rust: Preallocate the returned `Vec` in `utils::files::relativize_path`
Profiling `rhg files > /dev/null` on an old snapshot of mozilla-central
(with `perf` and the Firefox Profiler:
https://github.com/firefox-devtools/profiler/blob/main/docs-user/guide-perf-profiling.md)
showed non-trivial time spend in this function and in `realloc`.
This change makes the wall-clock time for that process on my machine
go from ~190 ms to ~150 ms.
Differential Revision: https://phab.mercurial-scm.org/D10199
--- a/rust/hg-core/src/utils/files.rs Tue Dec 01 12:59:33 2020 -0500
+++ b/rust/hg-core/src/utils/files.rs Sat Mar 13 08:59:03 2021 +0100
@@ -290,7 +290,13 @@
if cwd.as_ref().is_empty() {
Cow::Borrowed(path.as_bytes())
} else {
- let mut res: Vec<u8> = Vec::new();
+ // This is not all accurate as to how large `res` will actually be, but
+ // profiling `rhg files` on a large-ish repo shows it’s better than
+ // starting from a zero-capacity `Vec` and letting `extend` reallocate
+ // repeatedly.
+ let guesstimate = path.as_bytes().len();
+
+ let mut res: Vec<u8> = Vec::with_capacity(guesstimate);
let mut path_iter = path.as_bytes().split(|b| *b == b'/').peekable();
let mut cwd_iter =
cwd.as_ref().as_bytes().split(|b| *b == b'/').peekable();