From 7b9f64475ed51fbf8221ecb8c7f4fc9e71fa1d53 Mon Sep 17 00:00:00 2001 From: Mikail Bagishov Date: Sun, 8 Jan 2023 22:53:51 +0300 Subject: [PATCH] Change memory ordering in System wrapper example Currently, the `SeqCst` ordering is used, which seems unnecessary: + Even `Relaxed` ordering guarantees that all updates are atomic and are executed in total order + User code only reads atomic for monitoring purposes, no "happens-before" relationships with actual allocations and deallocations are needed for this If argumentation above is correct, I propose changing ordering to `Relaxed` to clarify that no synchronization is required here, and improve performance (if somebody copy-pastes this example into their code). --- library/std/src/alloc.rs | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/library/std/src/alloc.rs b/library/std/src/alloc.rs index c5a5991cc81c4..ec774e62debbf 100644 --- a/library/std/src/alloc.rs +++ b/library/std/src/alloc.rs @@ -93,7 +93,7 @@ pub use alloc_crate::alloc::*; /// /// ```rust /// use std::alloc::{System, GlobalAlloc, Layout}; -/// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst}; +/// use std::sync::atomic::{AtomicUsize, Ordering::Relaxed}; /// /// struct Counter; /// @@ -103,14 +103,14 @@ pub use alloc_crate::alloc::*; /// unsafe fn alloc(&self, layout: Layout) -> *mut u8 { /// let ret = System.alloc(layout); /// if !ret.is_null() { -/// ALLOCATED.fetch_add(layout.size(), SeqCst); +/// ALLOCATED.fetch_add(layout.size(), Relaxed); /// } /// ret /// } /// /// unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) { /// System.dealloc(ptr, layout); -/// ALLOCATED.fetch_sub(layout.size(), SeqCst); +/// ALLOCATED.fetch_sub(layout.size(), Relaxed); /// } /// } /// @@ -118,7 +118,7 @@ pub use alloc_crate::alloc::*; /// static A: Counter = Counter; /// /// fn main() { -/// println!("allocated bytes before main: {}", ALLOCATED.load(SeqCst)); +/// println!("allocated bytes before main: {}", ALLOCATED.load(Relaxed)); /// } /// ``` ///