From: Kristofer Karlsson via GitGitGadget Date: Wed, 01 Jul 2026 16:37:09 GMT Subject: [PATCH v5 08/10] commit-reach: terminate merge-base walk when one paint side is exhausted Message-ID: In-Reply-To: From: Kristofer Karlsson 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 --- .../technical/paint-down-to-common.adoc | 17 +++++++++++++++++ commit-reach.c | 18 +++++++++++++++--- t/t6600-test-reach.sh | 4 ++-- 3 files changed, 34 insertions(+), 5 deletions(-) diff --git a/Documentation/technical/paint-down-to-common.adoc b/Documentation/technical/paint-down-to-common.adoc index 74ba675643..8a8a7a930e 100644 --- a/Documentation/technical/paint-down-to-common.adoc +++ b/Documentation/technical/paint-down-to-common.adoc @@ -110,6 +110,9 @@ ends when one of the following conditions holds: 4. Single result: the caller only needs one merge base, one has been found, and the walk has entered the finite-generation region. + 5. 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 ~~~~~~~~~~~~~~~~~~~~~ @@ -120,6 +123,20 @@ 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. + Generation cutoff ~~~~~~~~~~~~~~~~~ Some callers (notably `remove_redundant()`) supply a `min_generation` diff --git a/commit-reach.c b/commit-reach.c index dbbbea641f..2b9249105c 100644 --- a/commit-reach.c +++ b/commit-reach.c @@ -132,6 +132,10 @@ static void paint_queue_put(struct paint_state *state, } } +/* + * Dequeue the next commit for the paint walk, or return NULL when + * no more merge bases can be discovered. + */ static struct commit *paint_queue_get(struct paint_state *state) { struct commit *commit = prio_queue_get(&state->queue); @@ -141,9 +145,17 @@ static struct commit *paint_queue_get(struct paint_state *state) commit->object.flags &= ~ENQUEUED; - if (!state->parent1_count && !state->parent2_count && - !state->mb_candidate_count) - return NULL; + if (!state->mb_candidate_count) { + /* only stale entries remain */ + if (!state->parent1_count && !state->parent2_count) + return NULL; + + /* one side is exhausted */ + if ((!state->parent1_count || !state->parent2_count) && + state->gen_ordered && + 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 11db9ec988..fd11febf1a 100755 --- a/t/t6600-test-reach.sh +++ b/t/t6600-test-reach.sh @@ -298,7 +298,7 @@ test_expect_success 'in_merge_bases_many:self' ' EOF echo "in_merge_bases_many(A,X):1" >expect && test_all_modes in_merge_bases_many && - test_paint_down_steps 45 2 25 3 + test_paint_down_steps 45 1 25 1 ' test_expect_success 'is_descendant_of:hit' ' @@ -415,7 +415,7 @@ test_expect_success 'merge-base --all commit-walk steps' ' >input && git rev-parse commit-9-1 >expect && run_all_modes git merge-base --all commit-9-9 commit-9-1 && - test_paint_down_steps 81 80 81 81 + test_paint_down_steps 81 9 57 81 ' test_expect_success 'merge-base --all with clock skew and v1 commit-graph (side-exhaustion)' ' -- gitgitgadget