Re: [PATCH v2 2/2] ref-filter: memoize --contains with generations

> 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 <peff@peff.net>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> ---
>  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))