[Bug middle-end/102566] [i386] GCC should emit LOCK BTS for simple bit-test-and-set operations with std::atomic

[marko.makela at mariadb dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102566

--- Comment #33 from Marko Mäkelä <marko.makela at mariadb dot com> ---
When it comes to toggling the most significant bit, std::atomic::fetch_xor()
could be translated to LOCK XADD which would be able to return all bits:

#include <atomic>
uint32_t toggle_by_add(std::atomic<uint32_t>& a)
{
  return a.fetch_add(1U<<31);
}
uint32_t toggle_by_xor(std::atomic<uint32_t>& a)
{
  return a.fetch_xor(1U<<31);
}