     during non-local gotos.



File: gccint.info,  Node: Allocation Order,  Next: Values in Registers,  Prev: Register Basics,  Up: Registers

10.8.2 Order of Allocation of Registers
---------------------------------------

Registers are allocated in order.

`REG_ALLOC_ORDER'
     If defined, an initializer for a vector of integers, containing the
     numbers of hard registers in the order in which GCC should prefer
     to use them (from most preferred to least).

     If this macro is not defined, registers are used lowest numbered
     first (all else being equal).

     One use of this macro is on machines where the highest numbered
     registers must always be saved and the save-multiple-registers
     instruction supports only sequences of consecutive registers.  On
     such machines, define `REG_ALLOC_ORDER' to be an initializer that
     lists the highest numbered allocable register first.

`ORDER_REGS_FOR_LOCAL_ALLOC'
     A C statement (sans semicolon) to choose the order in which to
     allocate hard registers for pseudo-registers local to a basic
     block.

     Store the desired register order in the array `reg_alloc_order'.
     Element 0 should be the register to allocate first; element 1, the
     next register; and so on.

     The macro body should not assume anything about the contents of
     `reg_alloc_order' before execution of the macro.

     On most machines, it is not necessary to define this macro.


File: gccint.info,  Node: Values in Registers,  Next: Leaf Functions,  Prev: Allocation Order,  Up: Registers

10.8.3 How Values Fit in Registers
----------------------------------

This section discusses the macros that describe which kinds of values
(specifically, which machine modes) each register can hold, and how many
consecutive registers are needed for a given mode.

`HARD_REGNO_NREGS (REGNO, MODE)'
     A C expression for the number of consecutive hard registers,
     starting at register number REGNO, required to hold a value of mode
     MODE.

     On a machine where all registers are exactly one word, a suitable
     definition of this macro is

          #define HARD_REGNO_NREGS(REGNO, MODE)            \
             ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1)  \
              / UNITS_PER_WORD)

`HARD_REGNO_MODE_OK (REGNO, MODE)'
     A C expression that is nonzero if it is permissible to store a
     value of mode MODE in hard register number REGNO (or in several
     registers starting with that one).  For a machine where all
     registers are equivalent, a suitable definition is

          #define HARD_REGNO_MODE_OK(REGNO, MODE) 1

     You need not include code to check for the numbers of fixed
     registers, because the allocation mechanism considers them to be
     always occupied.

     On some machines, double-precision values must be kept in even/odd
     register pairs.  You can implement that by defining this macro to
     reject odd register numbers for such modes.

     The minimum requirement for a mode to be OK in a register is that
     the `movMODE' instruction pattern support moves between the
     register and other hard register in the same class and that moving
     a value into the register and back out not alter it.

     Since the same instruction used to move `word_mode' will work for
     all narrower integer modes, it is not necessary on any machine for
     `HARD_REGNO_MODE_OK' to distinguish between these modes, provided
     you define patterns `movhi', etc., to take advantage of this.  This
     is useful because of the interaction between `HARD_REGNO_MODE_OK'
     and `MODES_TIEABLE_P'; it is very desirable for all integer modes
     to be tieable.

     Many machines have special registers for floating point arithmetic.
     Often people assume that floating point machine modes are allowed
     only in floating point registers.  This is not true.  Any
     registers that can hold integers can safely _hold_ a floating
     point machine mode, whether or not floating arithmetic can be done
     on it in those registers.  Integer move instructions can be used
     to move the values.

     On some machines, though, the converse is true: fixed-point machine
     modes may not go in floating registers.  This is true if the
     floating registers normalize any value stored in them, because
     storing a non-floating value there would garble it.  In this case,
     `HARD_REGNO_MODE_OK' should reject fixed-point machine modes in
     floating registers.  But if the floating registers do not
     automatically normalize, if you can store any bit pattern in one
     and retrieve it unchanged without a trap, then any machine mode
     may go in a floating register, so you can define this macro to say
     so.

     The primary significance of special floating registers is rather
     that they are the registers acceptable in floating point arithmetic
     instructions.  However, this is of no concern to
     `HARD_REGNO_MODE_OK'.  You handle it by writing the proper
     constraints for those instructions.

