We do it by using the sign bit of the atomic to indicate that there are no waiters waiting. That way, the counter only becomes a zero when all expected counters have counted down *and* there is at least one waiter. That means the countDown() code remains unchanged. On x86-64 and AArch64, there is no change in the number of instructions in the inline portion of wait() either. The non-inline portion uses a __atomic_and_fetch() instead of atomic_fetch_and() so compilers will generate LOCK AND for x86-64, LDCLR for AArch64 ARMv8.1, and AMOAND.W for RISC-V. This is more efficient than the Standard Libraries's current implementations, which use a separate, global atomic out of a pool, meaning that there could be a collision between two or more latches (or any other users std::atomic waiting, such as std::semaphore and std::barrier). Coupled with the fact that we futexWait() on the actual latch's address (something libc++ can't / won't do), this implementation should be overall much more efficient. Change-Id: Ib5ce7a497e034ebabb2cfffd1761b02a44d548d3 Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>