InfoMagic Source Code 1993 July/THE_SOURCE_CODE_CD_ROM.iso/gnu/gccdist/gcc/gcc.info-8

GNU Info File | 1992-09-10 | 47.4 KB | 1,142 lines

This is Info file gcc.info, produced by Makeinfo-1.47 from the input
file gcc.texinfo.

   This file documents the use and the internals of the GNU compiler.

   Copyright (C) 1988, 1989, 1990 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

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   Permission is granted to copy and distribute translations of this
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File: gcc.info,  Node: Standard Names,  Next: Pattern Ordering,  Prev: Constraints,  Up: Machine Desc

Standard Names for Patterns Used in 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 in 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.  The effect on the rest of the register is undefined.

     This class of patterns is special in several ways.  First of all,
     each of these names *must* be defined, because there is no other
     way to copy a datum from one place to another.

     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.  Look in `spur.md' to see how
     the requirement can be satisfied.

     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.

     The constraints on a `moveM' must allow any hard register to be
     moved to any other hard register (provided that
     `HARD_REGNO_MODE_OK' permits mode M in both registers).

     It is obligatory to support floating point `moveM' 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 `moveM'
     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 `moveM' instructions must be designed to avoid
     ever trying to reload into a floating point register.

`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.

`movsi_unaligned'
     Like `movsi' except that the memory reference will not trap if
     loading or storing into unaligned memory.  You must define this
     pattern if you can pass structures or unions in registers when they
     have less than a full word of alignment.

`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', `umulM3', `divM3', `udivM3', `modM3', `umodM3', `andM3', `iorM3', `xorM3'
     Similar, for other arithmetic operations.

     There are special considerations for register classes for
     logical-and instructions, affecting also the macro
     `PREFERRED_RELOAD_CLASS'. They apply not only to the patterns with
     these standard names, but to any patterns that will match such an
     instruction.  *Note Register Classes::.

`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.

`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.

`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.  Operand 2 has mode
     `SImode', not mode M.

`ashrM3', `lshlM3', `lshrM3', `rotlM3', `rotrM3'
     Other shift and rotate instructions.

     Logical and arithmetic left shift are the same.  Machines that do
     not allow negative shift counts often have only one instruction for
     shifting left.  On such machines, you should define a pattern named
     `ashlM3' and leave `lshlM3' undefined.

     There are special considerations for register classes for shift
     instructions, affecting also the macro `PREFERRED_RELOAD_CLASS'.
     They apply not only to the patterns with these standard names, but
     to any patterns that will match such an instruction.  *Note
     Register Classes::.

`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.

`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.

`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 ...)))

     Each such definition in the machine description, for integer mode
     M, must have a corresponding `tstM' pattern, because optimization
     can simplify the compare into a test when operand 1 is zero.

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

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

`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. The fourth operand is the known shared alignment of the source
     and destination, in the form of a `const_int' rtx.

`cmpstrM'
     Block compare instruction, with operands like `movstrM' except
     that the two memory blocks are compared byte by byte in
     lexicographic order.  The effect of the instruction is to set the
     condition codes.

`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.

`truncMN'
     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.

`extendMN'
     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_extendMN'
     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
     `SImode'. Operand 1 may have mode `QImode' or `SImode'; often
     `SImode' is allowed only for registers.  Operands 2 and 3 must be
     valid for `SImode'.

     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 `SImode') 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 `QImode'
     or `SImode'; often `SImode' is allowed only for registers.
     Operands 1 and 2 must be valid for `SImode'.

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

`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 must be omitted from the machine description.  You
     must tell the compiler exactly which value is stored by defining
     the macro `STORE_FLAG_VALUE'.

`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.

`call'
     Subroutine call instruction returning no value.  Operand 0 is the
     function to call; operand 1 is the number of bytes of arguments
     pushed (in mode `SImode', except it is normally 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.

`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.

`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.

`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.

`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 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.)

       5. A label that precedes the table itself.

     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' is defined then the first operand
     is an offset that counts from the address of the table; otherwise,
     it is an absolute address to jump to.

     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.


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

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

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.  These patterns
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.

