### 16.15 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.