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Remove unsafe in with_nix_path() for [u8] #1583

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merged 1 commit into from
Dec 21, 2021
Merged

Remove unsafe in with_nix_path() for [u8] #1583

merged 1 commit into from
Dec 21, 2021

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rtzoeller
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Remove use of unsafe in the implementation of with_nix_path() for [u8]. This also comes with a nice determinism win across input sizes, and is fairly performance neutral (slightly slower for small strings, much faster for large strings).

I suspect the performance degradation in the existing implementation is related to the following note in the CStr::from_ptr() documentation:

Note: This operation is intended to be a 0-cost cast but it is currently implemented with an up-front calculation of the length of the string. This is not guaranteed to always be the case.

Tested with cargo 1.57.0-nightly (7fbbf4e8f 2021-10-19), with variations of the following benchmarking code:

#[bench]
fn bench_with_nix_path_1024(b: &mut test::Bencher) {
    let bytes = std::hint::black_box([70u8; 1024]);
    b.iter(|| {
        bytes.with_nix_path(|cstr| {
            std::hint::black_box(&cstr);
        }).unwrap();
    })
}
Length Before Change After Change
16 37 ns/iter (+/- 0) 44 ns/iter (+/- 0)
64 39 ns/iter (+/- 0) 44 ns/iter (+/- 0)
256 84 ns/iter (+/- 0) 48 ns/iter (+/- 0)
1024 232 ns/iter (+/- 1) 50 ns/iter (+/- 1)
4095 796 ns/iter (+/- 8) 62 ns/iter (+/- 2)

@asomers asomers added this to the 0.24.0 milestone Nov 17, 2021
asomers
asomers approved these changes Nov 17, 2021
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Looks great!

@asomers
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asomers commented Dec 21, 2021

bors r+

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bors bot commented Dec 21, 2021

Build succeeded:

@bors bors bot merged commit 4f7119c into nix-rust:master Dec 21, 2021
@rtzoeller rtzoeller mentioned this pull request Feb 3, 2022
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@asomers asomers asomers approved these changes

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@rtzoeller @asomers