   The compiler does more with RTL than just create it from patterns
and recognize the patterns: it can perform arithmetic expression codes
when constant values for their operands can be determined.  As a result,
sometimes having one pattern can require other patterns.  For example,
the Vax has no `and' instruction, but it has `and not' instructions.
Here is the definition of one of them:

     (define_insn "andcbsi2"
       [(set (match_operand:SI 0 "general_operand" "")
             (and:SI (match_dup 0)
                     (not:SI (match_operand:SI
                               1 "general_operand" ""))))]
       ""
       "bicl2 %1,%0")

If operand 1 is an explicit integer constant, an instruction constructed
using that pattern can be simplified into an `and' like this:

     (set (reg:SI 41)
          (and:SI (reg:SI 41)
                  (const_int 0xffff7fff)))

(where the integer constant is the one's complement of what appeared in
the original instruction).

   To avoid a fatal error, the compiler must have a pattern that
recognizes such an instruction.  Here is what is used:

     (define_insn ""
       [(set (match_operand:SI 0 "general_operand" "")
             (and:SI (match_dup 0)
                     (match_operand:SI 1 "general_operand" "")))]
       "GET_CODE (operands[1]) == CONST_INT"
       "*
     { operands[1]
         = gen_rtx (CONST_INT, VOIDmode, ~INTVAL (operands[1]));
       return \"bicl2 %1,%0\";
     }")

Whereas a pattern to match a general `and' instruction is impossible to
support on the Vax, this pattern is possible because it matches only a
constant second argument: a special case that can be output as an `and
not' instruction.

   A "compare" instruction whose RTL looks like this:

     (set (cc0) (compare OPERAND (const_int 0)))

may be simplified by optimization into a "test" like this:

     (set (cc0) OPERAND)

So in the machine description, each "compare" pattern for an integer
mode must have a corresponding "test" pattern that will match the
result of such simplification.

   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.

   When an instruction has the constraint letter `o', the reload pass
may generate instructions which copy a nonoffsettable address into an
index register.  The idea is that the register can be used as a
replacement offsettable address.  In order for these generated
instructions to work, there must be patterns to copy any kind of valid
address into a register.

   Most older machine designs have "load address" instructions which do
just what is needed here.  Some RISC machines do not advertise such
instructions, but the possible addresses on these machines are very
limited, so it is easy to fake them.

   Auto-increment and auto-decrement addresses are an exception; there
need not be an instruction that can copy such an address into a
register, because reload handles these cases in a different manner.


File: gcc.info,  Node: Jump Patterns,  Next: Peephole Definitions,  Prev: Dependent Patterns,  Up: Machine Desc

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

   GNU CC 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_INSN (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 GNU CC always generates a pair of
consecutive RTL 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.


File: gcc.info,  Node: Peephole Definitions,  Next: Expander Definitions,  Prev: Jump Patterns,  Up: Machine Desc

Defining 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.

   A definition looks like this:

     (define_peephole
       [INSN-PATTERN-1
        INSN-PATTERN-2
        ...]
       "CONDITION"
       "TEMPLATE"
       "MACHINE-SPECIFIC INFO")

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' 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_INSN' to find the preceding insns (but be
careful to skip over any `note' insns that intervene).

   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_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: gcc.info,  Node: Expander Definitions,  Prev: Peephole Definitions,  Up: Machine Desc

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 just like the RTL template for a
     `define_peephole' in that it is a vector of RTL expressions each
     being one insn.

   * 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 GNU CC is run.  This is just like the condition of a
     `define_insn' that has a standard name.

   * 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 need 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.  GNU CC
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 operations
     (addition, multiplication, shifting, etc.).

     Do not emit any insns explicitly with `emit_insn' before failing.

   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_rtx (CONST_INT,
                                        VOIDmode, 65535)); ")

   *Note:* If the `define_expand' is used to serve a standard binary or
unary arithmetic 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'.


File: gcc.info,  Node: Machine Macros,  Next: Config,  Prev: Machine Desc,  Up: Top

Machine Description Macros
**************************

   The other half of the machine description is a C header file
conventionally given the name `tm-MACHINE.h'.  The file `tm.h' should
be a link to it.  The header file `config.h' includes `tm.h' and most
compiler source files include `config.h'.

* Menu:

* Run-time Target::     Defining `-m' options like `-m68000' and `-m68020'.
* Storage Layout::      Defining sizes and alignments of data types.
* Registers::           Naming and describing the hardware registers.
* Register Classes::    Defining the classes of hardware registers.
* Stack Layout::        Defining which way the stack grows and by how much.
* Library Calls::       Specifying how to call certain library routines.
* Addressing Modes::    Defining addressing modes valid for memory operands.
* Delayed Branch::      Do branches execute the following instruction?
* Condition Code::      Defining how insns update the condition code.
* Cross-compilation::   Handling floating point for cross-compilers.
* Misc::                Everything else.
* Assembler Format::    Defining how to write insns and pseudo-ops to output.


