llvm dev - Oct 2016 - [Support/RWMutex] [RFC] Use std::atomic to replace platform-specific implementation

Hi,
The old llvm::sys::RWMutex use platform-specific lock mechanism to
implement read/write lock. For example, SRWLock on Windows and pthread
read/write lock on *nix systems.

However, in comparison with the approach that implement read/write lock
base on std::atomic, the old methods not only has potential performance
issue(Sorry I can't provide concrete evidence, any benchmark result is
welcome) but also non-standard.

Before std::shared_mutex in C++17 got fully adopt by LLVM codebase, I think
we can use std::atomic + std::conditional_variable approach like this:
https://gist.github.com/mshockwave/b314eb78d4019e7e106e705e864e0398
to power RWMutex.

-- 
Bekket McClane
Department of Computer Science,
National Tsing Hua University
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