     On some machines, the floating registers are especially slow to
     access, so that it is better to store a value in a stack frame
     than in such a register if floating point arithmetic is not being
     done.  As long as the floating registers are not in class
     `GENERAL_REGS', they will not be used unless some pattern's
     constraint asks for one.

`MODES_TIEABLE_P (MODE1, MODE2)'
     A C expression that is nonzero if a value of mode MODE1 is
     accessible in mode MODE2 without copying.

     If `HARD_REGNO_MODE_OK (R, MODE1)' and `HARD_REGNO_MODE_OK (R,
     MODE2)' are always the same for any R, then `MODES_TIEABLE_P
     (MODE1, MODE2)' should be nonzero.  If they differ for any R, you
     should define this macro to return zero unless some other
     mechanism ensures the accessibility of the value in a narrower
     mode.

     You should define this macro to return nonzero in as many cases as
     possible since doing so will allow GCC to perform better register
     allocation.

`AVOID_CCMODE_COPIES'
     Define this macro if the compiler should avoid copies to/from
     `CCmode' registers.  You should only define this macro if support
     for copying to/from `CCmode' is incomplete.


File: gccint.info,  Node: Leaf Functions,  Next: Stack Registers,  Prev: Values in Registers,  Up: Registers

10.8.4 Handling Leaf Functions
------------------------------

On some machines, a leaf function (i.e., one which makes no calls) can
run more efficiently if it does not make its own register window.
Often this means it is required to receive its arguments in the
registers where they are passed by the caller, instead of the registers
where they would normally arrive.

   The special treatment for leaf functions generally applies only when
other conditions are met; for example, often they may use only those
registers for its own variables and temporaries.  We use the term "leaf
function" to mean a function that is suitable for this special
handling, so that functions with no calls are not necessarily "leaf
functions".

   GCC assigns register numbers before it knows whether the function is
suitable for leaf function treatment.  So it needs to renumber the
registers in order to output a leaf function.  The following macros
accomplish this.

`LEAF_REGISTERS'
     Name of a char vector, indexed by hard register number, which
     contains 1 for a register that is allowable in a candidate for leaf
     function treatment.

     If leaf function treatment involves renumbering the registers,
     then the registers marked here should be the ones before
     renumbering--those that GCC would ordinarily allocate.  The
     registers which will actually be used in the assembler code, after
     renumbering, should not be marked with 1 in this vector.

     Define this macro only if the target machine offers a way to
     optimize the treatment of leaf functions.

`LEAF_REG_REMAP (REGNO)'
     A C expression whose value is the register number to which REGNO
     should be renumbered, when a function is treated as a leaf
     function.

     If REGNO is a register number which should not appear in a leaf
     function before renumbering, then the expression should yield -1,
     which will cause the compiler to abort.

     Define this macro only if the target machine offers a way to
     optimize the treatment of leaf functions, and registers need to be
     renumbered to do this.

   `TARGET_ASM_FUNCTION_PROLOGUE' and `TARGET_ASM_FUNCTION_EPILOGUE'
must usually treat leaf functions specially.  They can test the C
variable `current_function_is_leaf' which is nonzero for leaf
functions.  `current_function_is_leaf' is set prior to local register
allocation and is valid for the remaining compiler passes.  They can
also test the C variable `current_function_uses_only_leaf_regs' which
is nonzero for leaf functions which only use leaf registers.
`current_function_uses_only_leaf_regs' is valid after reload and is
only useful if `LEAF_REGISTERS' is defined.


File: gccint.info,  Node: Stack Registers,  Prev: Leaf Functions,  Up: Registers

10.8.5 Registers That Form a Stack
----------------------------------

There are special features to handle computers where some of the
"registers" form a stack, as in the 80387 coprocessor for the 80386.
Stack registers are normally written by pushing onto the stack, and are
numbered relative to the top of the stack.

   Currently, GCC can only handle one group of stack-like registers, and
they must be consecutively numbered.

`STACK_REGS'
     Define this if the machine has any stack-like registers.

`FIRST_STACK_REG'
     The number of the first stack-like register.  This one is the top
     of the stack.