File: gcc.info,  Node: Run-time Target,  Next: Storage Layout,  Prev: Machine Macros,  Up: Machine Macros

Run-time Target Specification
=============================

`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 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 ANSI standard, so they are
     predefined regardless of whether `-ansi' 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.

`CPP_SPEC'
     A C string constant that tells the GNU CC driver program options to
     pass to CPP.  It can also specify how to translate options you
     give to GNU CC into options for GNU CC to pass to the CPP.

     Do not define this macro if it does not need to do anything.

`CC1_SPEC'
     A C string constant that tells the GNU CC driver program options to
     pass to CC1.  It can also specify how to translate options you
     give to GNU CC into options for GNU CC to pass to the CC1.

     Do not define this macro if it does not need to do anything.

`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'; for example:

          #define TARGET_68020 (target_flags & 1)

     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 `tm-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, and a number, which contains the bits to set in
     `target_flags'.  A negative number says to clear bits instead; the
     negative of the number is which bits to clear.  The actual option
     name is made by appending `-m' to the specified name.

     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", 1},      \
              { "68000", -1},     \
              { "", 1}}

`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.


File: gcc.info,  Node: Storage Layout,  Next: Registers,  Prev: Run-time Target,  Up: Machine Macros

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 if the most significant bit in a byte has the
     lowest number.  This means that bit-field instructions count from
     the most significant bit.  If the machine has no bit-field
     instructions, this macro is irrelevant.

     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 if the most significant byte in a word has the
     lowest number.

`WORDS_BIG_ENDIAN'
     Define this macro if, in a multiword object, the most significant
     word has the lowest number.

`BITS_PER_UNIT'
     Number of bits in an addressable storage unit (byte); normally 8.

`BITS_PER_WORD'
     Number of bits in a word; normally 32.

`UNITS_PER_WORD'
     Number of storage units in a word; normally 4.

`POINTER_SIZE'
     Width of a pointer, in bits.

`POINTER_BOUNDARY'
     Alignment required for pointers stored in memory, in bits.

`PARM_BOUNDARY'
     Normal alignment required for function parameters on the stack, in
     bits.  All stack parameters receive least this much alignment
     regardless of data type.  On most machines, this is the same as the
     size of an integer.

`MAX_PARM_BOUNDARY'
     Largest alignment required for any stack parameters, in bits.  If
     the data type of the parameter calls for more alignment than
     `PARM_BOUNDARY', then it is given extra padding up to this limit.

     Don't define this macro if it would be equal to `PARM_BOUNDARY';
     in other words, if the alignment of a stack parameter should not
     depend on its data type (as is the case on most machines).

`STACK_BOUNDARY'
     Define this macro if you wish to preserve a certain alignment for
     the stack pointer at all times.  The definition is a C expression
     for the desired alignment (measured in bits).

`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.

`CONSTANT_ALIGNMENT (CODE, TYPEALIGN)'
     A C expression to compute the alignment for a constant.  The
     argument TYPEALIGN is the alignment required for the constant's
     data type. CODE is the tree code of the constant itself.

     If this macro is not defined, the default is to use TYPEALIGN.  If
     you do define this macro, the value must be a multiple of
     TYPEALIGN.

     The purpose of defining this macro is usually to cause string
     constants to be word aligned so that `dhrystone' can be made to
     run faster.

`EMPTY_FIELD_BOUNDARY'
     Alignment in bits to be given to a structure bit field that
     follows an empty field such as `int : 0;'.

`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 if instructions will fail to work if given data not on
     the nominal alignment.  If instructions will merely go slower in
     that case, do not define this macro.

`PCC_BITFIELD_TYPE_MATTERS'
     Define this if you wish to imitate a certain bizarre behavior
     pattern of some instances of PCC: a bit field whose declared type
     is `int' has the same effect on the size and alignment of a
     structure as an actual `int' would have.

     If the macro is defined, then its definition should be a C
     expression; a nonzero value for the expression enables
     PCC-compatible behavior.

     Just what effect that is in GNU CC depends on other parameters,
     but on most machines it would force the structure's alignment and
     size to a multiple of 32 or `BIGGEST_ALIGNMENT' bits.

`MAX_FIXED_MODE_SIZE'
     An integer expression for 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.

`CHECK_FLOAT_VALUE (MODE, VALUE)'
     A C statement to validate the value VALUE (or 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 `SFmode' or `DFmode'.

     If VALUE is not valid, you should call `error' to print an error
     message 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.