[llvm] [DirectX] Legalize memcpy (PR #139173)

[Finn Plummer via llvm-commits]

================
@@ -246,6 +247,59 @@ downcastI64toI32InsertExtractElements(Instruction &I,
   }
 }
 
+static void emitMemcpyExpansion(IRBuilder<> &Builder, Value *Dst, Value *Src,
+                                ConstantInt *Length) {
+
+  uint64_t ByteLength = Length->getZExtValue();
+  if (ByteLength == 0)
+    return;
+
+  LLVMContext &Ctx = Builder.getContext();
+  const DataLayout &DL = Builder.GetInsertBlock()->getModule()->getDataLayout();
+
+  auto GetArrTyFromVal = [](Value *Val) {
+    if (auto *Alloca = dyn_cast<AllocaInst>(Val))
+      return dyn_cast<ArrayType>(Alloca->getAllocatedType());
+    if (auto *GlobalVar = dyn_cast<GlobalVariable>(Val))
+      return dyn_cast<ArrayType>(GlobalVar->getValueType());
+    llvm_unreachable(
----------------
inbelic wrote:

nit: Maybe an assert is better? This code path will be triggered if say `Val` is a ptr from, say, the function args or a gep. So I do think it is reachable, just never expected to.

https://github.com/llvm/llvm-project/pull/139173