commit-reach: early exit paint_down_to_common for single merge-base

From: Kristofer Karlsson <[email protected]>

When find_all is false and generation numbers are available, the priority queue pops in non-increasing generation order. The first doubly-painted commit is a valid best merge-base; no later commit can dominate it. Skip the expensive STALE drain in this case.

The early exit is guarded by three conditions: find_all must be false, the commit-graph must provide generation numbers, and the merge-base commit itself must have a finite generation (not GENERATION_NUMBER_INFINITY from being outside the commit-graph).

Add find_all parameter to repo_get_merge_bases_many_dirty() and thread it through to paint_down_to_common(). git merge-base (without --all) passes show_all=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

---
[RFC] commit-reach: skip STALE drain when only one merge-base needed

Context for what this is all about.

I am working with a very large git monorepo and have been investigating performance issue. After some digging I ended up looking more deeply into git merge-base. I saw it had an --all parameter but the default is to only return a single merge-base. Looking through the code and adding debug timing, I realized that although the total time to compute the merge-base was high, a very small amount of time was spent finding the initial merge-base value that was later returned.

The optimization is actually quite dramatic in a large repo - runtime went down from 5000ms to 50ms, so it's roughly a 100x optimization. This comes from an exploding frontier of STALE commits to drain.

Thus, my idea is simply to return early from the function once we know what will be returned. This only works if we find a candidate that we know will not be pruned later - but fortunately if we have a commit graph with generations we will visit commits in order such that it will actually not be pruned.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2109%2Fspkrka%2Fmerge-base-early-exit-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2109/spkrka/merge-base-early-exit-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2109

 builtin/merge-base.c  |   3 +-
 commit-reach.c        |  26 ++++++---
 commit-reach.h        |   5 +-
 t/t6010-merge-base.sh | 119 ++++++++++++++++++++++++++++++++++++++++++
 t/t6600-test-reach.sh |  40 ++++++++++++++
 5 files changed, 183 insertions(+), 10 deletions(-)

index c7ee97fa6a..6b9d42f596 100644
@@ -14,7 +14,8 @@ static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
 	struct commit_list *result = NULL, *r;
 
 	if (repo_get_merge_bases_many_dirty(the_repository, rev[0],
-					    rev_nr - 1, rev + 1, &result) < 0) {
+					    rev_nr - 1, rev + 1,
+					    show_all, &result) < 0) {
 		commit_list_free(result);
 		return -1;
 	}

index d3a9b3ed6f..c9d2d594de 100644
@@ -55,14 +55,16 @@ static int paint_down_to_common(struct repository *r,
 				struct commit **twos,
 				timestamp_t min_generation,
 				int ignore_missing_commits,
+				int find_all,
 				struct commit_list **result)
 {
 	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
 	int i;
+	int has_gens = min_generation || corrected_commit_dates_enabled(r);
 	timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
 	struct commit_list **tail = result;
 
-	if (!min_generation && !corrected_commit_dates_enabled(r))
+	if (!has_gens)
 		queue.compare = compare_commits_by_commit_date;
 
 	one->object.flags |= PARENT1;

@@ -97,6 +99,11 @@ static int paint_down_to_common(struct repository *r,
 			if (!(commit->object.flags & RESULT)) {
 				commit->object.flags |= RESULT;
 				tail = commit_list_append(commit, tail);
+				/* Generation-ordered queue: no later
+				 * commit can dominate this one. */
+				if (!find_all && has_gens &&
+				    generation < GENERATION_NUMBER_INFINITY)
+					break;
 			}
 			/* Mark parents of a found merge stale */
 			flags |= STALE;

@@ -136,6 +143,7 @@ static int paint_down_to_common(struct repository *r,
 static int merge_bases_many(struct repository *r,
 			    struct commit *one, int n,
 			    struct commit **twos,
+			    int find_all,
 			    struct commit_list **result)
 {
 	struct commit_list *list = NULL, **tail = result;

@@ -165,7 +173,7 @@ static int merge_bases_many(struct repository *r,
 				     oid_to_hex(&twos[i]->object.oid));
 	}
 
-	if (paint_down_to_common(r, one, n, twos, 0, 0, &list)) {
+	if (paint_down_to_common(r, one, n, twos, 0, 0, find_all, &list)) {
 		commit_list_free(list);
 		return -1;
 	}

@@ -246,7 +254,7 @@ static int remove_redundant_no_gen(struct repository *r,
 				min_generation = curr_generation;
 		}
 		if (paint_down_to_common(r, array[i], filled,
-					 work, min_generation, 0, &common)) {
+					 work, min_generation, 0, 1, &common)) {
 			clear_commit_marks(array[i], all_flags);
 			clear_commit_marks_many(filled, work, all_flags);
 			commit_list_free(common);

@@ -425,6 +433,7 @@ static int get_merge_bases_many_0(struct repository *r,
 				  size_t n,
 				  struct commit **twos,
 				  int cleanup,
+				  int find_all,
 				  struct commit_list **result)
 {
 	struct commit_list *list, **tail = result;

@@ -432,7 +441,7 @@ static int get_merge_bases_many_0(struct repository *r,
 	size_t cnt, i;
 	int ret;
 
-	if (merge_bases_many(r, one, n, twos, result) < 0)
+	if (merge_bases_many(r, one, n, twos, find_all, result) < 0)
 		return -1;
 	for (i = 0; i < n; i++) {
 		if (one == twos[i])

@@ -475,16 +484,17 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit **twos,
 			      struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 1, result);
+	return get_merge_bases_many_0(r, one, n, twos, 1, 1, result);
 }
 
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one,
 				    size_t n,
 				    struct commit **twos,
+				    int find_all,
 				    struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 0, result);
+	return get_merge_bases_many_0(r, one, n, twos, 0, find_all, result);
 }
 
 int repo_get_merge_bases(struct repository *r,

@@ -492,7 +502,7 @@ int repo_get_merge_bases(struct repository *r,
 			 struct commit *two,
 			 struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, 1, &two, 1, result);
+	return get_merge_bases_many_0(r, one, 1, &two, 1, 1, result);
 }
 
 /*

@@ -555,7 +565,7 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 
 	if (paint_down_to_common(r, commit,
 				 nr_reference, reference,
-				 generation, ignore_missing_commits, &bases))
+				 generation, ignore_missing_commits, 1, &bases))
 		ret = -1;
 	else if (commit->object.flags & PARENT2)
 		ret = 1;

index 6012402dfc..908b9539c5 100644
@@ -17,10 +17,13 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit *one, size_t n,
 			      struct commit **twos,
 			      struct commit_list **result);
-/* To be used only when object flags after this call no longer matter */
+/* To be used only when object flags after this call no longer matter.
+ * When find_all is false and generation numbers are available, returns
+ * after finding the first merge-base, skipping the STALE drain. */
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one, size_t n,
 				    struct commit **twos,
+				    int find_all,
 				    struct commit_list **result);
 
 int get_octopus_merge_bases(struct commit_list *in, struct commit_list **result);

index dc0421ed2f..51c23b7683 100755
@@ -882,4 +882,44 @@ test_expect_success 'rev-list --maximal-only matches merge-base --independent' '
 	test_cmp expect.sorted actual.sorted
 '
 
+# The following tests verify the early-exit optimisation in
+# paint_down_to_common when merge-base is invoked without --all.
+# Each test checks all four commit-graph configurations.
+
+merge_base_all_modes () {
+	test_when_finished rm -rf .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-full .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-half .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-no-gdat .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual
+}
+
+test_expect_success 'merge-base without --all (unique base)' '
+	git rev-parse commit-5-3 >expect &&
+	merge_base_all_modes commit-5-7 commit-8-3
+'
+
+test_expect_success 'merge-base without --all is one of --all results' '
+	test_when_finished rm -rf .git/objects/info/commit-graph &&
+
+	cp commit-graph-full .git/objects/info/commit-graph &&
+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
+	test_line_count = 1 single &&
+	grep -F -f single all &&
+
+	cp commit-graph-half .git/objects/info/commit-graph &&
+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
+	test_line_count = 1 single &&
+	grep -F -f single all
+'
+
 test_done

base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0

gitgitgadget

"Kristofer Karlsson via GitGitGadget" <[email protected]>

                        if (!(commit->object.flags & RESULT)) {
                                if (mark_result(r, &tail, commit,
						find_all, min_generation))
                                        break;
                        }

From: Kristofer Karlsson <[email protected]>

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 find_all is false 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 find_all parameter to repo_get_merge_bases_many_dirty() and thread it through to paint_down_to_common(). git merge-base (without --all) passes show_all=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

---
[RFC] commit-reach: skip STALE drain when only one merge-base needed

Context for what this is all about.

