[clang] [libcxx] [clang] [libc++] fix _Atomic c11 compare exchange does not update expected results (PR #78707)

[via cfe-commits]

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@@ -411,7 +412,48 @@ static void emitAtomicCmpXchg(CodeGenFunction &CGF, AtomicExpr *E, bool IsWeak,
 
   CGF.Builder.SetInsertPoint(StoreExpectedBB);
   // Update the memory at Expected with Old's value.
-  CGF.Builder.CreateStore(Old, Val1);
+llvm::Type *ExpectedType = ExpectedResult.getElementType();
+const llvm::DataLayout &DL = CGF.CGM.getDataLayout();
+uint64_t ExpectedSizeInBytes = DL.getTypeStoreSize(ExpectedType);
+
+if (ExpectedSizeInBytes == Size) {
+  // Sizes match: store directly
+  CGF.Builder.CreateStore(Old, ExpectedResult);
+
+} else {
+  // store only the first ExpectedSizeInBytes bytes of Old
+  llvm::Type *OldType = Old->getType();
+
+  llvm::Align SrcAlignLLVM = DL.getABITypeAlign(OldType);
+  llvm::Align DstAlignLLVM = DL.getABITypeAlign(ExpectedType);
+
+  clang::CharUnits SrcAlign = clang::CharUnits::fromQuantity(SrcAlignLLVM.value());
+  clang::CharUnits DstAlign = clang::CharUnits::fromQuantity(DstAlignLLVM.value());
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huixie90 wrote:

Since `Old` is the result of the compare exchange. I assume it has the same alignment of the atomic address itself. so I used the `Ptr.getAlignment()` here. would that be fine?

https://github.com/llvm/llvm-project/pull/78707