Using and Porting GNU CC - Example

Example of define_insn

Here 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 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.