[llvm-bugs] [Bug 26462] New: GCC/clang C11 _Atomic incompatibility

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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: unassignedclangbugs at nondot.org
          Reporter: jyknight at google.com
                CC: llvm-bugs at 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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