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Tue, 11 Aug 2026 02:29:04 -0700 (PDT) Received: from [127.0.0.1] ([4.155.196.103]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-392ee3eadd3sm1271709a91.12.2026.08.11.02.29.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 11 Aug 2026 02:29:04 -0700 (PDT) Message-Id: <4a6603731cf0256a6aaede225b19c536db9cff0e.1786440533.git.gitgitgadget@gmail.com> In-Reply-To: References: From: "Kristofer Karlsson via GitGitGadget" Date: Tue, 11 Aug 2026 09:28:50 +0000 Subject: [PATCH v8 08/10] commit-reach: terminate merge-base walk when one paint side is exhausted Fcc: Sent Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 To: git@vger.kernel.org Cc: Kristofer Karlsson , Kristofer Karlsson 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 ordered 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 optimization is gated by gen_ordered (which excludes v1 commit-graphs that use the date-ordering fallback) and by a generation check against topo_ceiling. topo_ceiling is GENERATION_NUMBER_INFINITY for v2 graphs and GENERATION_NUMBER_V1_MAX for v1 graphs, so that saturated commits are treated as unordered. Together these ensure the check only fires in the ordered region where topological ordering holds. The same topo_ceiling boundary is applied to the existing single-result early exit so that all generation-dependent gates express the same saturation-aware boundary consistently. 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 | 25 +++++++++++++++++-- commit-reach.c | 20 ++++++++++++--- t/t6600-test-reach.sh | 4 +-- 3 files changed, 41 insertions(+), 8 deletions(-) diff --git a/Documentation/technical/paint-down-to-common.adoc b/Documentation/technical/paint-down-to-common.adoc index acf32bacd4..2393bb03b6 100644 --- a/Documentation/technical/paint-down-to-common.adoc +++ b/Documentation/technical/paint-down-to-common.adoc @@ -76,7 +76,11 @@ its child. Commits not in the commit-graph have generation INFINITY; v1 commit-graphs saturate at V1_MAX. Both place commits in the unordered region. Any optimization that depends on generation -ordering must account for this saturation boundary. +ordering must account for this saturation boundary. The early +exit gates compare against a topological ceiling -- +`GENERATION_NUMBER_V1_MAX` for v1 graphs and +`GENERATION_NUMBER_INFINITY` for v2 graphs -- so that saturated +commits are treated as unordered. With generation ordering, values in the unordered region exceed those in the ordered region. The walk may therefore transition @@ -103,6 +107,9 @@ ends when one of the following conditions holds: a caller-supplied `min_generation` threshold. 4. Single result: the caller only needs one merge base, one has been found, and the walk has entered the ordered 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 ordered region. Stale entry condition ~~~~~~~~~~~~~~~~~~~~~ @@ -113,6 +120,16 @@ 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 ordered region, +where paint flags are final at visit time; in the unordered region, +a side that appears exhausted could reappear through late paint +propagation. + Generation cutoff ~~~~~~~~~~~~~~~~~ Some callers (notably `remove_redundant()`) supply a `min_generation` @@ -158,12 +175,16 @@ ordering via `compare_commits_by_commit_date`. Because commit dates are not monotonic (clock skew, rebases, etc.), the queue may visit commits out of topological order. -This disables the optimization that depends on generation ordering: +This disables the optimizations that depend on generation ordering: - *Single result*: the first merge-base candidate found may not be the shallowest, because a deeper ancestor with a higher commit date can be dequeued first. + - *Side exhaustion*: one paint side can appear to drain from the + queue while commits from that side are still waiting with lower + dates, causing premature termination. + Related documentation --------------------- diff --git a/commit-reach.c b/commit-reach.c index 0f5ffec36e..7c5bbe00c3 100644 --- a/commit-reach.c +++ b/commit-reach.c @@ -90,6 +90,7 @@ struct paint_state { size_t parent2_count; size_t mb_candidate_count; int gen_ordered; + timestamp_t topo_ceiling; }; static void paint_count_update(struct paint_state *state, @@ -150,9 +151,17 @@ static struct commit *paint_queue_get(struct paint_state *state) * still include this commit, so the last non-stale commit * sees a non-zero count and is returned for processing. */ - 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) < state->topo_ceiling) + return NULL; + } paint_count_update(state, commit->object.flags, -1); return commit; @@ -180,6 +189,9 @@ static int paint_down_to_common(struct repository *r, timestamp_t last_gen = GENERATION_NUMBER_INFINITY; struct commit_list **tail = result; + state.topo_ceiling = corrected_commit_dates_enabled(r) + ? GENERATION_NUMBER_INFINITY + : GENERATION_NUMBER_V1_MAX; if (!min_generation && !corrected_commit_dates_enabled(r)) { state.queue.compare = compare_commits_by_commit_date; state.gen_ordered = 0; @@ -222,7 +234,7 @@ static int paint_down_to_common(struct repository *r, */ if (!(mb_flags & MERGE_BASE_FIND_ALL) && state.gen_ordered && - generation < GENERATION_NUMBER_INFINITY) + generation < state.topo_ceiling) break; } /* Mark parents of a found merge stale */ diff --git a/t/t6600-test-reach.sh b/t/t6600-test-reach.sh index 9f3a8f4743..23417897c8 100755 --- a/t/t6600-test-reach.sh +++ b/t/t6600-test-reach.sh @@ -297,7 +297,7 @@ 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' ' @@ -414,7 +414,7 @@ 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 (side-exhaustion)' ' -- gitgitgadget