[llvm] [DirectX] legalize memset (PR #136244)

[Justin Bogner via llvm-commits]

================
@@ -239,6 +247,72 @@ downcastI64toI32InsertExtractElements(Instruction &I,
   }
 }
 
+void emitMemsetExpansion(IRBuilder<> &Builder, Value *Dst, Value *Val,
+                         ConstantInt *SizeCI,
+                         DenseMap<Value *, Value *> &ReplacedValues) {
+  LLVMContext &Ctx = Builder.getContext();
+  [[maybe_unused]] DataLayout DL =
+      Builder.GetInsertBlock()->getModule()->getDataLayout();
+  [[maybe_unused]] uint64_t OrigSize = SizeCI->getZExtValue();
+
+  AllocaInst *Alloca = dyn_cast<AllocaInst>(Dst);
+
+  assert(Alloca && "Expected memset on an Alloca");
+  assert(OrigSize == Alloca->getAllocationSize(DL)->getFixedValue() &&
+         "Expected for memset size to match DataLayout size");
+
+  Type *AllocatedTy = Alloca->getAllocatedType();
+  ArrayType *ArrTy = dyn_cast<ArrayType>(AllocatedTy);
+  assert(ArrTy && "Expected Alloca for an Array Type");
+
+  Type *ElemTy = ArrTy->getElementType();
+  uint64_t Size = ArrTy->getArrayNumElements();
+
+  [[maybe_unused]] uint64_t ElemSize = DL.getTypeStoreSize(ElemTy);
+
+  assert(ElemSize > 0 && "Size must be set");
+  assert(OrigSize == ElemSize * Size && "Size in bytes must match");
+
+  Value *TypedVal = Val;
+
+  if (Val->getType() != ElemTy) {
+    // Note for i8 replacements if we know them we should use them.
+    // Further if this is a constant ReplacedValues will return null
+    // so we will stick to TypedVal = Val
+    if (ReplacedValues[Val])
+      TypedVal = ReplacedValues[Val];
+    // This case Val is a ConstantInt so the cast folds away.
+    // However if we don't do the cast the store below ends up being
+    // an i8.
+    else
+      TypedVal = Builder.CreateIntCast(Val, ElemTy, false);
----------------
bogner wrote:

I find the comment block before the `else` kind of confusing. Maybe use braces and move the comments into the two blocks?
```suggestion
    if (ReplacedValues[Val]) {
      // Note for i8 replacements if we know them we should use them. Further if
      // this is a constant ReplacedValues will return null so we will stick to
      // TypedVal = Val
      TypedVal = ReplacedValues[Val];
    } else {
      // This case Val is a ConstantInt so the cast folds away. However if we
      // don't do the cast the store below ends up being an i8.
      TypedVal = Builder.CreateIntCast(Val, ElemTy, false);
    }
```

https://github.com/llvm/llvm-project/pull/136244