rust-lang / rust-lang/rust

possibly improve performance of SIMD candidate check in substring search

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#157,593 4 comments 6 reactions 1 assignee View on GitHub

@mazam-97 is already working on this.

Since Jun 8, 2026.

C-enhancement T-libs
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Description

I noticed that the candidate check for SIMD substring search

https://github.com/rust-lang/rust/blob/029c9e18dd1f4668e1d42bb187c1c263dfe20093/library/core/src/str/pattern.rs#L1843-L1855

is using small_slice_eq

https://github.com/rust-lang/rust/blob/029c9e18dd1f4668e1d42bb187c1c263dfe20093/library/core/src/str/pattern.rs#L1914-L1985

This was pulled from memchr a while back. But memchr got an improvement here a few years ago:

https://github.com/BurntSushi/memchr/commit/e77f0bf07ae3445d77776aabe233a9352dd753c5

std might consider adopting this. Note that memchr's infrastructure uses raw pointers everywhere for $reasons, so I'm unsure if that will matter since compilers are weird.

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