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INFO-DIR-SECTION Programming
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* gcc: (gcc).                  The GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the use and the internals of the GNU compiler.


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

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

   For most machines, 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_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 GNU CC 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: gcc.info,  Node: Looping Patterns,  Next: Insn Canonicalizations,  Prev: Jump Patterns,  Up: Machine Desc

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.

   GNU CC has three special named patterns to support low overhead
looping, `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 optimiser 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 initialisation
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: gcc.info,  Node: Insn Canonicalizations,  Next: Expander Definitions,  Prev: Looping Patterns,  Up: Machine Desc

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: gcc.info,  Node: Expander Definitions,  Next: Insn Splitting,  Prev: Insn Canonicalizations,  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 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 GNU CC 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.  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 (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: gcc.info,  Node: Insn Splitting,  Next: Peephole Definitions,  Prev: Expander Definitions,  Up: Machine Desc

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 non-zero, 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 non-zero, 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.

   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: gcc.info,  Node: Peephole Definitions,  Next: Insn Attributes,  Prev: Insn Splitting,  Up: Machine Desc

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: gcc.info,  Node: define_peephole,  Next: define_peephole2,  Up: Peephole Definitions

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: gcc.info,  Node: define_peephole2,  Prev: define_peephole,  Up: Peephole Definitions

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

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: gcc.info,  Node: Defining Attributes,  Next: Expressions,  Up: Insn Attributes

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'.

