Uh oh!
There was an error while loading. Please reload this page.
Optimize Vec::shrink_to_fit() - #21401
Merged
Merged
Conversation
This uses `Vec::shrink_to_fit()` internally so it's really benchmarking that.
`Vec::shrink_to_fit()` may be called on vectors that are already the correct length. Calling out to `reallocate()` in this case is a bad idea because there is no guarantee that `reallocate()` won't allocate a new buffer anyway, and based on performance seen in external benchmarks, it seems likely that it is in fact reallocating a new buffer. Before: test string::tests::bench_exact_size_shrink_to_fit ... bench: 45 ns/iter (+/- 2) After: test string::tests::bench_exact_size_shrink_to_fit ... bench: 26 ns/iter (+/- 1)
rust-highfive
commented
Jan 19, 2015
Contributor
(rust_highfive has picked a reviewer for you, use r? to override) |
Gankra
commented
Jan 19, 2015
Contributor
How does this impact code that actually needs to shrink? |
Gankra
commented
Jan 19, 2015
Contributor
(I imagine the overhead is negligable if not free, but am still curious) |
lilyball
commented
Jan 21, 2015
ContributorAuthor
@gankro I would imagine it's negligible too. It's a single comparison, of values that are already being used anyway so no cache faults or anything like that. I would welcome some actual profiling but I doubt I can get any meaningful measurements. |
nikomatsakis
commented
Jan 26, 2015
Contributor
bors added a commit
that referenced
this pull request
Jan 26, 2015
Don't reallocate when capacity is already equal to length `Vec::shrink_to_fit()` may be called on vectors that are already the correct length. Calling out to `reallocate()` in this case is a bad idea because there is no guarantee that `reallocate()` won't allocate a new buffer anyway, and based on performance seen in external benchmarks, it seems likely that it is in fact reallocating a new buffer. Before: test string::tests::bench_exact_size_shrink_to_fit ... bench: 45 ns/iter (+/- 2) After: test string::tests::bench_exact_size_shrink_to_fit ... bench: 26 ns/iter (+/- 1)
bors
commented
Jan 26, 2015
Collaborator
bors
commented
Jan 26, 2015
Collaborator
Kobzol pushed a commit
to Kobzol/rust
that referenced
this pull request
Jan 5, 2026
Add a README.md to proc-macro-srv-cli
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for freeto join this conversation on GitHub.
Already have an account?
Sign in to comment
5 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Don't reallocate when capacity is already equal to length
Vec::shrink_to_fit()may be called on vectors that are already thecorrect length. Calling out to
reallocate()in this case is a bad ideabecause there is no guarantee that
reallocate()won't allocate a newbuffer anyway, and based on performance seen in external benchmarks, it
seems likely that it is in fact reallocating a new buffer.
Before:
After: