Despite my pathological distrust of spin locks, the number just don't lie. I've put a small spin in __sp_mut::lock() on std::mutex::try_lock(), which is testing quite well. In my experience, putting in a yield for every failed iteration is also a major performance booster. This change makes one of the performance tests I was using (a highly contended one) run about 20 times faster.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@160967 91177308-0d34-0410-b5e6-96231b3b80d8
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