tsan: optimize DenseSlabAlloc
If lots of threads do lots of malloc/free and they overflow
per-pthread DenseSlabAlloc cache, it causes lots of contention:
31.97% race.old race.old [.] __sanitizer::StaticSpinMutex::LockSlow
17.61% race.old race.old [.] __tsan_read4
10.77% race.old race.old [.] __tsan::SlotLock
Optimize DenseSlabAlloc to use a lock-free stack of batches of nodes.
This way we don't take any locks in steady state at all and do only
1 push/pop per Refill/Drain.
Effect on the added benchmark:
$ TIME="%e %U %S %M" time ./test.old 36 5 2000000
34.51 978.22 175.67 5833592
32.53 891.73 167.03 5790036
36.17 1005.54 201.24 5802828
36.94 1004.76 226.58 5803188
$ TIME="%e %U %S %M" time ./test.new 36 5 2000000
26.44 720.99 13.45 5750704
25.92 721.98 13.58 5767764
26.33 725.15 13.41 5777936
25.93 713.49 13.41 5791796
Reviewed By: melver
Differential Revision: https://reviews.llvm.org/D130002