I am working with a very large git monorepo and have been investigating performance issues. After some digging I ended up looking more deeply into git merge-base. I saw it had an --all parameter but the default is to only return a single merge-base. Looking through the code and adding debug timing, I realized that although the total time to compute the merge-base was high, a very small amount of time was spent finding the initial merge-base value that was later returned.

The optimization is actually quite dramatic in a large repo - runtime went down from 5000ms to 50ms, so it's roughly a 100x optimization. This comes from an exploding frontier of STALE commits to drain.

Thus, my idea is simply to return early from the function once we know what will be returned. This only works if we find a candidate that we know will not be pruned later - but fortunately if we have a commit graph with generations we will visit commits in order such that it will actually not be pruned.

Changes since v1 (thanks Junio for the review):

* Dropped the has_gens variable entirely. If a commit has a finite generation then it is in the commit-graph, and so are all its ancestors — no additional check is needed to know the queue ordering is sound. Without a commit-graph every commit gets INFINITY and the guard never fires. This also avoids the misleading interaction with callers that pass non-zero min_generation without having generation data.

* Simplified the early exit guard from three conditions to two:
!find_all && generation < GENERATION_NUMBER_INFINITY.

* Fixed multi-line comment style per CodingGuidelines.

* Replaced "dominate" with concrete reasoning about queue ordering.

* Did not extract a helper function: after the simplifications above the inner block is four lines and reads naturally inline. The right boundary for a helper is not obvious (it could absorb just the result marking, or also the RESULT flag check, or also the PARENT1|PARENT2 test) and each level requires more local state passed by pointer. Happy to extract one if preferred.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2109%2Fspkrka%2Fmerge-base-early-exit-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2109/spkrka/merge-base-early-exit-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/2109

Range-diff vs v1:

1: 54cdf9bfd9 ! 1: f7b5c267f3 commit-reach: early exit paint_down_to_common for single merge-base
@@ Metadata
## Commit message ##
commit-reach: early exit paint_down_to_common for single merge-base

- When find_all is false and generation numbers are available, the
- priority queue pops in non-increasing generation order. The first
- doubly-painted commit is a valid best merge-base; no later commit
- can dominate it. Skip the expensive STALE drain in this case.
-
- The early exit is guarded by three conditions: find_all must be
- false, the commit-graph must provide generation numbers, and the
- merge-base commit itself must have a finite generation (not
- GENERATION_NUMBER_INFINITY from being outside the commit-graph).
+ 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 find_all is false 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 find_all parameter to repo_get_merge_bases_many_dirty() and thread it through to paint_down_to_common(). git merge-base
@@ commit-reach.c: static int paint_down_to_common(struct repository *r,
struct commit_list **result)
{
struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
- int i;
-+ int has_gens = min_generation || corrected_commit_dates_enabled(r);
- timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
- struct commit_list **tail = result;
-
-- if (!min_generation && !corrected_commit_dates_enabled(r))
-+ if (!has_gens)
- queue.compare = compare_commits_by_commit_date;
-
- one->object.flags |= PARENT1;
@@ commit-reach.c: static int paint_down_to_common(struct repository *r,
if (!(commit->object.flags & RESULT)) {
commit->object.flags |= RESULT; tail = commit_list_append(commit, tail);
-+ /* Generation-ordered queue: no later
-+ * commit can dominate this one. */
-+ if (!find_all && has_gens &&
++ /*
++ * The queue is generation-ordered; no
++ * remaining common ancestor can be a
++ * descendant of this one.
++ */
++ if (!find_all &&
+ generation < GENERATION_NUMBER_INFINITY)
+ break;
}
@@ commit-reach.h: int repo_get_merge_bases_many(struct repository *r,
struct commit **twos,
struct commit_list **result);
-/* To be used only when object flags after this call no longer matter */
-+/* To be used only when object flags after this call no longer matter. ++/*
++ * To be used only when object flags after this call no longer matter.
+ * When find_all is false and generation numbers are available, returns
-+ * after finding the first merge-base, skipping the STALE drain. */ ++ * after finding the first merge-base, skipping the STALE drain. ++ */
int repo_get_merge_bases_many_dirty(struct repository *r,
struct commit *one, size_t n, struct commit **twos,

 builtin/merge-base.c  |   3 +-
 commit-reach.c        |  26 ++++++---
 commit-reach.h        |   7 ++-
 t/t6010-merge-base.sh | 119 ++++++++++++++++++++++++++++++++++++++++++
 t/t6600-test-reach.sh |  40 ++++++++++++++
 5 files changed, 186 insertions(+), 9 deletions(-)

index c7ee97fa6a..6b9d42f596 100644
@@ -14,7 +14,8 @@ static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
 	struct commit_list *result = NULL, *r;
 
 	if (repo_get_merge_bases_many_dirty(the_repository, rev[0],
-					    rev_nr - 1, rev + 1, &result) < 0) {
+					    rev_nr - 1, rev + 1,
+					    show_all, &result) < 0) {
 		commit_list_free(result);
 		return -1;
 	}

index d3a9b3ed6f..b4ca00bb7e 100644
@@ -55,6 +55,7 @@ static int paint_down_to_common(struct repository *r,
 				struct commit **twos,
 				timestamp_t min_generation,
 				int ignore_missing_commits,
+				int find_all,
 				struct commit_list **result)
 {
 	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };

@@ -97,6 +98,14 @@ static int paint_down_to_common(struct repository *r,
 			if (!(commit->object.flags & RESULT)) {
 				commit->object.flags |= RESULT;
 				tail = commit_list_append(commit, tail);
+				/*
+				 * The queue is generation-ordered; no
+				 * remaining common ancestor can be a
+				 * descendant of this one.
+				 */
+				if (!find_all &&
+				    generation < GENERATION_NUMBER_INFINITY)
+					break;
 			}
 			/* Mark parents of a found merge stale */
 			flags |= STALE;

@@ -136,6 +145,7 @@ static int paint_down_to_common(struct repository *r,
 static int merge_bases_many(struct repository *r,
 			    struct commit *one, int n,
 			    struct commit **twos,
+			    int find_all,
 			    struct commit_list **result)
 {
 	struct commit_list *list = NULL, **tail = result;

@@ -165,7 +175,7 @@ static int merge_bases_many(struct repository *r,
 				     oid_to_hex(&twos[i]->object.oid));
 	}
 
-	if (paint_down_to_common(r, one, n, twos, 0, 0, &list)) {
+	if (paint_down_to_common(r, one, n, twos, 0, 0, find_all, &list)) {
 		commit_list_free(list);
 		return -1;
 	}

