This code perform atomic increment:

#![no_std] use core::sync::atomic::*; pub extern "C" fn atomic_usize_inc(ptr: &AtomicUsize) -> usize { let mut old = ptr.load(Ordering::Relaxed); loop { let new = old + 1; match ptr.compare_exchange_weak(old, new, Ordering::Relaxed, Ordering::Relaxed) { Ok(_old) => break _old, Err(_old) => old = _old, } } }

Expected asm:

example::atomic_usize_inc: 1:  ldr r1, [r0]  add r3, r1, #1  ldrex r2, [r0]  cmp r2, r1  itt ne  clrexne  bne 1b  strex r1, r3, [r0]  cbz r1, 2f  b 1b 2:  movs r0, r2

Produced asm godbolt:

example::atomic_usize_inc:  ldr r2, [r0] .LBB0_1:  ldrex r1, [r0]  cmp r1, r2  bne .LBB0_4  adds r2, #1  strex r3, r2, [r0]  cbnz r3, .LBB0_5  movs r2, #1  b .LBB0_6 .LBB0_4:  clrex .LBB0_5:  movs r2, #0 .LBB0_6:  cbnz r2, .LBB0_25  ldrex r2, [r0]  cmp r2, r1  bne .LBB0_10  adds r1, #1  strex r3, r1, [r0]  cbnz r3, .LBB0_11  movs r1, #1  b .LBB0_12 .LBB0_10:  clrex .LBB0_11:  movs r1, #0 .LBB0_12:  cbnz r1, .LBB0_24  ldrex r1, [r0]  cmp r1, r2  bne .LBB0_16  adds r2, #1  strex r3, r2, [r0]  cbnz r3, .LBB0_17  movs r2, #1  b .LBB0_18 .LBB0_16:  clrex .LBB0_17:  movs r2, #0 .LBB0_18:  cbnz r2, .LBB0_25  ldrex r2, [r0]  cmp r2, r1  bne .LBB0_22  adds r1, #1  strex r3, r1, [r0]  cbnz r3, .LBB0_23  movs r1, #1  cmp r1, #0  beq .LBB0_1  b .LBB0_24 .LBB0_22:  clrex .LBB0_23:  movs r1, #0  cmp r1, #0  beq .LBB0_1 .LBB0_24:  mov r1, r2 .LBB0_25:  mov r0, r1  bx lr

Code size is very important on Cortex-M targets (thumbv*) because some controllers have only 20KB flash!

Besides the increment instruction (adds r2, #1) was moved into ldrex/strex section. This code lost compare_exchange advantage: evaluation before ldrex to reduce tick count when Exclusive Monitor is set.

This code is useless because we can call fetch_add. But other tasks can requires compare_exchange_weak, for example atomic increment with max condition (pseudocode):

pub extern "C" fn atomic_usize_inc_with_max(ptr: &AtomicUsize) -> (bool, usize) { atomic { let old = ptr.load(Ordering::Relaxed); if old == max { return (false, old); } ptr.store(old + 1, Ordering::Relaxed); return (true, old); } }

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rustc --version --verbose:

rustc 1.50.0-nightly (1c389ffef 2020-11-24)