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

[Louis Dionne via cfe-commits]

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@@ -408,7 +409,15 @@ 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();
+  uint64_t OriginalSizeInBits = CGF.CGM.getDataLayout().getTypeSizeInBits(ExpectedType);
+  if (OriginalSizeInBits / 8 == Size) {
+    CGF.Builder.CreateStore(Old, ExpectedResult);
+  } else {
+    // How to just store N bytes to ExpectedResult ?
+    CGF.Builder.CreateStore(Old, ExpectedResult);
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ldionne wrote:

@bcardosolopes Hi Bruno! I'm chatting with @huixie90 right now and it seems like he needs to store an N bytes temporary object (that was used to perform the compare exchange above, but where only the first M bytes are used) into the actual result, which has exactly M bytes. We're novices in LLVM IR here -- can you suggest someone that would be able to answer a few questions like this one? Starting with you since Github suggested you as a reviewer for having modified these files recently :)

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