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define_insnHere is an actual example of an instruction pattern, for the 68000/68020.
(define_insn "tstsi"
[(set (cc0)
(match_operand:SI 0 "general_operand" "rm"))]
""
"*
{
if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
return \"tstl %0\";
return \"cmpl #0,%0\";
}")
This can also be written using braced strings:
(define_insn "tstsi"
[(set (cc0)
(match_operand:SI 0 "general_operand" "rm"))]
""
{
if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
return "tstl %0";
return "cmpl #0,%0";
})
This is an instruction that sets the condition codes based on the value of
a general operand. It has no condition, so any insn whose RTL description
has the form shown may be handled according to this pattern. The name
`tstsi' means “test a SImode value” and tells the RTL generation
pass that, when it is necessary to test such a value, an insn to do so
can be constructed using this pattern.
The output control string is a piece of C code which chooses which output template to return based on the kind of operand and the specific type of CPU for which code is being generated.
`"rm"' is an operand constraint. Its meaning is explained below.