`LAST_STACK_REG'
     The number of the last stack-like register.  This one is the
     bottom of the stack.


File: gccint.info,  Node: Register Classes,  Next: Stack and Calling,  Prev: Registers,  Up: Target Macros

10.9 Register Classes
=====================

On many machines, the numbered registers are not all equivalent.  For
example, certain registers may not be allowed for indexed addressing;
certain registers may not be allowed in some instructions.  These
machine restrictions are described to the compiler using "register
classes".

   You define a number of register classes, giving each one a name and
saying which of the registers belong to it.  Then you can specify
register classes that are allowed as operands to particular instruction
patterns.

   In general, each register will belong to several classes.  In fact,
one class must be named `ALL_REGS' and contain all the registers.
Another class must be named `NO_REGS' and contain no registers.  Often
the union of two classes will be another class; however, this is not
required.

   One of the classes must be named `GENERAL_REGS'.  There is nothing
terribly special about the name, but the operand constraint letters `r'
and `g' specify this class.  If `GENERAL_REGS' is the same as
`ALL_REGS', just define it as a macro which expands to `ALL_REGS'.

   Order the classes so that if class X is contained in class Y then X
has a lower class number than Y.

   The way classes other than `GENERAL_REGS' are specified in operand
constraints is through machine-dependent operand constraint letters.
You can define such letters to correspond to various classes, then use
them in operand constraints.

   You should define a class for the union of two classes whenever some
instruction allows both classes.  For example, if an instruction allows
either a floating point (coprocessor) register or a general register
for a certain operand, you should define a class `FLOAT_OR_GENERAL_REGS'
which includes both of them.  Otherwise you will get suboptimal code.

   You must also specify certain redundant information about the
register classes: for each class, which classes contain it and which
ones are contained in it; for each pair of classes, the largest class
contained in their union.

   When a value occupying several consecutive registers is expected in a
certain class, all the registers used must belong to that class.
Therefore, register classes cannot be used to enforce a requirement for
a register pair to start with an even-numbered register.  The way to
specify this requirement is with `HARD_REGNO_MODE_OK'.

   Register classes used for input-operands of bitwise-and or shift
instructions have a special requirement: each such class must have, for
each fixed-point machine mode, a subclass whose registers can transfer
that mode to or from memory.  For example, on some machines, the
operations for single-byte values (`QImode') are limited to certain
registers.  When this is so, each register class that is used in a
bitwise-and or shift instruction must have a subclass consisting of
registers from which single-byte values can be loaded or stored.  This
is so that `PREFERRED_RELOAD_CLASS' can always have a possible value to
return.

`enum reg_class'
     An enumeral type that must be defined with all the register class
     names as enumeral values.  `NO_REGS' must be first.  `ALL_REGS'
     must be the last register class, followed by one more enumeral
     value, `LIM_REG_CLASSES', which is not a register class but rather
     tells how many classes there are.

     Each register class has a number, which is the value of casting
     the class name to type `int'.  The number serves as an index in
     many of the tables described below.

`N_REG_CLASSES'
     The number of distinct register classes, defined as follows:

          #define N_REG_CLASSES (int) LIM_REG_CLASSES

`REG_CLASS_NAMES'
     An initializer containing the names of the register classes as C
     string constants.  These names are used in writing some of the
     debugging dumps.

`REG_CLASS_CONTENTS'
     An initializer containing the contents of the register classes, as
     integers which are bit masks.  The Nth integer specifies the
     contents of class N.  The way the integer MASK is interpreted is
     that register R is in the class if `MASK & (1 << R)' is 1.

     When the machine has more than 32 registers, an integer does not
     suffice.  Then the integers are replaced by sub-initializers,
     braced groupings containing several integers.  Each
     sub-initializer must be suitable as an initializer for the type
     `HARD_REG_SET' which is defined in `hard-reg-set.h'.  In this
     situation, the first integer in each sub-initializer corresponds to
     registers 0 through 31, the second integer to registers 32 through
     63, and so on.

`REGNO_REG_CLASS (REGNO)'
     A C expression whose value is a register class containing hard
     register REGNO.  In general there is more than one such class;
     choose a class which is "minimal", meaning that no smaller class
     also contains the register.

