Warning
Info
While benchmarking some code, I noticed that the performance of one benchmark was being affected by the other ones (when I commented out the other benchmarks, the first one became ~2x faster). I've minimized the code down to:
#![feature(test)]
extern crate test;
use test::Bencher;
#[bench]
fn bench(b: &mut Bencher) {
b.iter(|| {
[].is_ascii();
});
}
/*// `#[no_mangle]` to prevent this from being optimized out
#[no_mangle]
fn breaks_it(b: &mut Bencher) {
let bytes: &[u8] = &[];
test::bench::iter(&mut || {
bytes.is_ascii();
Vec::new().resize(0, ());
});
b.iter(|| {});
}*/
Running cargo bench on this gives a time of 0.23 ns/iter for bench. If you uncomment the function breaks_it, the time goes up to 1.60 ns/iter.
The time goes back to 0.23 ns/iter if you comment any part of breaks_it, so it seems that the performance difference only happens when breaks_it contains:
test::bench::iter, at least 1 directly, others can be through Bencher::iter.b, (apparently black_box(b) doesn't count, so it must be either b.iter() or b.bench())test::bench::iter, there's a call to Vec::resize, and a call to [u8]::is_ascii on a slice borrowed from outside the closure.The time also goes down to 0.23 ns/iter if I use the same closure for both calls to test::bench::iter, i.e.:
#[no_mangle]
fn breaks_it(b: &mut Bencher) {
let bytes: &[u8] = &[];
let mut inner = || {
bytes.is_ascii();
Vec::new().resize(0, ());
};
test::bench::iter(&mut inner);
b.iter(inner);
}
The time also goes back down if I wrap both closures in breaks_it or the closure in bench in Boxes, but goes back up if I wrap all of them.
The time also goes down if I inline any of the functions used (test::bench::iter, [u8]::is_ascii, Vec::resize), so perhaps them being in the standard library changes something.
Incidentally, 0.23 ns/iter is the same time I get benching an empty closure, so it looks like in that case, is_ascii is being optimized out.
This implies that there's something about these very specific conditions that prevents optimizations in a different function.
Does anyone know what could cause this spooky action?
My rust version is 1.89.0-nightly (64a124607 2025-05-30).
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