16.3 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.