@@ -246,7 +256,7 @@ static int remove_redundant_no_gen(struct repository *r,
 				min_generation = curr_generation;
 		}
 		if (paint_down_to_common(r, array[i], filled,
-					 work, min_generation, 0, &common)) {
+					 work, min_generation, 0, 1, &common)) {
 			clear_commit_marks(array[i], all_flags);
 			clear_commit_marks_many(filled, work, all_flags);
 			commit_list_free(common);

@@ -425,6 +435,7 @@ static int get_merge_bases_many_0(struct repository *r,
 				  size_t n,
 				  struct commit **twos,
 				  int cleanup,
+				  int find_all,
 				  struct commit_list **result)
 {
 	struct commit_list *list, **tail = result;

@@ -432,7 +443,7 @@ static int get_merge_bases_many_0(struct repository *r,
 	size_t cnt, i;
 	int ret;
 
-	if (merge_bases_many(r, one, n, twos, result) < 0)
+	if (merge_bases_many(r, one, n, twos, find_all, result) < 0)
 		return -1;
 	for (i = 0; i < n; i++) {
 		if (one == twos[i])

@@ -475,16 +486,17 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit **twos,
 			      struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 1, result);
+	return get_merge_bases_many_0(r, one, n, twos, 1, 1, result);
 }
 
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one,
 				    size_t n,
 				    struct commit **twos,
+				    int find_all,
 				    struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 0, result);
+	return get_merge_bases_many_0(r, one, n, twos, 0, find_all, result);
 }
 
 int repo_get_merge_bases(struct repository *r,

@@ -492,7 +504,7 @@ int repo_get_merge_bases(struct repository *r,
 			 struct commit *two,
 			 struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, 1, &two, 1, result);
+	return get_merge_bases_many_0(r, one, 1, &two, 1, 1, result);
 }
 
 /*

@@ -555,7 +567,7 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 
 	if (paint_down_to_common(r, commit,
 				 nr_reference, reference,
-				 generation, ignore_missing_commits, &bases))
+				 generation, ignore_missing_commits, 1, &bases))
 		ret = -1;
 	else if (commit->object.flags & PARENT2)
 		ret = 1;

index 6012402dfc..c3b570a5cc 100644
@@ -17,10 +17,15 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit *one, size_t n,
 			      struct commit **twos,
 			      struct commit_list **result);
-/* To be used only when object flags after this call no longer matter */
+/*
+ * To be used only when object flags after this call no longer matter.
+ * When find_all is false and generation numbers are available, returns
+ * after finding the first merge-base, skipping the STALE drain.
+ */
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one, size_t n,
 				    struct commit **twos,
+				    int find_all,
 				    struct commit_list **result);
 
 int get_octopus_merge_bases(struct commit_list *in, struct commit_list **result);

index dc0421ed2f..51c23b7683 100755
@@ -882,4 +882,44 @@ test_expect_success 'rev-list --maximal-only matches merge-base --independent' '
 	test_cmp expect.sorted actual.sorted
 '
 
+# The following tests verify the early-exit optimisation in
+# paint_down_to_common when merge-base is invoked without --all.
+# Each test checks all four commit-graph configurations.
+
+merge_base_all_modes () {
+	test_when_finished rm -rf .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-full .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-half .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-no-gdat .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual
+}
+
+test_expect_success 'merge-base without --all (unique base)' '
+	git rev-parse commit-5-3 >expect &&
+	merge_base_all_modes commit-5-7 commit-8-3
+'
+
+test_expect_success 'merge-base without --all is one of --all results' '
+	test_when_finished rm -rf .git/objects/info/commit-graph &&
+
+	cp commit-graph-full .git/objects/info/commit-graph &&
+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
+	test_line_count = 1 single &&
+	grep -F -f single all &&
+
+	cp commit-graph-half .git/objects/info/commit-graph &&
+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
+	test_line_count = 1 single &&
+	grep -F -f single all
+'
+
 test_done

base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0

gitgitgadget

From: Kristofer Karlsson <[email protected]>

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.

Introduce enum merge_base_flags with MERGE_BASE_FIND_ALL and MERGE_BASE_IGNORE_MISSING_COMMITS, replacing the two boolean parameters in paint_down_to_common(). Thread the flags through merge_bases_many(), get_merge_bases_many_0(), and the public repo_get_merge_bases_many_dirty() API. 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

---
[RFC] commit-reach: skip STALE drain when only one merge-base needed

Context for what this is all about.

I am working with a very large git monorepo and have been investigating performance issues. After some digging I ended up looking more deeply into git merge-base. I saw it had an --all parameter but the default is to only return a single merge-base. Looking through the code and adding debug timing, I realized that although the total time to compute the merge-base was high, a very small amount of time was spent finding the initial merge-base value that was later returned.

The optimization is actually quite dramatic in a large repo - runtime went down from 5000ms to 50ms, so it's roughly a 100x optimization. This comes from an exploding frontier of STALE commits to drain.

Thus, my idea is simply to return early from the function once we know what will be returned. This only works if we find a candidate that we know will not be pruned later - but fortunately if we have a commit graph with generations we will visit commits in order such that it will actually not be pruned.

Changes since v1 (thanks Junio for the review):

* Dropped the has_gens variable entirely. If a commit has a finite generation then it is in the commit-graph, and so are all its ancestors — no additional check is needed to know the queue ordering is sound. Without a commit-graph every commit gets INFINITY and the guard never fires. This also avoids the misleading interaction with callers that pass non-zero min_generation without having generation data.

* Simplified the early exit guard from three conditions to two:
!find_all && generation < GENERATION_NUMBER_INFINITY.

* Fixed multi-line comment style per CodingGuidelines.

* Replaced "dominate" with concrete reasoning about queue ordering.

* Did not extract a helper function: after the simplifications above the inner block is four lines and reads naturally inline. The right boundary for a helper is not obvious (it could absorb just the result marking, or also the RESULT flag check, or also the PARENT1|PARENT2 test) and each level requires more local state passed by pointer. Happy to extract one if preferred.

Changes since v2 (thanks Patrick for the suggestion):

* Replaced the boolean find_all and ignore_missing_commits parameters in paint_down_to_common() with a single enum merge_base_flags mb_flags, reducing the function from 8 to 7 parameters. The enum is defined in commit-reach.h with MERGE_BASE_FIND_ALL and MERGE_BASE_IGNORE_MISSING_COMMITS.

* Named the enum merge_base_flags rather than paint_down_to_common_flags since the flags express caller intent and are threaded through multiple layers including the public repo_get_merge_bases_many_dirty() API.

* Used mb_flags as the parameter name to avoid shadowing the existing local int flags (commit object flags) inside paint_down_to_common().

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2109%2Fspkrka%2Fmerge-base-early-exit-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2109/spkrka/merge-base-early-exit-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/2109

Range-diff vs v2:

1: f7b5c267f3 ! 1: e4dada892f commit-reach: early exit paint_down_to_common for single merge-base

     @@ Commit message
          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 find_all is false 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.
     +    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 find_all parameter to repo_get_merge_bases_many_dirty() and
     -    thread it through to paint_down_to_common().  git merge-base
     -    (without --all) passes show_all=0, triggering the early exit.
     +    Introduce enum merge_base_flags with MERGE_BASE_FIND_ALL and
     +    MERGE_BASE_IGNORE_MISSING_COMMITS, replacing the two boolean
     +    parameters in paint_down_to_common().  Thread the flags through
     +    merge_bases_many(), get_merge_bases_many_0(), and the public
     +    repo_get_merge_bases_many_dirty() API.  git merge-base (without
     +    --all) passes 0, triggering the early exit.
      
