[llvm] [RISCV][TII] Add and use new hook to optimize/canonicalize instructions after MachineCopyPropagation (PR #137973)

[Philip Reames via llvm-commits]

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@@ -3850,6 +3865,207 @@ MachineInstr *RISCVInstrInfo::commuteInstructionImpl(MachineInstr &MI,
   return TargetInstrInfo::commuteInstructionImpl(MI, NewMI, OpIdx1, OpIdx2);
 }
 
+bool RISCVInstrInfo::optimizeInstruction(MachineInstr &MI) const {
+  switch (MI.getOpcode()) {
+  default:
+    break;
+  case RISCV::OR:
+  case RISCV::XOR:
+      // Normalize:
+      // [x]or rd, zero, rs => [x]or rd, rs, zero
+      if (MI.getOperand(1).getReg() == RISCV::X0) {
+        MachineOperand MO1 = MI.getOperand(1);
+        MI.removeOperand(1);
+        MI.addOperand(MO1);
+      }
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preames wrote:

You have "Normalize" as a comment, maybe "Normalize (so we hit the next if-clause)".  Or something along those lines.  I didn't spot the fallthrough on first glance, though I agree it's easier to see after the use of commuteInstruction.  

https://github.com/llvm/llvm-project/pull/137973