17.3 Example of define_insn

Here is an example of an instruction pattern, taken from the machine description 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 describes an instruction which sets the condition codes based on the value of a general operand. It has no condition, so any insn with an RTL description of the form shown may be matched 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.