undefined | Better HN

0 pointsridiculous_fish3y ago0 comments

IME loops with induction variables (integer indexes) often produces better codegen than with iterators. Compare the these two Rust functions for inverting bits: https://rust.godbolt.org/z/cE4vPdbdY

This got improved in Rust 1.65 just this month, but the point stands.

edit: ARM64 compilation is even sillier. https://rust.godbolt.org/z/PEsbeGxWP

0 comments

akshaykarthik3y ago

Interestingly enough, with `-C opt-level=3` both functions yield the same assembly.

I wonder if there's some pass that's missing or not done at the lower opt level.

masklinn3y ago

Might be a problem of the number of pass repetitions, where O2 does not rerun the vectorisation after whatever manages to unroll everything but O3 does.

j / k navigate · click thread line to collapse