[clang] [clang] Avoid memcopy for small structure with padding under -ftrivial-auto-var-init (PR #71677)

[via cfe-commits]

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@@ -1244,29 +1244,28 @@ static void emitStoresForConstant(CodeGenModule &CGM, const VarDecl &D,
   // If the initializer is small, use a handful of stores.
   if (shouldSplitConstantStore(CGM, ConstantSize)) {
     if (auto *STy = dyn_cast<llvm::StructType>(Ty)) {
-      // FIXME: handle the case when STy != Loc.getElementType().
-      if (STy == Loc.getElementType()) {
-        for (unsigned i = 0; i != constant->getNumOperands(); i++) {
-          Address EltPtr = Builder.CreateStructGEP(Loc, i);
-          emitStoresForConstant(
-              CGM, D, EltPtr, isVolatile, Builder,
-              cast<llvm::Constant>(Builder.CreateExtractValue(constant, i)),
-              IsAutoInit);
-        }
-        return;
+      const llvm::StructLayout *Layout =
+          CGM.getDataLayout().getStructLayout(STy);
+      for (unsigned i = 0; i != constant->getNumOperands(); i++) {
+        CharUnits CurOff = CharUnits::fromQuantity(Layout->getElementOffset(i));
+        Address EltPtr = Builder.CreateConstInBoundsByteGEP(
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serge-sans-paille wrote:

We cannot do that because we want to access padding, something `CreateStructGEP` cannot do.

https://github.com/llvm/llvm-project/pull/71677