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Optimize AtomicBool::fetch_nand
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@ -539,17 +539,21 @@ pub fn fetch_nand(&self, val: bool, order: Ordering) -> bool {
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// We can't use atomic_nand here because it can result in a bool with
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// an invalid value. This happens because the atomic operation is done
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// with an 8-bit integer internally, which would set the upper 7 bits.
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// So we just use a compare-exchange loop instead, which is what the
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// intrinsic actually expands to anyways on many platforms.
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let mut old = self.load(Relaxed);
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loop {
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let new = !(old && val);
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match self.compare_exchange_weak(old, new, order, Relaxed) {
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Ok(_) => break,
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Err(x) => old = x,
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// So we just use fetch_xor or compare_exchange instead.
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if val {
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// !(x & true) == !x
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// We must invert the bool.
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self.fetch_xor(true, order)
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} else {
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// !(x & false) == true
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// We must set the bool to true. Instead of delegating to swap or fetch_or, use
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// compare_exchange instead in order to avoid unnecessary writes to memory, which
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// might minimize cache-coherence traffic.
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match self.compare_exchange(false, true, order, Ordering::Relaxed) {
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Ok(_) => false,
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Err(_) => true,
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}
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}
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old
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}
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/// Logical "or" with a boolean value.
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@ -24,10 +24,23 @@ fn bool_() {
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#[test]
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fn bool_and() {
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let a = AtomicBool::new(true);
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assert_eq!(a.fetch_and(false, SeqCst),true);
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assert_eq!(a.fetch_and(false, SeqCst), true);
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assert_eq!(a.load(SeqCst),false);
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}
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#[test]
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fn bool_nand() {
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let a = AtomicBool::new(false);
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assert_eq!(a.fetch_nand(false, SeqCst), false);
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assert_eq!(a.load(SeqCst), true);
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assert_eq!(a.fetch_nand(false, SeqCst), true);
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assert_eq!(a.load(SeqCst), true);
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assert_eq!(a.fetch_nand(true, SeqCst), true);
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assert_eq!(a.load(SeqCst), false);
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assert_eq!(a.fetch_nand(true, SeqCst), false);
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assert_eq!(a.load(SeqCst), true);
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}
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#[test]
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fn uint_and() {
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let x = AtomicUsize::new(0xf731);
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