From 5c5a5182c94d07409fac8cb40b2cdab488c140ff Mon Sep 17 00:00:00 2001 From: Stjepan Glavina Date: Fri, 7 Apr 2017 17:28:55 +0200 Subject: Optimize AtomicBool::fetch_nand --- src/libcore/sync/atomic.rs | 22 +++++++++++++--------- 1 file changed, 13 insertions(+), 9 deletions(-) (limited to 'src/libcore/sync') diff --git a/src/libcore/sync/atomic.rs b/src/libcore/sync/atomic.rs index 2e1058bfc34..dd0069502de 100644 --- a/src/libcore/sync/atomic.rs +++ b/src/libcore/sync/atomic.rs @@ -539,17 +539,21 @@ impl AtomicBool { // We can't use atomic_nand here because it can result in a bool with // an invalid value. This happens because the atomic operation is done // with an 8-bit integer internally, which would set the upper 7 bits. - // So we just use a compare-exchange loop instead, which is what the - // intrinsic actually expands to anyways on many platforms. - let mut old = self.load(Relaxed); - loop { - let new = !(old && val); - match self.compare_exchange_weak(old, new, order, Relaxed) { - Ok(_) => break, - Err(x) => old = x, + // So we just use fetch_xor or compare_exchange instead. + if val { + // !(x & true) == !x + // We must invert the bool. + self.fetch_xor(true, order) + } else { + // !(x & false) == true + // We must set the bool to true. Instead of delegating to swap or fetch_or, use + // compare_exchange instead in order to avoid unnecessary writes to memory, which + // might minimize cache-coherence traffic. + match self.compare_exchange(false, true, order, Ordering::Relaxed) { + Ok(_) => false, + Err(_) => true, } } - old } /// Logical "or" with a boolean value. -- cgit 1.4.1-3-g733a5