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IME loops with induction variables (integer indexes) often produces better codegen than with iterators. Compare the these two Rust functions for inverting bits:
by ridiculous_fish 4y ago
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 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 https://rust.godbolt.org/z/PEsbeGxWP
- akshaykarthik 4y agoInterestingly 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.
- masklinn 4y agoMight 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.