[llvm] [RISCV] Commute True in foldVMergeToMask (PR #156499)

[Craig Topper via llvm-commits]

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@@ -745,12 +745,24 @@ bool RISCVVectorPeephole::foldVMergeToMask(MachineInstr &MI) const {
   if (PassthruReg && !isKnownSameDefs(PassthruReg, FalseReg))
     return false;
 
+  std::optional<std::pair<unsigned, unsigned>> NeedsCommute;
+
   // If True has a passthru operand then it needs to be the same as vmerge's
   // False, since False will be used for the result's passthru operand.
   Register TruePassthru = True.getOperand(True.getNumExplicitDefs()).getReg();
   if (RISCVII::isFirstDefTiedToFirstUse(True.getDesc()) && TruePassthru &&
-      !isKnownSameDefs(TruePassthru, FalseReg))
-    return false;
+      !isKnownSameDefs(TruePassthru, FalseReg)) {
+    // If True's passthru != False, check if it uses False in another operand
+    // and try to commute it.
+    int OtherIdx = True.findRegisterUseOperandIdx(FalseReg, TRI);
+    if (OtherIdx == -1)
+      return false;
+    unsigned OpIdx1 = OtherIdx;
+    unsigned OpIdx2 = TargetInstrInfo::CommuteAnyOperandIndex;
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topperc wrote:

Shouldn't OpIdx2 be the index of the passthru operand?

https://github.com/llvm/llvm-project/pull/156499