gcc/doc/gccint/machine-descriptions/example-of-defineinsn.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: define_insn example
   7
   8 .. _example:
   9
  10 Example of define_insn
  11 **********************
  12
  13 Here is an example of an instruction pattern, taken from the machine
  14 description for the 68000/68020.
  15
  16 .. code-block::
  17
  18   (define_insn "tstsi"
  19     [(set (cc0)
  20           (match_operand:SI 0 "general_operand" "rm"))]
  21     ""
  22     "*
  23   {
  24     if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
  25       return \"tstl %0\";
  26     return \"cmpl #0,%0\";
  27   }")
  28
  29 This can also be written using braced strings:
  30
  31 .. code-block::
  32
  33   (define_insn "tstsi"
  34     [(set (cc0)
  35           (match_operand:SI 0 "general_operand" "rm"))]
  36     ""
  37   {
  38     if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
  39       return "tstl %0";
  40     return "cmpl #0,%0";
  41   })
  42
  43 This describes an instruction which sets the condition codes based on the
  44 value of a general operand.  It has no condition, so any insn with an RTL
  45 description of the form shown may be matched to this pattern.  The name
  46 :samp:`tstsi` means 'test a ``SImode`` value' and tells the RTL
  47 generation pass that, when it is necessary to test such a value, an insn
  48 to do so can be constructed using this pattern.
  49
  50 The output control string is a piece of C code which chooses which
  51 output template to return based on the kind of operand and the specific
  52 type of CPU for which code is being generated.
  53
  54 :samp:`"rm"` is an operand constraint.  Its meaning is explained below.