Assembler - GNU Compiler Collection (GCC) Internals
Description: GNU Compiler Collection (GCC) Internals

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### 10.17 Assembler Instructions as Expressions

The RTX code `asm_operands` represents a value produced by a user-specified assembler instruction. It is used to represent an `asm` statement with arguments. An `asm` statement with a single output operand, like this:

         asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z));

is represented using a single `asm_operands` RTX which represents the value that is stored in `outputvar`:

         (set rtx-for-outputvar
              (asm_operands "foo %1,%2,%0" "a" 0
                            [rtx-for-addition-result rtx-for-*z]
                            [(asm_input:m1 "g")
                             (asm_input:m2 "di")]))

Here the operands of the `asm_operands` RTX are the assembler template string, the output-operand's constraint, the index-number of the output operand among the output operands specified, a vector of input operand RTX's, and a vector of input-operand modes and constraints. The mode m1 is the mode of the sum `x+y`; m2 is that of `*z`.

When an `asm` statement has multiple output values, its insn has several such `set` RTX's inside of a `parallel`. Each `set` contains an `asm_operands`; all of these share the same assembler template and vectors, but each contains the constraint for the respective output operand. They are also distinguished by the output-operand index number, which is 0, 1, ... for successive output operands.