14.15 Defining RTL Sequences for Code Generation

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

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

A define_expand RTX has four operands:

• The name. Each define_expand must have a name, since the only use for it is to refer to it by name.
• The RTL template. This is a vector of RTL expressions representing a sequence of separate instructions. Unlike define_insn, there is no implicit surrounding PARALLEL.
• The condition, a string containing a C expression. This expression is used to express how the availability of this pattern depends on subclasses of target machine, selected by command-line options when GCC is run. This is just like the condition of a define_insn that has a standard name. Therefore, the condition (if present) may not depend on the data in the insn being matched, but only the target-machine-type flags. The compiler needs to test these conditions during initialization in order to learn exactly which named instructions are available in a particular run.
• The preparation statements, a string containing zero or more C statements which are to be executed before RTL code is generated from the RTL template.

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

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

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

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

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

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

DONE

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

FAIL

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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