          On a 2.2M-commit merge-heavy monorepo with commit-graph:
      
     @@ Commit message
          Signed-off-by: Kristofer Karlsson <[email protected]>
      
       ## builtin/merge-base.c ##
     -@@ builtin/merge-base.c: static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
     +@@
     + 
     + static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
     + {
     ++	enum merge_base_flags flags = show_all ? MERGE_BASE_FIND_ALL : 0;
       	struct commit_list *result = NULL, *r;
       
       	if (repo_get_merge_bases_many_dirty(the_repository, rev[0],
      -					    rev_nr - 1, rev + 1, &result) < 0) {
      +					    rev_nr - 1, rev + 1,
     -+					    show_all, &result) < 0) {
     ++					    flags, &result) < 0) {
       		commit_list_free(result);
       		return -1;
       	}
      
       ## commit-reach.c ##
      @@ commit-reach.c: static int paint_down_to_common(struct repository *r,
     + 				struct commit *one, int n,
       				struct commit **twos,
       				timestamp_t min_generation,
     - 				int ignore_missing_commits,
     -+				int find_all,
     +-				int ignore_missing_commits,
     ++				enum merge_base_flags mb_flags,
       				struct commit_list **result)
       {
       	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
     @@ commit-reach.c: static int paint_down_to_common(struct repository *r,
      +				 * remaining common ancestor can be a
      +				 * descendant of this one.
      +				 */
     -+				if (!find_all &&
     ++				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
      +				    generation < GENERATION_NUMBER_INFINITY)
      +					break;
       			}
       			/* Mark parents of a found merge stale */
       			flags |= STALE;
     +@@ commit-reach.c: static int paint_down_to_common(struct repository *r,
     + 				 * corrupt commits would already have been
     + 				 * dispatched with a `die()`.
     + 				 */
     +-				if (ignore_missing_commits)
     ++				if (mb_flags & MERGE_BASE_IGNORE_MISSING_COMMITS)
     + 					return 0;
     + 				return error(_("could not parse commit %s"),
     + 					     oid_to_hex(&p->object.oid));
      @@ commit-reach.c: static int paint_down_to_common(struct repository *r,
       static int merge_bases_many(struct repository *r,
       			    struct commit *one, int n,
       			    struct commit **twos,
     -+			    int find_all,
     ++			    enum merge_base_flags mb_flags,
       			    struct commit_list **result)
       {
       	struct commit_list *list = NULL, **tail = result;
     @@ commit-reach.c: static int merge_bases_many(struct repository *r,
       	}
       
      -	if (paint_down_to_common(r, one, n, twos, 0, 0, &list)) {
     -+	if (paint_down_to_common(r, one, n, twos, 0, 0, find_all, &list)) {
     ++	if (paint_down_to_common(r, one, n, twos, 0, mb_flags, &list)) {
       		commit_list_free(list);
       		return -1;
       	}
     @@ commit-reach.c: static int remove_redundant_no_gen(struct repository *r,
       		}
       		if (paint_down_to_common(r, array[i], filled,
      -					 work, min_generation, 0, &common)) {
     -+					 work, min_generation, 0, 1, &common)) {
     ++					 work, min_generation,
     ++					 MERGE_BASE_FIND_ALL, &common)) {
       			clear_commit_marks(array[i], all_flags);
       			clear_commit_marks_many(filled, work, all_flags);
       			commit_list_free(common);
     @@ commit-reach.c: static int get_merge_bases_many_0(struct repository *r,
       				  size_t n,
       				  struct commit **twos,
       				  int cleanup,
     -+				  int find_all,
     ++				  enum merge_base_flags mb_flags,
       				  struct commit_list **result)
       {
       	struct commit_list *list, **tail = result;
     @@ commit-reach.c: static int get_merge_bases_many_0(struct repository *r,
       	int ret;
       
      -	if (merge_bases_many(r, one, n, twos, result) < 0)
     -+	if (merge_bases_many(r, one, n, twos, find_all, result) < 0)
     ++	if (merge_bases_many(r, one, n, twos, mb_flags, result) < 0)
       		return -1;
       	for (i = 0; i < n; i++) {
       		if (one == twos[i])
     @@ commit-reach.c: int repo_get_merge_bases_many(struct repository *r,
       			      struct commit_list **result)
       {
      -	return get_merge_bases_many_0(r, one, n, twos, 1, result);
     -+	return get_merge_bases_many_0(r, one, n, twos, 1, 1, result);
     ++	return get_merge_bases_many_0(r, one, n, twos, 1,
     ++				     MERGE_BASE_FIND_ALL, result);
       }
       
       int repo_get_merge_bases_many_dirty(struct repository *r,
       				    struct commit *one,
       				    size_t n,
       				    struct commit **twos,
     -+				    int find_all,
     ++				    enum merge_base_flags mb_flags,
       				    struct commit_list **result)
       {
      -	return get_merge_bases_many_0(r, one, n, twos, 0, result);
     -+	return get_merge_bases_many_0(r, one, n, twos, 0, find_all, result);
     ++	return get_merge_bases_many_0(r, one, n, twos, 0, mb_flags, result);
       }
       
       int repo_get_merge_bases(struct repository *r,
     @@ commit-reach.c: int repo_get_merge_bases(struct repository *r,
       			 struct commit_list **result)
       {
      -	return get_merge_bases_many_0(r, one, 1, &two, 1, result);
     -+	return get_merge_bases_many_0(r, one, 1, &two, 1, 1, result);
     ++	return get_merge_bases_many_0(r, one, 1, &two, 1,
     ++				     MERGE_BASE_FIND_ALL, result);
       }
       
       /*
      @@ commit-reach.c: int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
     + 	struct commit_list *bases = NULL;
     + 	int ret = 0, i;
     + 	timestamp_t generation, max_generation = GENERATION_NUMBER_ZERO;
     ++	enum merge_base_flags mb_flags = MERGE_BASE_FIND_ALL;
     ++
     ++	if (ignore_missing_commits)
     ++		mb_flags |= MERGE_BASE_IGNORE_MISSING_COMMITS;
     + 
     + 	if (repo_parse_commit(r, commit))
     + 		return ignore_missing_commits ? 0 : -1;
     +@@ commit-reach.c: int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
       
       	if (paint_down_to_common(r, commit,
       				 nr_reference, reference,
      -				 generation, ignore_missing_commits, &bases))
     -+				 generation, ignore_missing_commits, 1, &bases))
     ++				 generation, mb_flags, &bases))
       		ret = -1;
       	else if (commit->object.flags & PARENT2)
       		ret = 1;
     @@ commit-reach.h: int repo_get_merge_bases_many(struct repository *r,
       			      struct commit **twos,
       			      struct commit_list **result);
      -/* To be used only when object flags after this call no longer matter */
     ++enum merge_base_flags {
     ++	MERGE_BASE_FIND_ALL               = (1 << 0),
     ++	MERGE_BASE_IGNORE_MISSING_COMMITS = (1 << 1),
     ++};
     ++
      +/*
      + * To be used only when object flags after this call no longer matter.
     -+ * When find_all is false and generation numbers are available, returns
     -+ * after finding the first merge-base, skipping the STALE drain.
     ++ * Without MERGE_BASE_FIND_ALL and with generation numbers available,
     ++ * returns after finding the first merge-base, skipping the STALE drain.
      + */
       int repo_get_merge_bases_many_dirty(struct repository *r,
       				    struct commit *one, size_t n,
       				    struct commit **twos,
     -+				    int find_all,
     ++				    enum merge_base_flags mb_flags,
       				    struct commit_list **result);
       
       int get_octopus_merge_bases(struct commit_list *in, struct commit_list **result);

 builtin/merge-base.c  |   4 +-
 commit-reach.c        |  36 +++++++++----
 commit-reach.h        |  12 ++++-
 t/t6010-merge-base.sh | 119 ++++++++++++++++++++++++++++++++++++++++++
 t/t6600-test-reach.sh |  40 ++++++++++++++
 5 files changed, 200 insertions(+), 11 deletions(-)

index c7ee97fa6a..a87011c6cd 100644
@@ -11,10 +11,12 @@
 
 static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
 {
+	enum merge_base_flags flags = show_all ? MERGE_BASE_FIND_ALL : 0;
 	struct commit_list *result = NULL, *r;
 
 	if (repo_get_merge_bases_many_dirty(the_repository, rev[0],
-					    rev_nr - 1, rev + 1, &result) < 0) {
+					    rev_nr - 1, rev + 1,
+					    flags, &result) < 0) {
 		commit_list_free(result);
 		return -1;
 	}

index d3a9b3ed6f..5a52be90a6 100644
@@ -54,7 +54,7 @@ static int paint_down_to_common(struct repository *r,
 				struct commit *one, int n,
 				struct commit **twos,
 				timestamp_t min_generation,
-				int ignore_missing_commits,
+				enum merge_base_flags mb_flags,
 				struct commit_list **result)
 {
 	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };

