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Hint the maximum length permitted by invariant of slices #77023

Merged
merged 1 commit into from
Oct 4, 2020
Merged

Hint the maximum length permitted by invariant of slices #77023

merged 1 commit into from
Oct 4, 2020

Commits on Oct 4, 2020

  1. Assume slice len is bounded by allocation size 

    Uses assume to check the length against a constant upper bound. The
inlined result then informs the optimizer of the sound value range.

This was tried with unreachable_unchecked before which introduces a
branch. This has the advantage of not being executed in sound code but
complicates basic blocks. It resulted in ~2% increased compile time in
some worst cases.

Add a codegen test for the assumption, testing the issue from rust-lang#67186
    @HeroicKatora
    HeroicKatora committed Oct 4, 2020
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