[clang] [Clang] Remove 3-element vector load and store special handling (PR #104661)

[Matt Arsenault via cfe-commits]

================
@@ -2002,20 +2002,19 @@ llvm::Value *CodeGenFunction::EmitLoadOfScalar(Address Addr, bool Volatile,
       return EmitFromMemory(V, Ty);
     }
 
-    // Handle vectors of size 3 like size 4 for better performance.
-    const llvm::Type *EltTy = Addr.getElementType();
-    const auto *VTy = cast<llvm::FixedVectorType>(EltTy);
-
-    if (!CGM.getCodeGenOpts().PreserveVec3Type && VTy->getNumElements() == 3) {
-
-      llvm::VectorType *vec4Ty =
-          llvm::FixedVectorType::get(VTy->getElementType(), 4);
-      Address Cast = Addr.withElementType(vec4Ty);
-      // Now load value.
-      llvm::Value *V = Builder.CreateLoad(Cast, Volatile, "loadVec4");
-
-      // Shuffle vector to get vec3.
-      V = Builder.CreateShuffleVector(V, ArrayRef<int>{0, 1, 2}, "extractVec");
+    // Handles vectors of sizes that are likely to be expanded to a larger size
+    // to optimize performance.
+    auto *VTy = cast<llvm::FixedVectorType>(Addr.getElementType());
+    auto *NewVecTy =
+        CGM.getABIInfo().getOptimalVectorMemoryType(VTy, getLangOpts());
+
+    if (VTy != NewVecTy) {
+      Address Cast = Addr.withElementType(NewVecTy);
+      llvm::Value *V = Builder.CreateLoad(Cast, Volatile, "loadVecN");
+      unsigned OldNumElements = VTy->getNumElements();
+      SmallVector<int, 4> Mask(OldNumElements);
----------------
arsenm wrote:

```suggestion
      SmallVector<int, 16> Mask(OldNumElements);
```

https://github.com/llvm/llvm-project/pull/104661