@@ -97,6 +97,14 @@ static int paint_down_to_common(struct repository *r,
 			if (!(commit->object.flags & RESULT)) {
 				commit->object.flags |= RESULT;
 				tail = commit_list_append(commit, tail);
+				/*
+				 * The queue is generation-ordered; no
+				 * remaining common ancestor can be a
+				 * descendant of this one.
+				 */
+				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
+				    generation < GENERATION_NUMBER_INFINITY)
+					break;
 			}
 			/* Mark parents of a found merge stale */
 			flags |= STALE;

@@ -118,7 +126,7 @@ static int paint_down_to_common(struct repository *r,
 				 * corrupt commits would already have been
 				 * dispatched with a `die()`.
 				 */
-				if (ignore_missing_commits)
+				if (mb_flags & MERGE_BASE_IGNORE_MISSING_COMMITS)
 					return 0;
 				return error(_("could not parse commit %s"),
 					     oid_to_hex(&p->object.oid));

@@ -136,6 +144,7 @@ static int paint_down_to_common(struct repository *r,
 static int merge_bases_many(struct repository *r,
 			    struct commit *one, int n,
 			    struct commit **twos,
+			    enum merge_base_flags mb_flags,
 			    struct commit_list **result)
 {
 	struct commit_list *list = NULL, **tail = result;

@@ -165,7 +174,7 @@ static int merge_bases_many(struct repository *r,
 				     oid_to_hex(&twos[i]->object.oid));
 	}
 
-	if (paint_down_to_common(r, one, n, twos, 0, 0, &list)) {
+	if (paint_down_to_common(r, one, n, twos, 0, mb_flags, &list)) {
 		commit_list_free(list);
 		return -1;
 	}

@@ -246,7 +255,8 @@ static int remove_redundant_no_gen(struct repository *r,
 				min_generation = curr_generation;
 		}
 		if (paint_down_to_common(r, array[i], filled,
-					 work, min_generation, 0, &common)) {
+					 work, min_generation,
+					 MERGE_BASE_FIND_ALL, &common)) {
 			clear_commit_marks(array[i], all_flags);
 			clear_commit_marks_many(filled, work, all_flags);
 			commit_list_free(common);

@@ -425,6 +435,7 @@ static int get_merge_bases_many_0(struct repository *r,
 				  size_t n,
 				  struct commit **twos,
 				  int cleanup,
+				  enum merge_base_flags mb_flags,
 				  struct commit_list **result)
 {
 	struct commit_list *list, **tail = result;

@@ -432,7 +443,7 @@ static int get_merge_bases_many_0(struct repository *r,
 	size_t cnt, i;
 	int ret;
 
-	if (merge_bases_many(r, one, n, twos, result) < 0)
+	if (merge_bases_many(r, one, n, twos, mb_flags, result) < 0)
 		return -1;
 	for (i = 0; i < n; i++) {
 		if (one == twos[i])

@@ -475,16 +486,18 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit **twos,
 			      struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 1, result);
+	return get_merge_bases_many_0(r, one, n, twos, 1,
+				     MERGE_BASE_FIND_ALL, result);
 }
 
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one,
 				    size_t n,
 				    struct commit **twos,
+				    enum merge_base_flags mb_flags,
 				    struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 0, result);
+	return get_merge_bases_many_0(r, one, n, twos, 0, mb_flags, result);
 }
 
 int repo_get_merge_bases(struct repository *r,

@@ -492,7 +505,8 @@ int repo_get_merge_bases(struct repository *r,
 			 struct commit *two,
 			 struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, 1, &two, 1, result);
+	return get_merge_bases_many_0(r, one, 1, &two, 1,
+				     MERGE_BASE_FIND_ALL, result);
 }
 
 /*

@@ -537,6 +551,10 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 	struct commit_list *bases = NULL;
 	int ret = 0, i;
 	timestamp_t generation, max_generation = GENERATION_NUMBER_ZERO;
+	enum merge_base_flags mb_flags = MERGE_BASE_FIND_ALL;
+
+	if (ignore_missing_commits)
+		mb_flags |= MERGE_BASE_IGNORE_MISSING_COMMITS;
 
 	if (repo_parse_commit(r, commit))
 		return ignore_missing_commits ? 0 : -1;

@@ -555,7 +573,7 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 
 	if (paint_down_to_common(r, commit,
 				 nr_reference, reference,
-				 generation, ignore_missing_commits, &bases))
+				 generation, mb_flags, &bases))
 		ret = -1;
 	else if (commit->object.flags & PARENT2)
 		ret = 1;

index 6012402dfc..41607d8952 100644
@@ -17,10 +17,20 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit *one, size_t n,
 			      struct commit **twos,
 			      struct commit_list **result);
-/* To be used only when object flags after this call no longer matter */
+enum merge_base_flags {
+	MERGE_BASE_FIND_ALL               = (1 << 0),
+	MERGE_BASE_IGNORE_MISSING_COMMITS = (1 << 1),
+};
+
+/*
+ * To be used only when object flags after this call no longer matter.
+ * Without MERGE_BASE_FIND_ALL and with generation numbers available,
+ * returns after finding the first merge-base, skipping the STALE drain.
+ */
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one, size_t n,
 				    struct commit **twos,
+				    enum merge_base_flags mb_flags,
 				    struct commit_list **result);
 
 int get_octopus_merge_bases(struct commit_list *in, struct commit_list **result);

index dc0421ed2f..51c23b7683 100755
@@ -882,4 +882,44 @@ test_expect_success 'rev-list --maximal-only matches merge-base --independent' '
 	test_cmp expect.sorted actual.sorted
 '
 
+# The following tests verify the early-exit optimisation in
+# paint_down_to_common when merge-base is invoked without --all.
+# Each test checks all four commit-graph configurations.
+
+merge_base_all_modes () {
+	test_when_finished rm -rf .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-full .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-half .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-no-gdat .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual
+}
+
+test_expect_success 'merge-base without --all (unique base)' '
+	git rev-parse commit-5-3 >expect &&
+	merge_base_all_modes commit-5-7 commit-8-3
+'
+
+test_expect_success 'merge-base without --all is one of --all results' '
+	test_when_finished rm -rf .git/objects/info/commit-graph &&
+
+	cp commit-graph-full .git/objects/info/commit-graph &&
+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
+	test_line_count = 1 single &&
+	grep -F -f single all &&
+
+	cp commit-graph-half .git/objects/info/commit-graph &&
+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
+	test_line_count = 1 single &&
+	grep -F -f single all
+'
+
 test_done

base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0

gitgitgadget

Thanks for making these changes. I think it would make sense to split this up into two commits though, where the first commit only introduces the new enum (without the new flag) and the second commit then adds the new flag and the performance optimization.

