[RISCV] Refactor vecPolicyOp skip logic in doPeepholeMaskedRVV. NFC

[luke]

Summary

We can just explicitly check if the new unmasked pseudo takes a policy
op, rather than implicitly relying on I->UnmaskedTUPseudo ==
I->UnmaskedPseudo. Split out from another patch to make the diff more
readable.

[frasercrmck]

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

3186

I know this is an intermediary patch but I was surprised to see a new check on RISCVII::hasVecPolicyOp(MaskedMCID.TSFlags) being introduced just 4 lines above an existing one. Could we move the setting of TailPolicyOpIdx into there?

[luke]

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

3186

Ah you’re right, that makes much more sense. Will do

[luke]

Move assignment into if

[reames]

LGTM - That really helps readability.

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

bool RISCVDAGToDAGISel::doPeepholeMaskedRVV(SDNode *N) {
 
   if (!usesAllOnesMask(N, MaskOpIdx))
     return false;
 
   // Retrieve the tail policy operand index, if any.
   std::optional<unsigned> TailPolicyOpIdx;
   const MCInstrDesc &MaskedMCID = TII->get(N->getMachineOpcode());
 
+  if (RISCVII::hasVecPolicyOp(MaskedMCID.TSFlags))
+    TailPolicyOpIdx = getVecPolicyOpIdx(N, MaskedMCID);
+
   bool UseTUPseudo = false;
   if (RISCVII::hasVecPolicyOp(MaskedMCID.TSFlags)) {
     // Some operations are their own TU.
     if (I->UnmaskedTUPseudo == I->UnmaskedPseudo) {
       UseTUPseudo = true;
     } else {
-      TailPolicyOpIdx = getVecPolicyOpIdx(N, MaskedMCID);
       if (!(N->getConstantOperandVal(*TailPolicyOpIdx) &
             RISCVII::TAIL_AGNOSTIC)) {
         // Keep the true-masked instruction when there is no unmasked TU
         // instruction
         if (I->UnmaskedTUPseudo == I->MaskedPseudo)
           return false;
         UseTUPseudo = true;
       }
     }
   }
 
   unsigned Opc = UseTUPseudo ? I->UnmaskedTUPseudo : I->UnmaskedPseudo;
+  const MCInstrDesc &MCID = TII->get(Opc);
 
   // If this instruction is tail agnostic, the unmasked instruction should not
   // have a tied destination.
 #ifndef NDEBUG
-  const MCInstrDesc &MCID = TII->get(Opc);
   bool HasTiedDest = RISCVII::isFirstDefTiedToFirstUse(MCID);
   assert((UseTUPseudo == HasTiedDest) && "Unexpected pseudo to transform to");
 #endif
 
   SmallVector<SDValue, 8> Ops;
   // Skip the merge operand at index 0 if !UseTUPseudo.
   for (unsigned I = !UseTUPseudo, E = N->getNumOperands(); I != E; I++) {
-    // Skip the mask, the policy, and the Glue.
+    // Skip the mask, the policy (if the unmasked doesn't have a policy op), and
+    // the Glue.
     SDValue Op = N->getOperand(I);
-    if (I == MaskOpIdx || I == TailPolicyOpIdx ||
+    if (I == MaskOpIdx ||
+        (I == TailPolicyOpIdx && !RISCVII::hasVecPolicyOp(MCID.TSFlags)) ||
         Op.getValueType() == MVT::Glue)
       continue;
     Ops.push_back(Op);
   }
 
   // Transitively apply any node glued to our new node.
   const auto *Glued = N->getGluedNode();
   if (auto *TGlued = Glued->getGluedNode())