`BASE_REG_CLASS'
     A macro whose definition is the name of the class to which a valid
     base register must belong.  A base register is one used in an
     address which is the register value plus a displacement.

`MODE_BASE_REG_CLASS (MODE)'
     This is a variation of the `BASE_REG_CLASS' macro which allows the
     selection of a base register in a mode depenedent manner.  If MODE
     is VOIDmode then it should return the same value as
     `BASE_REG_CLASS'.

`INDEX_REG_CLASS'
     A macro whose definition is the name of the class to which a valid
     index register must belong.  An index register is one used in an
     address where its value is either multiplied by a scale factor or
     added to another register (as well as added to a displacement).

`REG_CLASS_FROM_LETTER (CHAR)'
     A C expression which defines the machine-dependent operand
     constraint letters for register classes.  If CHAR is such a
     letter, the value should be the register class corresponding to
     it.  Otherwise, the value should be `NO_REGS'.  The register
     letter `r', corresponding to class `GENERAL_REGS', will not be
     passed to this macro; you do not need to handle it.

`REGNO_OK_FOR_BASE_P (NUM)'
     A C expression which is nonzero if register number NUM is suitable
     for use as a base register in operand addresses.  It may be either
     a suitable hard register or a pseudo register that has been
     allocated such a hard register.

`REGNO_MODE_OK_FOR_BASE_P (NUM, MODE)'
     A C expression that is just like `REGNO_OK_FOR_BASE_P', except that
     that expression may examine the mode of the memory reference in
     MODE.  You should define this macro if the mode of the memory
     reference affects whether a register may be used as a base
     register.  If you define this macro, the compiler will use it
     instead of `REGNO_OK_FOR_BASE_P'.

`REGNO_OK_FOR_INDEX_P (NUM)'
     A C expression which is nonzero if register number NUM is suitable
     for use as an index register in operand addresses.  It may be
     either a suitable hard register or a pseudo register that has been
     allocated such a hard register.

     The difference between an index register and a base register is
     that the index register may be scaled.  If an address involves the
     sum of two registers, neither one of them scaled, then either one
     may be labeled the "base" and the other the "index"; but whichever
     labeling is used must fit the machine's constraints of which
     registers may serve in each capacity.  The compiler will try both
     labelings, looking for one that is valid, and will reload one or
     both registers only if neither labeling works.

`PREFERRED_RELOAD_CLASS (X, CLASS)'
     A C expression that places additional restrictions on the register
     class to use when it is necessary to copy value X into a register
     in class CLASS.  The value is a register class; perhaps CLASS, or
     perhaps another, smaller class.  On many machines, the following
     definition is safe:

          #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS

     Sometimes returning a more restrictive class makes better code.
     For example, on the 68000, when X is an integer constant that is
     in range for a `moveq' instruction, the value of this macro is
     always `DATA_REGS' as long as CLASS includes the data registers.
     Requiring a data register guarantees that a `moveq' will be used.

     If X is a `const_double', by returning `NO_REGS' you can force X
     into a memory constant.  This is useful on certain machines where
     immediate floating values cannot be loaded into certain kinds of
     registers.

`PREFERRED_OUTPUT_RELOAD_CLASS (X, CLASS)'
     Like `PREFERRED_RELOAD_CLASS', but for output reloads instead of
     input reloads.  If you don't define this macro, the default is to
     use CLASS, unchanged.

`LIMIT_RELOAD_CLASS (MODE, CLASS)'
     A C expression that places additional restrictions on the register
     class to use when it is necessary to be able to hold a value of
     mode MODE in a reload register for which class CLASS would
     ordinarily be used.

     Unlike `PREFERRED_RELOAD_CLASS', this macro should be used when
     there are certain modes that simply can't go in certain reload
     classes.

     The value is a register class; perhaps CLASS, or perhaps another,
     smaller class.

     Don't define this macro unless the target machine has limitations
     which require the macro to do something nontrivial.