That'd help quite a bit with the review as it splits up the changes into a refactoring-only change without any intended semantic change, and another commit that then does result in a user-visible change.

Patrick

Context for what this is all about.

I am working with a very large git monorepo and have been investigating performance issues. After some digging I ended up looking more deeply into git merge-base. I saw it had an --all parameter but the default is to only return a single merge-base. Looking through the code and adding debug timing, I realized that although the total time to compute the merge-base was high, a very small amount of time was spent finding the initial merge-base value that was later returned.

The optimization is actually quite dramatic in a large repo - runtime went down from 5000ms to 50ms, so it's roughly a 100x optimization. This comes from an exploding frontier of STALE commits to drain.

Thus, my idea is simply to return early from the function once we know what will be returned. This only works if we find a candidate that we know will not be pruned later - but fortunately if we have a commit graph with generations we will visit commits in order such that it will actually not be pruned.

Changes since v1 (thanks Junio for the review):

* Dropped the has_gens variable entirely. If a commit has a finite generation then it is in the commit-graph, and so are all its ancestors — no additional check is needed to know the queue ordering is sound. Without a commit-graph every commit gets INFINITY and the guard never fires. This also avoids the misleading interaction with callers that pass non-zero min_generation without having generation data.

* Simplified the early exit guard from three conditions to two: !find_all && generation < GENERATION_NUMBER_INFINITY.

* Fixed multi-line comment style per CodingGuidelines.

* Replaced "dominate" with concrete reasoning about queue ordering.

* Did not extract a helper function: after the simplifications above the inner block is four lines and reads naturally inline. The right boundary for a helper is not obvious (it could absorb just the result marking, or also the RESULT flag check, or also the PARENT1|PARENT2 test) and each level requires more local state passed by pointer. Happy to extract one if preferred.

Changes since v2 (thanks Patrick for the suggestion):

* Split into two commits: the first is a pure refactoring that introduces enum merge_base_flags and replaces the boolean ignore_missing_commits parameter, the second adds the new MERGE_BASE_FIND_ALL flag and the early exit optimization.

* Replaced the boolean find_all and ignore_missing_commits parameters in paint_down_to_common() with a single enum merge_base_flags mb_flags, reducing the function from 8 to 7 parameters. The enum is threaded through merge_bases_many(), get_merge_bases_many_0(), and the public repo_get_merge_bases_many_dirty() API.

* Named the enum merge_base_flags rather than paint_down_to_common_flags since the flags express caller intent and are threaded through multiple layers including the public API.

* Used mb_flags as the parameter name to avoid shadowing the existing local int flags (commit object flags) inside paint_down_to_common().

Kristofer Karlsson (2):
commit-reach: introduce merge_base_flags enum
commit-reach: early exit paint_down_to_common for single merge-base

 builtin/merge-base.c  |  4 +++-
 commit-reach.c        | 36 +++++++++++++++++++++++++++---------
 commit-reach.h        | 12 +++++++++++-
 t/t6600-test-reach.sh | 40 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 81 insertions(+), 11 deletions(-)

base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2109%2Fspkrka%2Fmerge-base-early-exit-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2109/spkrka/merge-base-early-exit-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/2109

Range-diff vs v3:

1: e4dada892f ! 1: 12d9e1c85f commit-reach: early exit paint_down_to_common for single merge-base

     @@ Metadata
      Author: Kristofer Karlsson <[email protected]>
      
       ## Commit message ##
     -    commit-reach: early exit paint_down_to_common for single merge-base
     +    commit-reach: introduce merge_base_flags enum
      
     -    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.
     +    Replace the boolean ignore_missing_commits parameter in
     +    paint_down_to_common() with an enum merge_base_flags, and thread
     +    the flags through merge_bases_many(), get_merge_bases_many_0(),
     +    and the public repo_get_merge_bases_many_dirty() API.
      
     -    Introduce enum merge_base_flags with MERGE_BASE_FIND_ALL and
     -    MERGE_BASE_IGNORE_MISSING_COMMITS, replacing the two boolean
     -    parameters in paint_down_to_common().  Thread the flags through
     -    merge_bases_many(), get_merge_bases_many_0(), and the public
     -    repo_get_merge_bases_many_dirty() API.  git merge-base (without
     -    --all) passes 0, triggering the early exit.
     +    This makes callsites with boolean parameters easier to read and
     +    prepares the function for additional flags in a subsequent commit.
      
     -    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
     +    No functional change: the single caller that used
     +    ignore_missing_commits (repo_in_merge_bases_many) now sets
     +    MERGE_BASE_IGNORE_MISSING_COMMITS in the flags word, and all
     +    other callers pass 0.
      
          Signed-off-by: Kristofer Karlsson <[email protected]>
      
       ## builtin/merge-base.c ##
     -@@
     - 
     - static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
     - {
     -+	enum merge_base_flags flags = show_all ? MERGE_BASE_FIND_ALL : 0;
     +@@ builtin/merge-base.c: static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
       	struct commit_list *result = NULL, *r;
       
       	if (repo_get_merge_bases_many_dirty(the_repository, rev[0],
      -					    rev_nr - 1, rev + 1, &result) < 0) {
      +					    rev_nr - 1, rev + 1,
     -+					    flags, &result) < 0) {
     ++					    0, &result) < 0) {
       		commit_list_free(result);
       		return -1;
       	}
     @@ commit-reach.c: static int paint_down_to_common(struct repository *r,
       				struct commit_list **result)
       {
       	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
     -@@ commit-reach.c: static int paint_down_to_common(struct repository *r,
     - 			if (!(commit->object.flags & RESULT)) {
     - 				commit->object.flags |= RESULT;
     - 				tail = commit_list_append(commit, tail);
     -+				/*
     -+				 * The queue is generation-ordered; no
     -+				 * remaining common ancestor can be a
     -+				 * descendant of this one.
     -+				 */
     -+				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
     -+				    generation < GENERATION_NUMBER_INFINITY)
     -+					break;
     - 			}
     - 			/* Mark parents of a found merge stale */
     - 			flags |= STALE;
      @@ commit-reach.c: static int paint_down_to_common(struct repository *r,
       				 * corrupt commits would already have been
       				 * dispatched with a `die()`.
     @@ commit-reach.c: static int merge_bases_many(struct repository *r,
       		commit_list_free(list);
       		return -1;
       	}
     -@@ commit-reach.c: static int remove_redundant_no_gen(struct repository *r,
     - 				min_generation = curr_generation;
     - 		}
     - 		if (paint_down_to_common(r, array[i], filled,
     --					 work, min_generation, 0, &common)) {
     -+					 work, min_generation,
     -+					 MERGE_BASE_FIND_ALL, &common)) {
     - 			clear_commit_marks(array[i], all_flags);
     - 			clear_commit_marks_many(filled, work, all_flags);
     - 			commit_list_free(common);
      @@ commit-reach.c: static int get_merge_bases_many_0(struct repository *r,
       				  size_t n,
       				  struct commit **twos,
     @@ commit-reach.c: int repo_get_merge_bases_many(struct repository *r,
       			      struct commit_list **result)
       {
      -	return get_merge_bases_many_0(r, one, n, twos, 1, result);
     -+	return get_merge_bases_many_0(r, one, n, twos, 1,
     -+				     MERGE_BASE_FIND_ALL, result);
     ++	return get_merge_bases_many_0(r, one, n, twos, 1, 0, result);
       }
       
       int repo_get_merge_bases_many_dirty(struct repository *r,
     @@ commit-reach.c: int repo_get_merge_bases(struct repository *r,
       			 struct commit_list **result)
       {
      -	return get_merge_bases_many_0(r, one, 1, &two, 1, result);
     -+	return get_merge_bases_many_0(r, one, 1, &two, 1,
     -+				     MERGE_BASE_FIND_ALL, result);
     ++	return get_merge_bases_many_0(r, one, 1, &two, 1, 0, result);
       }
       
