commit-reach: add general graph traversal find_reachable()#2142
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Add find_reachable(), a generalization of the memoized DFS in can_all_from_reach_with_flag(). The benefits are two-fold: 1. make the code more uniform - fewer core graph traversals to maintain and reason about. 2. optimization for ref-filter --contains Most converted callers get equal or slightly better performance, since this standardizes on the generally "best" implementation. The big win is ref-filter --contains/--no-contains. Instead of calling commit_contains() for each ref, we batch it into batched calls to find_reachable(), changing the time complexity from O(N * D) to O(D + N) where D is the reachable graph depth and N is the number of refs. Signed-off-by: Kristofer Karlsson <krka@spotify.com>
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In 2018, Stolee consolidated commit walks into commit-reach.c and
extracted can_all_from_reach_with_flag() from upload-pack's
ok_to_give_up() with the observation that we can reuse its
commit walking logic for many other callers (ba3ca1e).
In 4fbcca4 it also got optimized with a memoized DFS so
subsequent from-commits benefit from shared ancestry
(very cool optimization!).
This patch continues that idea by generalizing the algorithm into
find_reachable() and rolling it out to the remaining callers. Most
conversions are just code reuse with preserved performance. The big
win is ref-filter branch --contains, where batching N per-ref DFS
walks into a single call with shared RESULT memoization gives
14.5x on gitgitgadget/git.
This makes can_all_from_reach(), contains_tag_algo and its
infrastructure redundant — all deleted. The contains_cache commit
slab is replaced by temporary flag bits on commit->object.flags.
Benchmarks on gitgitgadget/git (v2.48, ~85k commits, 370 branches,
730 tags), median of 5-10 sequential runs on a quiet machine:
The branch --contains speedup comes from the O(N*D)->O(D+N) batch
change. tag --contains is neutral because the old contains_tag_algo
already had per-commit slab caching. merge-base --is-ancestor is
neutral since the bottleneck is commit-graph object loading, not
the walk pattern.