https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71660
--- Comment #7 from Peter Cordes <peter at cordes dot ca> --- C++11 std::atomic<> is correct, and the change was necessary. 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. What needs to change is the C11 stdatomic default alignment for 64-bit objects in and out of structs. (This includes _Atomic struct {int a,b;};) Currently, atomic_llong **is not atomic** in gcc, only in g++. I attached a testcase showing tearing to the still-unfixed C11 bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65146#c4). (It was filed at the same time as the C++11 bug that led to the change in std::atomic.) re: long double: it can't be lock-free in -m32. 10-byte x87 load / store instructions are not guaranteed to be atomic, and in fact even on real Intel CPUs are done as two separate load or store uops. 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.