       /*
     @@ commit-reach.c: int repo_in_merge_bases_many(struct repository *r, struct commit
       	struct commit_list *bases = NULL;
       	int ret = 0, i;
       	timestamp_t generation, max_generation = GENERATION_NUMBER_ZERO;
     -+	enum merge_base_flags mb_flags = MERGE_BASE_FIND_ALL;
     ++	enum merge_base_flags mb_flags = 0;
      +
      +	if (ignore_missing_commits)
      +		mb_flags |= MERGE_BASE_IGNORE_MISSING_COMMITS;
     @@ commit-reach.h: int repo_get_merge_bases_many(struct repository *r,
       			      struct commit *one, size_t n,
       			      struct commit **twos,
       			      struct commit_list **result);
     --/* To be used only when object flags after this call no longer matter */
      +enum merge_base_flags {
     -+	MERGE_BASE_FIND_ALL               = (1 << 0),
     -+	MERGE_BASE_IGNORE_MISSING_COMMITS = (1 << 1),
     ++	MERGE_BASE_IGNORE_MISSING_COMMITS = (1 << 0),
      +};
      +
     -+/*
     -+ * To be used only when object flags after this call no longer matter.
     -+ * Without MERGE_BASE_FIND_ALL and with generation numbers available,
     -+ * returns after finding the first merge-base, skipping the STALE drain.
     -+ */
     + /* To be used only when object flags after this call no longer matter */
       int repo_get_merge_bases_many_dirty(struct repository *r,
       				    struct commit *one, size_t n,
       				    struct commit **twos,
     @@ commit-reach.h: int repo_get_merge_bases_many(struct repository *r,
       				    struct commit_list **result);
       
       int get_octopus_merge_bases(struct commit_list *in, struct commit_list **result);
     -
     - ## t/t6010-merge-base.sh ##
     -@@ t/t6010-merge-base.sh: test_expect_success 'merge-base --octopus --all for complex tree' '
     - 	test_cmp expected actual
     - '
     - 
     -+# The following tests verify that "git merge-base" (without --all)
     -+# returns the same result with and without a commit-graph.
     -+# This exercises the early-exit optimisation in paint_down_to_common
     -+# that skips the STALE drain when generation numbers are available.
     -+
     -+test_expect_success 'setup for commit-graph tests' '
     -+	git init graph-repo &&
     -+	(
     -+		cd graph-repo &&
     -+
     -+		# Build a forked DAG:
     -+		#
     -+		#     L1---L2  (left)
     -+		#    /
     -+		#   S
     -+		#    \
     -+		#     R1---R2  (right)
     -+		#
     -+		test_commit GS &&
     -+		git checkout -b left &&
     -+		test_commit L1 &&
     -+		test_commit L2 &&
     -+		git checkout GS &&
     -+		git checkout -b right &&
     -+		test_commit GR1 &&
     -+		test_commit GR2
     -+	)
     -+'
     -+
     -+test_expect_success 'merge-base without commit-graph' '
     -+	(
     -+		cd graph-repo &&
     -+		rm -f .git/objects/info/commit-graph &&
     -+		git merge-base left right >actual &&
     -+		git rev-parse GS >expected &&
     -+		test_cmp expected actual
     -+	)
     -+'
     -+
     -+test_expect_success 'merge-base with commit-graph' '
     -+	(
     -+		cd graph-repo &&
     -+		git commit-graph write --reachable &&
     -+		git merge-base left right >actual &&
     -+		git rev-parse GS >expected &&
     -+		test_cmp expected actual
     -+	)
     -+'
     -+
     -+test_expect_success 'merge-base --all with commit-graph' '
     -+	(
     -+		cd graph-repo &&
     -+		git merge-base --all left right >actual &&
     -+		git rev-parse GS >expected &&
     -+		test_cmp expected actual
     -+	)
     -+'
     -+
     -+test_expect_success 'merge-base agrees with --all for single result' '
     -+	(
     -+		cd graph-repo &&
     -+		git commit-graph write --reachable &&
     -+		git merge-base left right >actual.single &&
     -+		git merge-base --all left right >actual.all &&
     -+		test_cmp actual.all actual.single
     -+	)
     -+'
     -+
     -+test_expect_success 'setup for deep chain commit-graph test' '
     -+	git init deep-repo &&
     -+	(
     -+		cd deep-repo &&
     -+
     -+		# Build a deep forked DAG:
     -+		#
     -+		#   L1--L2--...--L20  (left)
     -+		#  /
     -+		# S
     -+		#  \
     -+		#   R1--R2--...--R20  (right)
     -+		#
     -+		test_commit DS &&
     -+		git checkout -b left &&
     -+		for i in $(test_seq 1 20)
     -+		do
     -+			test_commit DL$i || return 1
     -+		done &&
     -+		git checkout DS &&
     -+		git checkout -b right &&
     -+		for i in $(test_seq 1 20)
     -+		do
     -+			test_commit DR$i || return 1
     -+		done
     -+	)
     -+'
     -+
     -+test_expect_success 'deep chain: merge-base matches with and without commit-graph' '
     -+	(
     -+		cd deep-repo &&
     -+		rm -f .git/objects/info/commit-graph &&
     -+		git merge-base left right >actual.no-graph &&
     -+		git rev-parse DS >expected &&
     -+		test_cmp expected actual.no-graph &&
     -+		git commit-graph write --reachable &&
     -+		git merge-base left right >actual.graph &&
     -+		test_cmp expected actual.graph
     -+	)
     -+'
     -+
     -+test_expect_success 'deep chain: --all and non---all agree with commit-graph' '
     -+	(
     -+		cd deep-repo &&
     -+		git commit-graph write --reachable &&
     -+		git merge-base left right >actual.single &&
     -+		git merge-base --all left right >actual.all &&
     -+		test_cmp actual.all actual.single
     -+	)
     -+'
     -+
     - test_done
     -
     - ## t/t6600-test-reach.sh ##
     -@@ t/t6600-test-reach.sh: test_expect_success 'rev-list --maximal-only matches merge-base --independent' '
     - 	test_cmp expect.sorted actual.sorted
     - '
     - 
     -+# The following tests verify the early-exit optimisation in
     -+# paint_down_to_common when merge-base is invoked without --all.
     -+# Each test checks all four commit-graph configurations.
     -+
     -+merge_base_all_modes () {
     -+	test_when_finished rm -rf .git/objects/info/commit-graph &&
     -+	git merge-base "$@" >actual &&
     -+	test_cmp expect actual &&
     -+	cp commit-graph-full .git/objects/info/commit-graph &&
     -+	git merge-base "$@" >actual &&
     -+	test_cmp expect actual &&
     -+	cp commit-graph-half .git/objects/info/commit-graph &&
     -+	git merge-base "$@" >actual &&
     -+	test_cmp expect actual &&
     -+	cp commit-graph-no-gdat .git/objects/info/commit-graph &&
     -+	git merge-base "$@" >actual &&
     -+	test_cmp expect actual
     -+}
     -+
     -+test_expect_success 'merge-base without --all (unique base)' '
     -+	git rev-parse commit-5-3 >expect &&
     -+	merge_base_all_modes commit-5-7 commit-8-3
     -+'
     -+
     -+test_expect_success 'merge-base without --all is one of --all results' '
     -+	test_when_finished rm -rf .git/objects/info/commit-graph &&
     -+
     -+	cp commit-graph-full .git/objects/info/commit-graph &&
     -+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
     -+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
     -+	test_line_count = 1 single &&
     -+	grep -F -f single all &&
     -+
     -+	cp commit-graph-half .git/objects/info/commit-graph &&
     -+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
     -+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
     -+	test_line_count = 1 single &&
     -+	grep -F -f single all
     -+'
     -+
     - test_done

-: ---------- > 2: 19f1605067 commit-reach: early exit paint_down_to_common for single merge-base

gitgitgadget

From: Kristofer Karlsson <[email protected]>

Replace the boolean ignore_missing_commits parameter in paint_down_to_common() with an enum merge_base_flags, and thread the flags through merge_bases_many(), get_merge_bases_many_0(), and the public repo_get_merge_bases_many_dirty() API.

