From: Kristofer Karlsson Date: Wed, 24 Jun 2026 12:14:13 +0000 (+0000) Subject: commit-reach: terminate merge-base walk when one paint side is exhausted X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=290729c083392cf7e5ed79c9ab0b6b4435456fad;p=thirdparty%2Fgit.git commit-reach: terminate merge-base walk when one paint side is exhausted Add an early termination check to paint_down_to_common() using the per-side counters introduced earlier. Once the walk enters the finite-generation region, terminate early when one side's exclusive count drops to zero -- no new merge-base can form without both paint sides meeting. The check also waits for pending_merge_bases to reach zero, ensuring all merge-base candidates have been dequeued and recorded before exiting. The INFINITY gate ensures correctness: commits without a commit-graph entry have GENERATION_NUMBER_INFINITY and are ordered by commit date, which is not topologically reliable. The optimization only fires once the walk enters the finite-generation region where ordering guarantees hold. Step counts measured with trace2 on git.git with commit-graph: merge-base --all v2.0.0 v2.55.0-rc1: before: 72264 steps after: 44589 steps merge-base --all v2.55.0-rc1 v2.55.0-rc1~5: before: 110 steps after: 7 steps Helped-by: Derrick Stolee Helped-by: Elijah Newren Signed-off-by: Kristofer Karlsson Signed-off-by: Junio C Hamano --- diff --git a/Documentation/technical/paint-down-to-common.adoc b/Documentation/technical/paint-down-to-common.adoc index 0f4e1892a5..983dfcf233 100644 --- a/Documentation/technical/paint-down-to-common.adoc +++ b/Documentation/technical/paint-down-to-common.adoc @@ -94,6 +94,9 @@ ends when one of the following conditions holds: 1. The queue is empty. 2. The queue contains only stale entries. + 3. Side exhaustion: no pure PARENT1 or pure PARENT2 commits + remain in the queue, no pending merge-base candidates exist, + and the walk has entered the finite-generation region. Stale entry condition ~~~~~~~~~~~~~~~~~~~~~ @@ -104,6 +107,20 @@ existing candidates by proving one is an ancestor of another, but `remove_redundant()` handles that as a post-processing step, so it is safe to exit early. +Side-exhaustion condition +~~~~~~~~~~~~~~~~~~~~~~~~~ +A new merge-base requires commits from both sides to meet. When one +side's exclusive counter reaches zero and there are no pending +merge-base candidates, no future traversal step can produce a new +candidate. + +This optimization only activates in the finite-generation region +where topological ordering holds. In that region, children are +always visited before parents, so paint flags are final at visit +time and an exhausted side cannot reappear. In the INFINITY region, +commit-date ordering can violate this guarantee, so the check is +skipped. + Related documentation --------------------- diff --git a/commit-reach.c b/commit-reach.c index e0d9874f99..f79d0b64d6 100644 --- a/commit-reach.c +++ b/commit-reach.c @@ -133,17 +133,30 @@ static void paint_queue_put(struct paint_state *state, static struct commit *paint_queue_get(struct paint_state *state) { - struct commit *commit; + struct commit *commit = prio_queue_get(&state->queue); - if (!state->p1_count && !state->p2_count && - !state->pending_merge_bases) + if (!commit) return NULL; - commit = prio_queue_get(&state->queue); - if (commit) { - commit->object.flags &= ~ENQUEUED; - paint_count_update(state, commit->object.flags, -1); + commit->object.flags &= ~ENQUEUED; + + if (!state->pending_merge_bases) { + if (!state->p1_count && !state->p2_count) + return NULL; + /* + * Side exhaustion: a new merge-base can only form + * when both PARENT1-only and PARENT2-only commits + * remain in the queue. In the finite-generation + * region the queue is ordered topologically, so + * no future step can add paint to visited commits + * and an exhausted side cannot reappear. + */ + if ((!state->p1_count || !state->p2_count) && + commit_graph_generation(commit) < GENERATION_NUMBER_INFINITY) + return NULL; } + + paint_count_update(state, commit->object.flags, -1); return commit; } diff --git a/t/t6600-test-reach.sh b/t/t6600-test-reach.sh index c1109fb42f..03175befb3 100755 --- a/t/t6600-test-reach.sh +++ b/t/t6600-test-reach.sh @@ -332,12 +332,12 @@ test_expect_success 'merge-base --all commit-walk steps' ' cp commit-graph-full .git/objects/info/commit-graph && GIT_TRACE2_EVENT="$(pwd)/trace-full.txt" \ git merge-base --all commit-9-9 commit-9-1 >actual && - test_trace2_data paint_down_to_common steps 80 actual && - test_trace2_data paint_down_to_common steps 81