13.17 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.