| Issue |
132998
|
| Summary |
ABI break for `_Atomic` in C++23 when using libstdc++
|
| Labels |
ABI,
c++23,
libstdc++,
diverges-from:gcc
|
| Assignees |
|
| Reporter |
zygoloid
|
In C++23 onwards, [`<stdatomic.h>` `#define`s `_Atomic(T)` as `std::atomic<T>`](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0943r6.html). This results in an ABI break when using libstdc++. Example:
```c++
#include <stdatomic.h>
struct A { int m, n; char c; };
static_assert(sizeof(_Atomic(A)) == 16);
```
When using Clang with libstdc++, [the built-in `_Atomic(A)` has size 16 but `std::atomic<A>` has size 12](https://godbolt.org/z/Y9G7sYcsE) on for example x86_64-linux-gnu. That means that including the `<stdatomic.h>` header in C++23, or switching from C++20 to C++23 when already including `<stdatomic.h>`, changes the ABI for the type that is spelled `_Atomic(A)`.
In some sense this is the same issue as #26836, but we're now seeing different ABI behavior with two Clang builds depending on a `-std=` flag / a `#include`, which seems worse than before.
![]()
_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
