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Optimize Once::doit when initialization is already completed #13349

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Optimize Once::doit when initialization is already completed #13349

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10 changes: 10 additions & 0 deletions src/libsync/one.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -63,7 +63,17 @@ impl Once {
///
/// When this function returns, it is guaranteed that some initialization
/// has run and completed (it may not be the closure specified).
#[inline(always)]
pub fn doit(&self, f: ||) {
// Optimize common path: load is much cheaper than fetch_add.
if self.cnt.load(atomics::SeqCst) < 0 {
return
}

self.doit_slow(f);
}

fn doit_slow(&self, f: ||) {
// Implementation-wise, this would seem like a fairly trivial primitive.
// The stickler part is where our mutexes currently require an
// allocation, and usage of a `Once` should't leak this allocation.
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