GNU Compiler Collection (GCC) Internals: define_peephole

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#### 17.18.1 RTL to Text Peephole Optimizers

A definition looks like this:

    (define_peephole
      [insn-pattern-1
       insn-pattern-2
       …]
      "condition"
      "template"
      "optional-insn-attributes")

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The effect of this optimization is to change

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

into

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

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

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

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

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

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