`SECONDARY_RELOAD_CLASS (CLASS, MODE, X)'
`SECONDARY_INPUT_RELOAD_CLASS (CLASS, MODE, X)'
`SECONDARY_OUTPUT_RELOAD_CLASS (CLASS, MODE, X)'
     Many machines have some registers that cannot be copied directly
     to or from memory or even from other types of registers.  An
     example is the `MQ' register, which on most machines, can only be
     copied to or from general registers, but not memory.  Some
     machines allow copying all registers to and from memory, but
     require a scratch register for stores to some memory locations
     (e.g., those with symbolic address on the RT, and those with
     certain symbolic address on the Sparc when compiling PIC).  In
     some cases, both an intermediate and a scratch register are
     required.

     You should define these macros to indicate to the reload phase
     that it may need to allocate at least one register for a reload in
     addition to the register to contain the data.  Specifically, if
     copying X to a register CLASS in MODE requires an intermediate
     register, you should define `SECONDARY_INPUT_RELOAD_CLASS' to
     return the largest register class all of whose registers can be
     used as intermediate registers or scratch registers.

     If copying a register CLASS in MODE to X requires an intermediate
     or scratch register, `SECONDARY_OUTPUT_RELOAD_CLASS' should be
     defined to return the largest register class required.  If the
     requirements for input and output reloads are the same, the macro
     `SECONDARY_RELOAD_CLASS' should be used instead of defining both
     macros identically.

     The values returned by these macros are often `GENERAL_REGS'.
     Return `NO_REGS' if no spare register is needed; i.e., if X can be
     directly copied to or from a register of CLASS in MODE without
     requiring a scratch register.  Do not define this macro if it
     would always return `NO_REGS'.

     If a scratch register is required (either with or without an
     intermediate register), you should define patterns for
     `reload_inM' or `reload_outM', as required (*note Standard
     Names::.  These patterns, which will normally be implemented with
     a `define_expand', should be similar to the `movM' patterns,
     except that operand 2 is the scratch register.

     Define constraints for the reload register and scratch register
     that contain a single register class.  If the original reload
     register (whose class is CLASS) can meet the constraint given in
     the pattern, the value returned by these macros is used for the
     class of the scratch register.  Otherwise, two additional reload
     registers are required.  Their classes are obtained from the
     constraints in the insn pattern.

     X might be a pseudo-register or a `subreg' of a pseudo-register,
     which could either be in a hard register or in memory.  Use
     `true_regnum' to find out; it will return -1 if the pseudo is in
     memory and the hard register number if it is in a register.

     These macros should not be used in the case where a particular
     class of registers can only be copied to memory and not to another
     class of registers.  In that case, secondary reload registers are
     not needed and would not be helpful.  Instead, a stack location
     must be used to perform the copy and the `movM' pattern should use
     memory as an intermediate storage.  This case often occurs between
     floating-point and general registers.

`SECONDARY_MEMORY_NEEDED (CLASS1, CLASS2, M)'
     Certain machines have the property that some registers cannot be
     copied to some other registers without using memory.  Define this
     macro on those machines to be a C expression that is nonzero if
     objects of mode M in registers of CLASS1 can only be copied to
     registers of class CLASS2 by storing a register of CLASS1 into
     memory and loading that memory location into a register of CLASS2.

     Do not define this macro if its value would always be zero.

`SECONDARY_MEMORY_NEEDED_RTX (MODE)'
     Normally when `SECONDARY_MEMORY_NEEDED' is defined, the compiler
     allocates a stack slot for a memory location needed for register
     copies.  If this macro is defined, the compiler instead uses the
     memory location defined by this macro.

     Do not define this macro if you do not define
     `SECONDARY_MEMORY_NEEDED'.

`SECONDARY_MEMORY_NEEDED_MODE (MODE)'
     When the compiler needs a secondary memory location to copy
     between two registers of mode MODE, it normally allocates
     sufficient memory to hold a quantity of `BITS_PER_WORD' bits and
     performs the store and load operations in a mode that many bits
     wide and whose class is the same as that of MODE.

     This is right thing to do on most machines because it ensures that
     all bits of the register are copied and prevents accesses to the
     registers in a narrower mode, which some machines prohibit for
     floating-point registers.

