boost shared_ptr should detect __sync support in clang

[vlovich@…]

clang (at least as of 3.4) has support for GCC sync functions. shared_ptr should properly detect that clang has such support (at least on ARM such support is not detected properly).

[Peter Dimov]

Most versions of clang appear to provide a working <atomic>, so we'll probably use that instead of the obsolete !__sync intrinsics.

​https://github.com/boostorg/smart_ptr/commit/fed15ad8c50921cbfc0365156e4e4f4454dc1b91 allows you to turn std::atomic support on manually by #define BOOST_SP_USE_STD_ATOMIC. This will eventually become the default when std::atomic is autodetected to be present.

[Peter Dimov]

As it turns out, sp_has_sync.hpp should already detect the __sync support because Clang defines the macro __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4. What specific problem are you having?

[vlovich@…]

The version is the reported field is incorrect (apologies if I supplied it). This bug report was for 1.51. As you point out it's probably fixed in newer boosts.