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### 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.