     However, this default behavior is not correct on some machines,
     such as the DEC Alpha, that store short integers in floating-point
     registers differently than in integer registers.  On those
     machines, the default widening will not work correctly and you
     must define this macro to suppress that widening in some cases.
     See the file `alpha.h' for details.

     Do not define this macro if you do not define
     `SECONDARY_MEMORY_NEEDED' or if widening MODE to a mode that is
     `BITS_PER_WORD' bits wide is correct for your machine.

`SMALL_REGISTER_CLASSES'
     On some machines, it is risky to let hard registers live across
     arbitrary insns.  Typically, these machines have instructions that
     require values to be in specific registers (like an accumulator),
     and reload will fail if the required hard register is used for
     another purpose across such an insn.

     Define `SMALL_REGISTER_CLASSES' to be an expression with a nonzero
     value on these machines.  When this macro has a nonzero value, the
     compiler will try to minimize the lifetime of hard registers.

     It is always safe to define this macro with a nonzero value, but
     if you unnecessarily define it, you will reduce the amount of
     optimizations that can be performed in some cases.  If you do not
     define this macro with a nonzero value when it is required, the
     compiler will run out of spill registers and print a fatal error
     message.  For most machines, you should not define this macro at
     all.

`CLASS_LIKELY_SPILLED_P (CLASS)'
     A C expression whose value is nonzero if pseudos that have been
     assigned to registers of class CLASS would likely be spilled
     because registers of CLASS are needed for spill registers.

     The default value of this macro returns 1 if CLASS has exactly one
     register and zero otherwise.  On most machines, this default
     should be used.  Only define this macro to some other expression
     if pseudos allocated by `local-alloc.c' end up in memory because
     their hard registers were needed for spill registers.  If this
     macro returns nonzero for those classes, those pseudos will only
     be allocated by `global.c', which knows how to reallocate the
     pseudo to another register.  If there would not be another
     register available for reallocation, you should not change the
     definition of this macro since the only effect of such a
     definition would be to slow down register allocation.

`CLASS_MAX_NREGS (CLASS, MODE)'
     A C expression for the maximum number of consecutive registers of
     class CLASS needed to hold a value of mode MODE.

     This is closely related to the macro `HARD_REGNO_NREGS'.  In fact,
     the value of the macro `CLASS_MAX_NREGS (CLASS, MODE)' should be
     the maximum value of `HARD_REGNO_NREGS (REGNO, MODE)' for all
     REGNO values in the class CLASS.

     This macro helps control the handling of multiple-word values in
     the reload pass.

`CLASS_CANNOT_CHANGE_MODE'
     If defined, a C expression for a class that contains registers for
     which the compiler may not change modes arbitrarily.

`CLASS_CANNOT_CHANGE_MODE_P(FROM, TO)'
     A C expression that is true if, for a register in
     `CLASS_CANNOT_CHANGE_MODE', the requested mode punning is invalid.

     For the example, loading 32-bit integer or floating-point objects
     into floating-point registers on the Alpha extends them to 64 bits.
     Therefore loading a 64-bit object and then storing it as a 32-bit
     object does not store the low-order 32 bits, as would be the case
     for a normal register.  Therefore, `alpha.h' defines
     `CLASS_CANNOT_CHANGE_MODE' as `FLOAT_REGS' and
     `CLASS_CANNOT_CHANGE_MODE_P' restricts mode changes to same-size
     modes.

     Compare this to IA-64, which extends floating-point values to 82
     bits, and stores 64-bit integers in a different format than 64-bit
     doubles.  Therefore `CLASS_CANNOT_CHANGE_MODE_P' is always true.

   Three other special macros describe which operands fit which
constraint letters.

`CONST_OK_FOR_LETTER_P (VALUE, C)'
     A C expression that defines the machine-dependent operand
     constraint letters (`I', `J', `K', ... `P') that specify
     particular ranges of integer values.  If C is one of those
     letters, the expression should check that VALUE, an integer, is in
     the appropriate range and return 1 if so, 0 otherwise.  If C is
     not one of those letters, the value should be 0 regardless of
     VALUE.

`CONST_DOUBLE_OK_FOR_LETTER_P (VALUE, C)'
     A C expression that defines the machine-dependent operand
     constraint letters that specify particular ranges of
     `const_double' values (`G' or `H').

