[llvm] [InstCombine] [X86] pblendvb intrinsics must be replaced by select when possible (PR #137322)

[Simon Pilgrim via llvm-commits]

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@@ -801,6 +802,71 @@ bool VectorCombine::foldInsExtBinop(Instruction &I) {
   return true;
 }
 
+bool VectorCombine::foldBitOpOfBitcasts(Instruction &I) {
+  // Match: bitop(bitcast(x), bitcast(y)) -> bitcast(bitop(x, y))
+  auto *BinOp = dyn_cast<BinaryOperator>(&I);
+  if (!BinOp || !BinOp->isBitwiseLogicOp())
+    return false;
+
+  Value *LHS = BinOp->getOperand(0);
+  Value *RHS = BinOp->getOperand(1);
+
+  // Both operands must be bitcasts
+  auto *LHSCast = dyn_cast<BitCastInst>(LHS);
+  auto *RHSCast = dyn_cast<BitCastInst>(RHS);
+  if (!LHSCast || !RHSCast)
+    return false;
+
+  Value *LHSSrc = LHSCast->getOperand(0);
+  Value *RHSSrc = RHSCast->getOperand(0);
+
+  // Source types must match
+  if (LHSSrc->getType() != RHSSrc->getType())
+    return false;
+
+  if (!LHSSrc->getType()->getScalarType()->isIntegerTy())
+    return false;
+
+  // Only handle vector types
+  auto *SrcVecTy = dyn_cast<FixedVectorType>(LHSSrc->getType());
+  auto *DstVecTy = dyn_cast<FixedVectorType>(I.getType());
+  if (!SrcVecTy || !DstVecTy)
+    return false;
+
+  // Same total bit width
+  assert(SrcVecTy->getPrimitiveSizeInBits() ==
+             DstVecTy->getPrimitiveSizeInBits() &&
+         "Bitcast should preserve total bit width");
+
+  // Cost Check :
+  // OldCost = bitlogic + 2*bitcasts
+  // NewCost = bitlogic + bitcast
+  auto OldCost =
----------------
RKSimon wrote:

(style) auto -> InstructionCost

https://github.com/llvm/llvm-project/pull/137322