Refactor scope latches to reduce matching #1059
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The former `enum ScopeLatch` forced a `match` during both `increment` and `set` (decrement), even though both variants only need to update an `AtomicUsize` most of the time. rayon-rs#1057 helped hide that for `increment`, but `set` branching still showed up in perf profiles. Now this is refactored to a unified `CountLatch` that has a direct field for its `counter` used in the frequent case, and then an internal enum for the one-time notification variants. Therefore, most of its updates will have no `match` reached at all. The only other use of the former `CountLatch` was the one-shot termination latch in `WorkerThread`, so that's now renamed to `OnceLatch`.
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The former
enum ScopeLatchforced amatchduring bothincrementand
set(decrement), even though both variants only need to update anAtomicUsizemost of the time. #1057 helped hide that forincrement,but
setbranching still showed up in perf profiles.Now this is refactored to a unified
CountLatchthat has a direct fieldfor its
counterused in the frequent case, and then an internal enumfor the one-time notification variants. Therefore, most of its updates
will have no
matchreached at all.The only other use of the former
CountLatchwas the one-shottermination latch in
WorkerThread, so that's now renamed toOnceLatch.