     If C is one of those letters, the expression should check that
     VALUE, an RTX of code `const_double', is in the appropriate range
     and return 1 if so, 0 otherwise.  If C is not one of those
     letters, the value should be 0 regardless of VALUE.

     `const_double' is used for all floating-point constants and for
     `DImode' fixed-point constants.  A given letter can accept either
     or both kinds of values.  It can use `GET_MODE' to distinguish
     between these kinds.

`EXTRA_CONSTRAINT (VALUE, C)'
     A C expression that defines the optional machine-dependent
     constraint letters that can be used to segregate specific types of
     operands, usually memory references, for the target machine.  Any
     letter that is not elsewhere defined and not matched by
     `REG_CLASS_FROM_LETTER' may be used.  Normally this macro will not
     be defined.

     If it is required for a particular target machine, it should
     return 1 if VALUE corresponds to the operand type represented by
     the constraint letter C.  If C is not defined as an extra
     constraint, the value returned should be 0 regardless of VALUE.

     For example, on the ROMP, load instructions cannot have their
     output in r0 if the memory reference contains a symbolic address.
     Constraint letter `Q' is defined as representing a memory address
     that does _not_ contain a symbolic address.  An alternative is
     specified with a `Q' constraint on the input and `r' on the
     output.  The next alternative specifies `m' on the input and a
     register class that does not include r0 on the output.


File: gccint.info,  Node: Stack and Calling,  Next: Varargs,  Prev: Register Classes,  Up: Target Macros

10.10 Stack Layout and Calling Conventions
==========================================

This describes the stack layout and calling conventions.

* Menu:

* Frame Layout::
* Exception Handling::
* Stack Checking::
* Frame Registers::
* Elimination::
* Stack Arguments::
* Register Arguments::
* Scalar Return::
* Aggregate Return::
* Caller Saves::
* Function Entry::
* Profiling::
* Tail Calls::


File: gccint.info,  Node: Frame Layout,  Next: Exception Handling,  Up: Stack and Calling

10.10.1 Basic Stack Layout
--------------------------

Here is the basic stack layout.

`STACK_GROWS_DOWNWARD'
     Define this macro if pushing a word onto the stack moves the stack
     pointer to a smaller address.

     When we say, "define this macro if ...," it means that the
     compiler checks this macro only with `#ifdef' so the precise
     definition used does not matter.

`STACK_PUSH_CODE'
     This macro defines the operation used when something is pushed on
     the stack.  In RTL, a push operation will be `(set (mem
     (STACK_PUSH_CODE (reg sp))) ...)'

     The choices are `PRE_DEC', `POST_DEC', `PRE_INC', and `POST_INC'.
     Which of these is correct depends on the stack direction and on
     whether the stack pointer points to the last item on the stack or
     whether it points to the space for the next item on the stack.

     The default is `PRE_DEC' when `STACK_GROWS_DOWNWARD' is defined,
     which is almost always right, and `PRE_INC' otherwise, which is
     often wrong.

`FRAME_GROWS_DOWNWARD'
     Define this macro if the addresses of local variable slots are at
     negative offsets from the frame pointer.

`ARGS_GROW_DOWNWARD'
     Define this macro if successive arguments to a function occupy
     decreasing addresses on the stack.

`STARTING_FRAME_OFFSET'
     Offset from the frame pointer to the first local variable slot to
     be allocated.

     If `FRAME_GROWS_DOWNWARD', find the next slot's offset by
     subtracting the first slot's length from `STARTING_FRAME_OFFSET'.
     Otherwise, it is found by adding the length of the first slot to
     the value `STARTING_FRAME_OFFSET'.

`STACK_POINTER_OFFSET'
     Offset from the stack pointer register to the first location at
     which outgoing arguments are placed.  If not specified, the
     default value of zero is used.  This is the proper value for most
     machines.

     If `ARGS_GROW_DOWNWARD', this is the offset to the location above
     the first location at which outgoing arguments are placed.

`FIRST_PARM_OFFSET (FUNDECL)'
     Offset from the argument pointer register to the first argument's
     address.  On some machines it may depend on the data type of the
     function.

