[llvm] [DirectX] Clean up extra vectors when lowering to buffer store (PR #116721)

[Justin Bogner via llvm-commits]

================
@@ -531,23 +531,46 @@ class OpLowerer {
         return make_error<StringError>(
             "typedBufferStore data must be a vector of 4 elements",
             inconvertibleErrorCode());
-      Value *Data0 =
-          IRB.CreateExtractElement(Data, ConstantInt::get(Int32Ty, 0));
-      Value *Data1 =
-          IRB.CreateExtractElement(Data, ConstantInt::get(Int32Ty, 1));
-      Value *Data2 =
-          IRB.CreateExtractElement(Data, ConstantInt::get(Int32Ty, 2));
-      Value *Data3 =
-          IRB.CreateExtractElement(Data, ConstantInt::get(Int32Ty, 3));
-
-      std::array<Value *, 8> Args{Handle, Index0, Index1, Data0,
-                                  Data1,  Data2,  Data3,  Mask};
+
+      // Since we're post-scalarizer, we likely have a vector that's constructed
+      // solely for the argument of the store.
+      std::array<Value *, 4> DataElements{nullptr, nullptr, nullptr, nullptr};
+      auto *IEI = dyn_cast<InsertElementInst>(Data);
+      while (IEI) {
+        auto *IndexOp = dyn_cast<ConstantInt>(IEI->getOperand(2));
+        if (!IndexOp)
+          break;
+        size_t IndexVal = IndexOp->getZExtValue();
+        assert(IndexVal < 4 && "Too many elements for buffer store");
----------------
bogner wrote:

I don't think so, this is fairly likely to crash in a non-asserts build if it comes up anyways, and this is a fairly obvious place we'd need to update if we were to make such a change. Note that we'd have to expand into multiple calls to `dx.op.BufferStore` here if we were to support larger vectors in HLSL, so this isn't just some implementation detail we'd be likely to forget about.

https://github.com/llvm/llvm-project/pull/116721