[llvm] [RISCV] Fold PseudoVMV_V_V with undef passthru, handling policy (PR #106943)

[Philip Reames via llvm-commits]

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@@ -472,6 +473,38 @@ bool RISCVVectorPeephole::ensureDominates(const MachineOperand &MO,
   return true;
 }
 
+/// If a PseudoVMV_V_V's passthru is undef then we can replace it with its input
+bool RISCVVectorPeephole::foldUndefPassthruVMV_V_V(MachineInstr &MI) {
+  if (RISCV::getRVVMCOpcode(MI.getOpcode()) != RISCV::VMV_V_V)
+    return false;
+  if (MI.getOperand(1).getReg() != RISCV::NoRegister)
+    return false;
+
+  // If the input was a pseudo with a policy operand, we can give it a tail
+  // agnostic policy if MI's undef tail subsumes the input's.
+  MachineInstr *Src = MRI->getVRegDef(MI.getOperand(2).getReg());
+  if (Src && !Src->hasUnmodeledSideEffects() &&
+      MRI->hasOneUse(MI.getOperand(2).getReg()) &&
+      RISCVII::hasVLOp(Src->getDesc().TSFlags) &&
+      RISCVII::hasVecPolicyOp(Src->getDesc().TSFlags) &&
+      getSEWLMULRatio(MI) == getSEWLMULRatio(*Src)) {
+    const MachineOperand &MIVL = MI.getOperand(3);
+    const MachineOperand &SrcVL =
+        Src->getOperand(RISCVII::getVLOpNum(Src->getDesc()));
+
+    MachineOperand &SrcPolicy =
+        Src->getOperand(RISCVII::getVecPolicyOpNum(Src->getDesc()));
+
+    if (isVLKnownLE(MIVL, SrcVL))
+      SrcPolicy.setImm(SrcPolicy.getImm() | RISCVII::TAIL_AGNOSTIC);
+  }
+
+  MRI->replaceRegWith(MI.getOperand(0).getReg(), Src->getOperand(0).getReg());
----------------
preames wrote:

Src may have multiple defs, use MI.getOperand(2).getReg() instead.

https://github.com/llvm/llvm-project/pull/106943