'OSMemoryBarrier' is deprecated in macOS 10.12

[khaledhosny]

Building with latest XCode I got lots of these warnings:

warning: 'OSMemoryBarrier' is deprecated: first deprecated in macOS 10.12 - Use std::atomic_thread_fence() from <atomic> instead [-Wdeprecated-declarations]
warning: 'OSAtomicAdd32Barrier' is deprecated: first deprecated in macOS 10.12 - Use std::atomic_fetch_add() from <atomic> instead [-Wdeprecated-declarations]
warning: 'OSAtomicCompareAndSwap64Barrier' is deprecated: first deprecated in macOS 10.12 - Use std::atomic_compare_exchange_strong() from <atomic> instead [-Wdeprecated-declarations]

[behdad]

Feel free to patch conditionally. Thanks.

[behdad]

Maybe we should support C++11 and use atomic on all platforms?

[khaledhosny]

But this would make us dependent on the C++ standard library on all platforms. C11 has a similar C API, may be we can use it if supported by platform/compiler and fallback to existing code otherwise?

[behdad]

But this would make us dependent on the C++ standard library on all platforms.

Right. Not sure if that's still a concern.

C11 has a similar C API, may be we can use it if supported by platform/compiler and fallback to existing code otherwise?

Whatever works. I do prefer not changing this much as it seems to have been working.

[behdad]

What's suggested work item here?

[khaledhosny]

Someone familiar with the new C/C++ atomics adds support for them. It should be e.g. used by default when supported and else fall back to the existing implementations. I might give it a try at some point, unless some one beat me to it.

[behdad]

There's an alternative approach in https://bugs.freedesktop.org/show_bug.cgi?id=90305

Let's give that one a try? I'll find some time to look into #486 as well.

[behdad]

Also, to make TSan happy, we should replace get_unsafe to do an atomic fetch with memory_order_relaxed ordering instead of being a non-atomic read.

[waywardmonkeys]

My changes in #676 remove this warning by using the compiler primitives rather than the macOS APIs.