[Bug c++/94649] New: 16-byte aligned atomic_compare_exchange doesn not generate cmpxcg16b on x86_64

[avi@cloudius-systems.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94649

            Bug ID: 94649
           Summary: 16-byte aligned atomic_compare_exchange doesn not
                    generate cmpxcg16b on x86_64
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: avi@cloudius-systems.com
  Target Milestone: ---

The code

#include <atomic>

struct alignas(16) a {
    long x;
    long y;
};

bool cmpxchg(std::atomic<a>& data, a expected, a newval) {
    return std::atomic_compare_exchange_weak(&data, &expected, newval);
}

compiles with -O3 -mcx16 generates a "lock cmpxchg16b" instruction with clang,
but not with gcc, where it generates a library call to
__atomic_compare_exchange_16. This is a missed optimization.