GlobalISel generic combiner example

a.cpp

// llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
bool CombinerHelper::matchBinOpSameVal(MachineInstr &MI) {
  return matchEqualDefs(MI.getOperand(1), MI.getOperand(2)) &&
         canReplaceReg(MI.getOperand(0).getReg(), MI.getOperand(1).getReg(),
                       MRI);
}
bool CombinerHelper::replaceSingleDefInstWithOperand(MachineInstr &MI,
                                                     unsigned OpIdx) {
  assert(MI.getNumExplicitDefs() == 1 && "Expected one explicit def?");
  Register OldReg = MI.getOperand(0).getReg();
  Register Replacement = MI.getOperand(OpIdx).getReg();
  assert(canReplaceReg(OldReg, Replacement, MRI) && "Cannot replace register?");
  MI.eraseFromParent();
  replaceRegWith(MRI, OldReg, Replacement);
  return true;
}

a.td

// llvm/include/llvm/Target/GlobalISel/Combine.td
// Fold (x op x) - > x
def binop_same_val: GICombineRule<
  (defs root:$root),
  (match (wip_match_opcode G_AND, G_OR):$root,
    [{ return Helper.matchBinOpSameVal(*${root}); }]),
  (apply [{ return Helper.replaceSingleDefInstWithOperand(*${root}, 1); }])
>;