From 4f3722051092bfe33ec87836e8dd4d6d155c1fc1 Mon Sep 17 00:00:00 2001 From: Thom Chiovoloni Date: Thu, 8 Oct 2020 17:04:32 -0700 Subject: [PATCH] Implement the same optimization in windows/time --- library/std/src/sys/windows/time.rs | 36 ++++++++++++++--------------- 1 file changed, 17 insertions(+), 19 deletions(-) diff --git a/library/std/src/sys/windows/time.rs b/library/std/src/sys/windows/time.rs index 900260169c767..91e4f7654840d 100644 --- a/library/std/src/sys/windows/time.rs +++ b/library/std/src/sys/windows/time.rs @@ -165,7 +165,7 @@ fn intervals2dur(intervals: u64) -> Duration { mod perf_counter { use super::NANOS_PER_SEC; - use crate::sync::atomic::{AtomicUsize, Ordering::SeqCst}; + use crate::sync::atomic::{AtomicU64, Ordering}; use crate::sys::c; use crate::sys::cvt; use crate::sys_common::mul_div_u64; @@ -197,27 +197,25 @@ mod perf_counter { } fn frequency() -> c::LARGE_INTEGER { - static mut FREQUENCY: c::LARGE_INTEGER = 0; - static STATE: AtomicUsize = AtomicUsize::new(0); - + // Either the cached result of `QueryPerformanceFrequency` or `0` for + // uninitialized. Storing this as a single `AtomicU64` allows us to use + // `Relaxed` operations, as we are only interested in the effects on a + // single memory location. + static FREQUENCY: AtomicU64 = AtomicU64::new(0); + + let cached = FREQUENCY.load(Ordering::Relaxed); + // If a previous thread has filled in this global state, use that. + if cached != 0 { + return cached as c::LARGE_INTEGER; + } + // ... otherwise learn for ourselves ... + let mut frequency = 0; unsafe { - // If a previous thread has filled in this global state, use that. - if STATE.load(SeqCst) == 2 { - return FREQUENCY; - } - - // ... otherwise learn for ourselves ... - let mut frequency = 0; cvt(c::QueryPerformanceFrequency(&mut frequency)).unwrap(); - - // ... and attempt to be the one thread that stores it globally for - // all other threads - if STATE.compare_exchange(0, 1, SeqCst, SeqCst).is_ok() { - FREQUENCY = frequency; - STATE.store(2, SeqCst); - } - frequency } + + FREQUENCY.store(frequency as u64, Ordering::Relaxed); + frequency } fn query() -> c::LARGE_INTEGER {