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Get rid of bounds check in slice::chunks_exact() and related function… #75936

Merged
merged 3 commits into from
Aug 31, 2020
Merged

Get rid of bounds check in slice::chunks_exact() and related function… #75936

merged 3 commits into from
Aug 31, 2020

Commits on Aug 30, 2020

  1. Add (non-public) slice::split_at_unchecked() and split_at_mut_uncheck… 

    …ed()

These are unsafe variants of the non-unchecked functions and don't do
any bounds checking.

For the time being these are not public and only a preparation for the
following commit. Making it public and stabilization can follow later
and be discussed in rust-lang#76014 .
    @sdroege
    sdroege committed Aug 30, 2020
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  2. Get rid of bounds check in slice::chunks_exact() and related function… 

    …s during construction

LLVM can't figure out in

    let rem = self.len() % chunk_size;
    let len = self.len() - rem;
    let (fst, snd) = self.split_at(len);

and

    let rem = self.len() % chunk_size;
    let (fst, snd) = self.split_at(rem);

that the index passed to split_at() is smaller than the slice length and
adds a bounds check plus panic for it.

Apart from removing the overhead of the bounds check this also allows
LLVM to optimize code around the ChunksExact iterator better.
    @sdroege
    sdroege committed Aug 30, 2020
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  3. Improve documentation of slice::get_unchecked() / split_at_unchecked() 

    Thanks to Ivan Tham, who gave the majority of these suggestions during
their review.
    @sdroege
    sdroege committed Aug 30, 2020
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