https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71660
--- Comment #8 from Thiago Macieira <thiago at kde dot org> --- (In reply to Peter Cordes from comment #7) > 8B alignment is required for 8B objects to be efficiently lock-free (using > SSE load / store for .load() and .store(), see > https://stackoverflow.com/questions/36624881/why-is-integer-assignment-on-a- > naturally-aligned-variable-atomic), and to avoid a factor of ~100 slowdown > if lock cmpxchg8b is split across a cache-line boundary. Unfortunately, the issue is not efficiency, but compatibility. The change broke ABI for roughly 50% of structs containing atomic<64bit>. I understand being fast, but not at the expense of silently breaking code at runtime. > alignof(long double) in 32-bit is different from alignof(long double) in > 64-bit. std::atomic<long double> or _Atomic long double should always have > the same alignment as long double. In and out of structs? That's the whole problem: inside structs, the alignment is 4 for historical reasons.
