From: Tamir Duberstein Date: Wed, 10 Jun 2026 12:20:49 GMT Subject: Re: [PATCH v2 2/2] ref-filter: memoize --contains with generations Message-ID: In-Reply-To: On Wed, Jun 10, 2026 at 4:47 AM Karthik Nayak wrote: > > Tamir Duberstein writes: > > > git branch and git for-each-ref call repo_is_descendant_of() for > > each candidate selected by --contains or --no-contains. Each call > > starts a new graph walk, so refs with shared history repeatedly > > traverse the same commits. > > > > ffc4b8012d (tag: speed up --contains calculation, 2011-06-11) > > introduced a depth-first walk for git tag that caches positive and > > negative answers across candidates. ee2bd06b0f (ref-filter: implement > > '--contains' option, 2015-07-07) preserved both implementations when > > ref-filter learned --contains. > > > > The memoized walk is not always faster. Without generation numbers, > > a negative check can walk to the root even when the breadth-first > > merge-base walk finds a nearby divergence. With generation numbers, > > the depth-first walk can stop below the oldest target while still > > reusing answers across candidates. > > > > Keep the existing memoized selection for git tag. Select it for other > > ref-filter callers when generation numbers are enabled, and retain > > the breadth-first walk otherwise. > > > > When generation numbers are unavailable, repo_is_descendant_of() can > > return -1 if ancestry cannot be read. The ref-filter Boolean interface > > treated that error as a match. Check it and exit instead. The memoized > > path already dies on the same parse failure, so both selected paths now > > fail rather than return a result. > > > > Add p1500 cases for up to 8,192 packed refs along one first-parent > > history and for sibling refs near the tip with generation numbers > > forced off. > > > > On a checkout with 62,174 remote-tracking refs and generation numbers > > enabled, I ran: > > > > hyperfine --warmup 0 --runs 3 \ > > --command-name parent \ > > '"$parent" branch -r --contains c78ae85f3ce7e >/dev/null' \ > > --command-name this-commit \ > > '"$this" branch -r --contains c78ae85f3ce7e >/dev/null' > > > > The results were: > > > > parent this commit > > elapsed 104.365 s 467.7 ms > > user 93.702 s 220.2 ms > > system 0.723 s 182.7 ms > > > > The wall-time standard deviations were 11.356 seconds and 133.8 > > milliseconds, respectively. Separate runs without redirection produced > > the same output with SHA-256 > > 2466f6e2b72aa16b1a2126eddb81c8a1b2764ee251204ac034c191a925aa896f. > > > > Both revisions were built with the default -O2 flags using Apple > > clang 21.0.0 on macOS 26.5. The machine was a MacBook Pro (Mac16,6) > > with a 16-core Apple M4 Max (12 performance and four efficiency > > cores) and 128 GB RAM. > > > > Link: https://lore.kernel.org/git/1445163904-24611-1-git-send-email-Karthik.188@gmail.com/ > > Link: https://lore.kernel.org/r/20230324191009.GA536967@coredump.intra.peff.net > > Link: https://lore.kernel.org/git/20260527070510.3510836-1-krka@spotify.com/ > > Link: https://lore.kernel.org/r/20260608223430.GA340696@coredump.intra.peff.net > > Suggested-by: Jeff King > > Signed-off-by: Tamir Duberstein > > --- > > commit-reach.c | 13 +++++++++-- > > commit-reach.h | 7 ++++++ > > t/perf/p1500-graph-walks.sh | 49 +++++++++++++++++++++++++++++++++++++++++- > > t/t6301-for-each-ref-errors.sh | 22 +++++++++++++++++++ > > 4 files changed, 88 insertions(+), 3 deletions(-) > > > > diff --git a/commit-reach.c b/commit-reach.c > > index 65b618959b..83a48004ef 100644 > > --- a/commit-reach.c > > +++ b/commit-reach.c > > @@ -821,9 +821,18 @@ static enum contains_result contains_tag_algo(struct commit *candidate, > > int commit_contains(struct ref_filter *filter, struct commit *commit, > > struct commit_list *list, struct contains_cache *cache) > > { > > - if (filter->with_commit_tag_algo) > > + int result; > > + > > + if (!list) > > + return 1; > > + if (filter->with_commit_tag_algo || > > + generation_numbers_enabled(the_repository)) > > What's stopping us from dropping `filter->with_commit_tag_algo` > completely and then doing? > > if (generation_numbers_enabled(the_repository)) > return contains_algo(commit, list, cache) == CONTAINS_YES; > return repo_is_descendant_of(the_repository, commit, list); Jeff raised this distinction during the v1 review: https://lore.kernel.org/r/20260608223430.GA340696@coredump.intra.peff.net/ `with_commit_tag_algo` preserves the existing behavior of `git tag` when generation numbers are unavailable. `git tag --contains` has used the memoized walk since ffc4b8012d (tag: speed up --contains calculation, 2011-06-11). Dropping the flag would send it back through repeated `repo_is_descendant_of()` walks in repositories without usable generation numbers. The condition in v2 implements the rule discussed there: retain the existing memoized path for `git tag`, and use it for other ref-filter callers when generation numbers make the depth-first walk reliably advantageous. This is probably my fault for breaking the threading between this and v1. Sorry about that.