QSemaphore: Make 64-bit wide

The implementation can be more efficient with two 32-bit fields instead
of just one, but it depends on whether operations on 64-bit atomics are
locked or not.

Quick check for the following architectures does not reveal any
problems, but someone needs to investigate more. Notably, GCC and Clang
generate a call to libatomic's __atomic_load_n / __atomic_store_n / etc.

32-bit                  std::atomic<uint64_t>
Arch            ::is_always_lock_free   .is_lock_free()
ARMv7-A         false                   true
i386            false                   true
MIPS            false                   true
PPC             false                   true
RISC-V          false                   true
SPARC           false                   true

Change-Id: Ia2273af1172d493092d6fffd163251a99064c51b
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

0 files changed, 0 insertions, 0 deletions