commit-reach: early exit paint_down_to_common for single merge-base
Commits not in the commit-graph get GENERATION_NUMBER_INFINITY and sort to the top of the priority queue. After those, commits with finite generation numbers are popped in non-increasing order. When MERGE_BASE_FIND_ALL is not set the first doubly-painted commit with a finite generation is therefore a best merge-base: no commit still in the queue can be a descendant of it. Skip the expensive STALE drain in this case. Add MERGE_BASE_FIND_ALL to the merge_base_flags enum. Callers that need every merge-base (repo_get_merge_bases_many, repo_get_merge_bases, repo_in_merge_bases_many, remove_redundant_no_gen) pass the flag to preserve existing behavior. git merge-base (without --all) passes 0, triggering the early exit. On a 2.2M-commit merge-heavy monorepo with commit-graph: HEAD vs ~500: 5,229ms -> 24ms HEAD vs ~1000: 4,214ms -> 39ms HEAD vs ~5000: 3,799ms -> 46ms HEAD vs ~10000: 3,827ms -> 61ms Signed-off-by: Kristofer Karlsson <krka@spotify.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Kristofer Karlsson committed
May 11, 2026 at 12:59 UTC
93e5b1680e30650ceca889a56429a055f17033bd
4 files changed
+63
-6
builtin/merge-base.c
+2
-1
@@ -11,11 +11,12 @@
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static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
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{
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+ enum merge_base_flags flags = show_all ? MERGE_BASE_FIND_ALL : 0;
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struct commit_list *result = NULL, *r;
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if (repo_get_merge_bases_many_dirty(the_repository, rev[0],
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rev_nr - 1, rev + 1,
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- 0, &result) < 0) {
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+ flags, &result) < 0) {
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commit_list_free(result);
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return -1;
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}
commit-reach.c
+15
-4
@@ -97,6 +97,14 @@ static int paint_down_to_common(struct repository *r,
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if (!(commit->object.flags & RESULT)) {
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commit->object.flags |= RESULT;
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tail = commit_list_append(commit, tail);
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+ /*
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+ * The queue is generation-ordered; no
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+ * remaining common ancestor can be a
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+ * descendant of this one.
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+ */
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+ if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
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+ generation < GENERATION_NUMBER_INFINITY)
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+ break;
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}
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/* Mark parents of a found merge stale */
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flags |= STALE;
@@ -247,7 +255,8 @@ static int remove_redundant_no_gen(struct repository *r,
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min_generation = curr_generation;
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}
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if (paint_down_to_common(r, array[i], filled,
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- work, min_generation, 0, &common)) {
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+ work, min_generation,
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+ MERGE_BASE_FIND_ALL, &common)) {
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clear_commit_marks(array[i], all_flags);
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clear_commit_marks_many(filled, work, all_flags);
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commit_list_free(common);
@@ -477,7 +486,8 @@ int repo_get_merge_bases_many(struct repository *r,
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struct commit **twos,
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struct commit_list **result)
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{
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- return get_merge_bases_many_0(r, one, n, twos, 1, 0, result);
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+ return get_merge_bases_many_0(r, one, n, twos, 1,
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+ MERGE_BASE_FIND_ALL, result);
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}
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int repo_get_merge_bases_many_dirty(struct repository *r,
@@ -495,7 +505,8 @@ int repo_get_merge_bases(struct repository *r,
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struct commit *two,
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struct commit_list **result)
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{
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- return get_merge_bases_many_0(r, one, 1, &two, 1, 0, result);
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+ return get_merge_bases_many_0(r, one, 1, &two, 1,
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+ MERGE_BASE_FIND_ALL, result);
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}
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/*
@@ -540,7 +551,7 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
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struct commit_list *bases = NULL;
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int ret = 0, i;
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timestamp_t generation, max_generation = GENERATION_NUMBER_ZERO;
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- enum merge_base_flags mb_flags = 0;
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+ enum merge_base_flags mb_flags = MERGE_BASE_FIND_ALL;
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if (ignore_missing_commits)
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mb_flags |= MERGE_BASE_IGNORE_MISSING_COMMITS;
commit-reach.h
+6
-1
@@ -19,9 +19,14 @@ int repo_get_merge_bases_many(struct repository *r,
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struct commit_list **result);
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enum merge_base_flags {
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MERGE_BASE_IGNORE_MISSING_COMMITS = (1 << 0),
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+ MERGE_BASE_FIND_ALL = (1 << 1),
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};
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-/* To be used only when object flags after this call no longer matter */
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+/*
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+ * To be used only when object flags after this call no longer matter.
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+ * Without MERGE_BASE_FIND_ALL and with generation numbers available,
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+ * returns after finding the first merge-base, skipping the STALE drain.
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+ */
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int repo_get_merge_bases_many_dirty(struct repository *r,
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struct commit *one, size_t n,
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struct commit **twos,
t/t6600-test-reach.sh
+40
@@ -882,4 +882,44 @@ test_expect_success 'rev-list --maximal-only matches merge-base --independent' '
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test_cmp expect.sorted actual.sorted
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'
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+# The following tests verify the early-exit optimisation in
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+# paint_down_to_common when merge-base is invoked without --all.
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+# Each test checks all four commit-graph configurations.
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+
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+merge_base_all_modes () {
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+ test_when_finished rm -rf .git/objects/info/commit-graph &&
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+ git merge-base "$@" >actual &&
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+ test_cmp expect actual &&
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+ cp commit-graph-full .git/objects/info/commit-graph &&
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+ git merge-base "$@" >actual &&
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+ test_cmp expect actual &&
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+ cp commit-graph-half .git/objects/info/commit-graph &&
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+ git merge-base "$@" >actual &&
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+ test_cmp expect actual &&
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+ cp commit-graph-no-gdat .git/objects/info/commit-graph &&
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+ git merge-base "$@" >actual &&
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+ test_cmp expect actual
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+}
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+
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+test_expect_success 'merge-base without --all (unique base)' '
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+ git rev-parse commit-5-3 >expect &&
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+ merge_base_all_modes commit-5-7 commit-8-3
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+'
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+
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+test_expect_success 'merge-base without --all is one of --all results' '
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+ test_when_finished rm -rf .git/objects/info/commit-graph &&
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+
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+ cp commit-graph-full .git/objects/info/commit-graph &&
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+ git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
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+ git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
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+ test_line_count = 1 single &&
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+ grep -F -f single all &&
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+
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+ cp commit-graph-half .git/objects/info/commit-graph &&
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+ git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
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+ git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
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+ test_line_count = 1 single &&
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+ grep -F -f single all
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+'
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+
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test_done