[llvm] [DirectX] replace byte splitting via vector bitcast with scalar (PR #140167)

[Finn Plummer via llvm-commits]

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@@ -419,40 +421,96 @@ static void updateFnegToFsub(Instruction &I,
   ToRemove.push_back(&I);
 }
 
+static void
+legalizeGetHighLowi64Bytes(Instruction &I,
+                           SmallVectorImpl<Instruction *> &ToRemove,
+                           DenseMap<Value *, Value *> &ReplacedValues) {
+  if (auto *BitCast = dyn_cast<BitCastInst>(&I)) {
+    if (BitCast->getDestTy() ==
+            FixedVectorType::get(Type::getInt32Ty(I.getContext()), 2) &&
+        BitCast->getSrcTy()->isIntegerTy(64)) {
+      ToRemove.push_back(BitCast);
+      ReplacedValues[BitCast] = BitCast->getOperand(0);
+      return;
+    }
+  }
+
+  if (auto *Extract = dyn_cast<ExtractElementInst>(&I)) {
+    auto *VecTy = dyn_cast<FixedVectorType>(Extract->getVectorOperandType());
+    if (VecTy && VecTy->getElementType()->isIntegerTy(32) &&
+        VecTy->getNumElements() == 2) {
+      if (auto *Index = dyn_cast<ConstantInt>(Extract->getIndexOperand())) {
+        unsigned Idx = Index->getZExtValue();
+        IRBuilder<> Builder(&I);
+        assert(dyn_cast<BitCastInst>(Extract->getVectorOperand()));
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inbelic wrote:

Should this be an if condition instead? It seems pretty easy to hit outside of this specific scenario

https://github.com/llvm/llvm-project/pull/140167