     If `ARGS_GROW_DOWNWARD', this is the offset to the location above
     the first argument's address.

`STACK_DYNAMIC_OFFSET (FUNDECL)'
     Offset from the stack pointer register to an item dynamically
     allocated on the stack, e.g., by `alloca'.

     The default value for this macro is `STACK_POINTER_OFFSET' plus the
     length of the outgoing arguments.  The default is correct for most
     machines.  See `function.c' for details.

`DYNAMIC_CHAIN_ADDRESS (FRAMEADDR)'
     A C expression whose value is RTL representing the address in a
     stack frame where the pointer to the caller's frame is stored.
     Assume that FRAMEADDR is an RTL expression for the address of the
     stack frame itself.

     If you don't define this macro, the default is to return the value
     of FRAMEADDR--that is, the stack frame address is also the address
     of the stack word that points to the previous frame.

`SETUP_FRAME_ADDRESSES'
     If defined, a C expression that produces the machine-specific code
     to setup the stack so that arbitrary frames can be accessed.  For
     example, on the Sparc, we must flush all of the register windows
     to the stack before we can access arbitrary stack frames.  You
     will seldom need to define this macro.

`BUILTIN_SETJMP_FRAME_VALUE'
     If defined, a C expression that contains an rtx that is used to
     store the address of the current frame into the built in `setjmp'
     buffer.  The default value, `virtual_stack_vars_rtx', is correct
     for most machines.  One reason you may need to define this macro
     is if `hard_frame_pointer_rtx' is the appropriate value on your
     machine.

`RETURN_ADDR_RTX (COUNT, FRAMEADDR)'
     A C expression whose value is RTL representing the value of the
     return address for the frame COUNT steps up from the current
     frame, after the prologue.  FRAMEADDR is the frame pointer of the
     COUNT frame, or the frame pointer of the COUNT - 1 frame if
     `RETURN_ADDR_IN_PREVIOUS_FRAME' is defined.

     The value of the expression must always be the correct address when
     COUNT is zero, but may be `NULL_RTX' if there is not way to
     determine the return address of other frames.

`RETURN_ADDR_IN_PREVIOUS_FRAME'
     Define this if the return address of a particular stack frame is
     accessed from the frame pointer of the previous stack frame.

`INCOMING_RETURN_ADDR_RTX'
     A C expression whose value is RTL representing the location of the
     incoming return address at the beginning of any function, before
     the prologue.  This RTL is either a `REG', indicating that the
     return value is saved in `REG', or a `MEM' representing a location
     in the stack.

     You only need to define this macro if you want to support call
     frame debugging information like that provided by DWARF 2.

     If this RTL is a `REG', you should also define
     `DWARF_FRAME_RETURN_COLUMN' to `DWARF_FRAME_REGNUM (REGNO)'.

`INCOMING_FRAME_SP_OFFSET'
     A C expression whose value is an integer giving the offset, in
     bytes, from the value of the stack pointer register to the top of
     the stack frame at the beginning of any function, before the
     prologue.  The top of the frame is defined to be the value of the
     stack pointer in the previous frame, just before the call
     instruction.

     You only need to define this macro if you want to support call
     frame debugging information like that provided by DWARF 2.

`ARG_POINTER_CFA_OFFSET (FUNDECL)'
     A C expression whose value is an integer giving the offset, in
     bytes, from the argument pointer to the canonical frame address
     (cfa).  The final value should coincide with that calculated by
     `INCOMING_FRAME_SP_OFFSET'.  Which is unfortunately not usable
     during virtual register instantiation.

     The default value for this macro is `FIRST_PARM_OFFSET (fundecl)',
     which is correct for most machines; in general, the arguments are
     found immediately before the stack frame.  Note that this is not
     the case on some targets that save registers into the caller's
     frame, such as SPARC and rs6000, and so such targets need to
     define this macro.

     You only need to define this macro if the default is incorrect,
     and you want to support call frame debugging information like that
     provided by DWARF 2.

`SMALL_STACK'
     Define this macro if the stack size for the target is very small.
     This has the effect of disabling gcc's built-in `alloca', though
     `__builtin_alloca' is not affected.

