https://llvm.org/bugs/show_bug.cgi?id=26462
Bug ID: 26462
Summary: GCC/clang C11 _Atomic incompatibility
Product: clang
Version: unspecified
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Frontend
Assignee: unassignedclangb...@nondot.org
Reporter: jykni...@google.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Clang and GCC have incompatible ABIs for _Atomic, on non-power-of-2-sized
types.
Simple demonstration of the difference:
struct A3 { char val[3]; };
_Atomic struct A3 a3;
// GCC:
_Static_assert(sizeof(a3) == 3, "");
_Static_assert(_Alignof(a3) == 1, "");
// Clang:
_Static_assert(sizeof(a3) == 4, "");
_Static_assert(_Alignof(a3) == 4, "");
GCC's logic for _Atomic is: For types which have a size of exactly 1, 2, 4, 8,
or 16 bytes, increase the alignment to be at least the size. Never change the
size of the type.
libstdc++'s std::atomic uses the same logic as GCC, but it's implemented inline
in the header, as GCC doesn't support C11 atomics in C++ mode. Thus, libstdc++
under clang also uses GCC's rule.
Clang has the following rule: if the size of a type is less than a
target-specific variable "MaxAtomicPromoteWidth" (0, 4, 8, or 16 bytes on
current targets), round the size up to the next power of two, and SET the
alignment to the size.
libc++'s std::atomic uses clang's C11 atomics support (which clang supports as
an extension in C++ mode), and thus gets the same behavior...but only when
built with clang. When libc++ is built with GCC, it uses an alternative
implementation which doesn't ever increase the alignment/size.
So, the current situation:
- C11 _Atomic is incompatible between Clang and GCC.
- libstdc++'s std::atomic is compatible between Clang and GCC.
- libc++'s std::atomic is incompatible between Clang and GCC.
Furthermore, I believe C11 and C++ atomics are intended to be compatible with
eachother. And that's not true with clang and libstdc++, nor with gcc and
libc++.
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