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Optimize Vec::insert for the case where index == len. #98755

Merged
merged 1 commit into from Jul 3, 2022
Merged

Optimize Vec::insert for the case where index == len. #98755

merged 1 commit into from Jul 3, 2022
+9 −6

Commits on Jun 30, 2022

  1. Optimize Vec::insert for the case where index == len. 

    By skipping the call to `copy` with a zero length. This makes it closer
to `push`.

I did this recently for `SmallVec`
(servo/rust-smallvec#282) and it was a big perf win in
one case. Although I don't have a specific use case in mind, it seems
worth doing it for `Vec` as well.

Things to note:
- In the `index < len` case, the number of conditions checked is
  unchanged.
- In the `index == len` case, the number of conditions checked increases
  by one, but the more expensive zero-length copy is avoided.
- In the `index > len` case the code now reserves space for the extra
  element before panicking. This seems like an unimportant change.
    @nnethercote
    nnethercote committed Jun 30, 2022
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