Auto merge of #128234 - jcsp:retain-empty-case, r=tgross35 · patricklam/verify-rust-std@f70ce7f (original) (raw)

Skip to content

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Sign up

Appearance settings

Commit f70ce7f

Auto merge of rust-lang#128234 - jcsp:retain-empty-case, r=tgross35

Optimize empty case in Vec::retain While profiling some code that happens to call Vec::retain() in a tight loop, I noticed more runtime than expected in retain, even in a bench case where the vector was always empty. When I wrapped my call to retain in `if !myvec.is_empty()` I saw faster execution compared with doing retain on an empty vector. On closer inspection, Vec::retain is doing set_len(0) on itself even when the vector is empty, and then resetting the length again in BackshiftOnDrop::drop. Unscientific screengrab of a flamegraph illustrating how we end up spending time in set_len and drop: ![image](https://github.com/user-attachments/assets/ebc72ace-84a0-4432-9b6f-1b3c96d353ba)

File tree

1 file changed

lines changed

1 file changed

lines changed

Lines changed: 6 additions & 0 deletions

Original file line number Diff line number Diff line change
@@ -1710,6 +1710,12 @@ impl<T, A: Allocator> Vec<T, A> {
1710 1710 F: FnMut(&mut T) -> bool,
1711 1711 {
1712 1712 let original_len = self.len();
1713 +
1714 +if original_len == 0 {
1715 +// Empty case: explicit return allows better optimization, vs letting compiler infer it
1716 +return;
1717 +}
1718 +
1713 1719 // Avoid double drop if the drop guard is not executed,
1714 1720 // since we may make some holes during the process.
1715 1721 unsafe { self.set_len(0) };