libcxx: add comment explaining why umtx is only used for 64bits
Requested by: imp Reviewed by: arichardson, emaste Sponsored by: The FreeBSD Foundation MFC after: 1 week Differential revision: https://reviews.freebsd.org/D38178
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@ -78,6 +78,11 @@ static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const vo
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}
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#elif defined(__FreeBSD__) && __SIZEOF_LONG__ == 8
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/*
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* Since __cxx_contention_t is int64_t even on 32bit FreeBSD
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* platforms, we have to use umtx ops that work on the long type, and
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* limit its use to architectures where long and int64_t are synonyms.
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*/
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static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
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__cxx_contention_t __val)
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