[RISCV] Commute True in foldVMergeToMask

In order to fold a vmerge into a pseudo, the pseudo's passthru needs to be the same as vmerge's false operand.

If they don't match we can try and commute the instruction if possible, e.g. here we can commute v9 and v8 to fold the vmerge:

    vsetvli zero, a0, e32, m1, ta, ma
    vfmadd.vv v9, v10, v8
    vsetvli zero, zero, e32, m1, tu, ma
    vmerge.vvm v8, v8, v9, v0

    vsetvli zero, a0, e32, m1, tu, mu
    vfmacc.vv v8, v9, v10, v0.t

Previously this wasn't possible because we did the peephole in SelectionDAG, but now that it's been migrated to MachineInstr in #144076 we can reuse the commuting infrastructure in TargetInstrInfo.

This fixes the extra vmv.v.v in the "mul" example here: #123069 (comment)

It should also allow us to remove the isel patterns described in #141885 later.

Author: lukel97

llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp

@@ -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;
+    if (!TII->findCommutedOpIndices(True, OpIdx1, OpIdx2))
+      return false;
+    NeedsCommute = {OpIdx1, OpIdx2};
+  }
 
   // Make sure it doesn't raise any observable fp exceptions, since changing the
   // active elements will affect how fflags is set.
@@ -796,6 +808,14 @@ bool RISCVVectorPeephole::foldVMergeToMask(MachineInstr &MI) const {
   if (!ensureDominates(MaskOp, True))
     return false;
 
+  if (NeedsCommute) {
+    auto [OpIdx1, OpIdx2] = *NeedsCommute;
+    [[maybe_unused]] bool Commuted =
+        TII->commuteInstruction(True, /*NewMI=*/false, OpIdx1, OpIdx2);
+    assert(Commuted && "Failed to commute True?");
+    Info = RISCV::lookupMaskedIntrinsicByUnmasked(True.getOpcode());
+  }
+
   True.setDesc(TII->get(Info->MaskedPseudo));
 
   // Insert the mask operand.

llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll

@@ -1219,10 +1219,8 @@ define <vscale x 2 x i32> @unfoldable_mismatched_sew(<vscale x 2 x i32> %passthr
 define <vscale x 2 x float> @commute_vfmadd(<vscale x 2 x float> %passthru, <vscale x 2 x float> %x, <vscale x 2 x float> %y, <vscale x 2 x i1> %mask, i32 zeroext %evl) {
 ; CHECK-LABEL: commute_vfmadd:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    vsetvli zero, a0, e32, m1, ta, ma
-; CHECK-NEXT:    vfmadd.vv v9, v10, v8
-; CHECK-NEXT:    vsetvli zero, zero, e32, m1, tu, ma
-; CHECK-NEXT:    vmerge.vvm v8, v8, v9, v0
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, tu, mu
+; CHECK-NEXT:    vfmacc.vv v8, v9, v10, v0.t
 ; CHECK-NEXT:    ret
   %fmul = fmul contract <vscale x 2 x float> %x, %y
   %fadd = fadd contract <vscale x 2 x float> %fmul, %passthru