Godbolt link
When a NonZeroUsize is created through new_unchecked and wrapped in a Some, the optimizer does not recognize that the value is always Some. This can be fixed by adding a std::intrinsics::assume(value != 0) line before creating the NonZero type.
use std::num::NonZeroUsize;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::Relaxed;
pub static X: AtomicUsize = AtomicUsize::new(1);
pub unsafe fn get() -> Option<NonZeroUsize> {
let x = X.load(Relaxed);
Some(NonZeroUsize::new_unchecked(x))
}
pub unsafe fn get2() -> usize {
match get() {
Some(x) => x.get(),
None => unreachable!(), // not optimized out
}
}Lots of panicking code gets generated in this example.
Fixed by:
#![feature(core_intrinsics)]
use std::num::NonZeroUsize;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::Relaxed;
pub static X: AtomicUsize = AtomicUsize::new(1);
pub unsafe fn get() -> Option<NonZeroUsize> {
let x = X.load(Relaxed);
std::intrinsics::assume(x != 0); // gets rid of the panicking code
Some(NonZeroUsize::new_unchecked(x))
}
pub unsafe fn get2() -> usize {
match get() {
Some(x) => x.get(),
None => unreachable!(), // optimized away
}
}I would guess that adding assume inside of new_unchecked would be ok, since it's already UB if it's not zero.
Related: #51346. Posted as a new issue, because it can be fixed without any LLVM pass changes.
Godbolt link
When a
NonZeroUsizeis created throughnew_uncheckedand wrapped in aSome, the optimizer does not recognize that the value is alwaysSome. This can be fixed by adding astd::intrinsics::assume(value != 0)line before creating the NonZero type.Lots of panicking code gets generated in this example.
Fixed by:
I would guess that adding
assumeinside ofnew_uncheckedwould be ok, since it's already UB if it's not zero.Related: #51346. Posted as a new issue, because it can be fixed without any LLVM pass changes.