This is not a joke: 3.7X speedup from removing a call to sleep

​https://bugs.webkit.org/show_bug.cgi?id=86702



Reviewed by Eric Seidel.



The speedup was on a GC benchmark, with a custom VM caching layer

not in TOT yet.



Instruments showed most GC threads spending the majority of their

time sleeping instead of doing useful work. Removing the call to

sleep sped up the benchmark.



wtf/TCSpinLock.h:

(TCMalloc_SlowLock): Since a spin lock is only ever held for a short

amount of time, don't sleep for a long amount of time waiting for it

to unlock -- yielding to the scheduler is sufficient.



If we find a case where someone is sitting on a spin lock for over 2ms,

we should fix the spin lock holder, not the spin lock.