This makes callsites with boolean parameters easier to read and prepares the function for additional flags in a subsequent commit.

No functional change: the single caller that used ignore_missing_commits (repo_in_merge_bases_many) now sets MERGE_BASE_IGNORE_MISSING_COMMITS in the flags word, and all other callers pass 0.

---
 builtin/merge-base.c |  3 ++-
 commit-reach.c       | 23 +++++++++++++++--------
 commit-reach.h       |  5 +++++
 3 files changed, 22 insertions(+), 9 deletions(-)

index c7ee97fa6a..9b50b4660e 100644
@@ -14,7 +14,8 @@ static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
 	struct commit_list *result = NULL, *r;
 
 	if (repo_get_merge_bases_many_dirty(the_repository, rev[0],
-					    rev_nr - 1, rev + 1, &result) < 0) {
+					    rev_nr - 1, rev + 1,
+					    0, &result) < 0) {
 		commit_list_free(result);
 		return -1;
 	}

index d3a9b3ed6f..766ba1156a 100644
@@ -54,7 +54,7 @@ static int paint_down_to_common(struct repository *r,
 				struct commit *one, int n,
 				struct commit **twos,
 				timestamp_t min_generation,
-				int ignore_missing_commits,
+				enum merge_base_flags mb_flags,
 				struct commit_list **result)
 {
 	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };

@@ -118,7 +118,7 @@ static int paint_down_to_common(struct repository *r,
 				 * corrupt commits would already have been
 				 * dispatched with a `die()`.
 				 */
-				if (ignore_missing_commits)
+				if (mb_flags & MERGE_BASE_IGNORE_MISSING_COMMITS)
 					return 0;
 				return error(_("could not parse commit %s"),
 					     oid_to_hex(&p->object.oid));

@@ -136,6 +136,7 @@ static int paint_down_to_common(struct repository *r,
 static int merge_bases_many(struct repository *r,
 			    struct commit *one, int n,
 			    struct commit **twos,
+			    enum merge_base_flags mb_flags,
 			    struct commit_list **result)
 {
 	struct commit_list *list = NULL, **tail = result;

@@ -165,7 +166,7 @@ static int merge_bases_many(struct repository *r,
 				     oid_to_hex(&twos[i]->object.oid));
 	}
 
-	if (paint_down_to_common(r, one, n, twos, 0, 0, &list)) {
+	if (paint_down_to_common(r, one, n, twos, 0, mb_flags, &list)) {
 		commit_list_free(list);
 		return -1;
 	}

@@ -425,6 +426,7 @@ static int get_merge_bases_many_0(struct repository *r,
 				  size_t n,
 				  struct commit **twos,
 				  int cleanup,
+				  enum merge_base_flags mb_flags,
 				  struct commit_list **result)
 {
 	struct commit_list *list, **tail = result;

@@ -432,7 +434,7 @@ static int get_merge_bases_many_0(struct repository *r,
 	size_t cnt, i;
 	int ret;
 
-	if (merge_bases_many(r, one, n, twos, result) < 0)
+	if (merge_bases_many(r, one, n, twos, mb_flags, result) < 0)
 		return -1;
 	for (i = 0; i < n; i++) {
 		if (one == twos[i])

@@ -475,16 +477,17 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit **twos,
 			      struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 1, result);
+	return get_merge_bases_many_0(r, one, n, twos, 1, 0, result);
 }
 
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one,
 				    size_t n,
 				    struct commit **twos,
+				    enum merge_base_flags mb_flags,
 				    struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 0, result);
+	return get_merge_bases_many_0(r, one, n, twos, 0, mb_flags, result);
 }
 
 int repo_get_merge_bases(struct repository *r,

@@ -492,7 +495,7 @@ int repo_get_merge_bases(struct repository *r,
 			 struct commit *two,
 			 struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, 1, &two, 1, result);
+	return get_merge_bases_many_0(r, one, 1, &two, 1, 0, result);
 }
 
 /*

@@ -537,6 +540,10 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 	struct commit_list *bases = NULL;
 	int ret = 0, i;
 	timestamp_t generation, max_generation = GENERATION_NUMBER_ZERO;
+	enum merge_base_flags mb_flags = 0;
+
+	if (ignore_missing_commits)
+		mb_flags |= MERGE_BASE_IGNORE_MISSING_COMMITS;
 
 	if (repo_parse_commit(r, commit))
 		return ignore_missing_commits ? 0 : -1;

@@ -555,7 +562,7 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 
 	if (paint_down_to_common(r, commit,
 				 nr_reference, reference,
-				 generation, ignore_missing_commits, &bases))
+				 generation, mb_flags, &bases))
 		ret = -1;
 	else if (commit->object.flags & PARENT2)
 		ret = 1;

index 6012402dfc..a3f2cd80eb 100644
@@ -17,10 +17,15 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit *one, size_t n,
 			      struct commit **twos,
 			      struct commit_list **result);
+enum merge_base_flags {
+	MERGE_BASE_IGNORE_MISSING_COMMITS = (1 << 0),
+};
+
 /* To be used only when object flags after this call no longer matter */
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one, size_t n,
 				    struct commit **twos,
+				    enum merge_base_flags mb_flags,
 				    struct commit_list **result);
 
 int get_octopus_merge_bases(struct commit_list *in, struct commit_list **result);

gitgitgadget

From: Kristofer Karlsson <[email protected]>

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

---
 builtin/merge-base.c  |  3 ++-
 commit-reach.c        | 19 +++++++++++++++----
 commit-reach.h        |  7 ++++++-
 t/t6600-test-reach.sh | 40 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 63 insertions(+), 6 deletions(-)

index 9b50b4660e..a87011c6cd 100644
@@ -11,11 +11,12 @@
 
 static int show_merge_base(struct commit **rev, size_t rev_nr, int show_all)
 {
+	enum merge_base_flags flags = show_all ? MERGE_BASE_FIND_ALL : 0;
 	struct commit_list *result = NULL, *r;
 
 	if (repo_get_merge_bases_many_dirty(the_repository, rev[0],
 					    rev_nr - 1, rev + 1,
-					    0, &result) < 0) {
+					    flags, &result) < 0) {
 		commit_list_free(result);
 		return -1;
 	}

index 766ba1156a..5a52be90a6 100644
@@ -97,6 +97,14 @@ static int paint_down_to_common(struct repository *r,
 			if (!(commit->object.flags & RESULT)) {
 				commit->object.flags |= RESULT;
 				tail = commit_list_append(commit, tail);
+				/*
+				 * The queue is generation-ordered; no
+				 * remaining common ancestor can be a
+				 * descendant of this one.
+				 */
+				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
+				    generation < GENERATION_NUMBER_INFINITY)
+					break;
 			}
 			/* Mark parents of a found merge stale */
 			flags |= STALE;

@@ -247,7 +255,8 @@ static int remove_redundant_no_gen(struct repository *r,
 				min_generation = curr_generation;
 		}
 		if (paint_down_to_common(r, array[i], filled,
-					 work, min_generation, 0, &common)) {
+					 work, min_generation,
+					 MERGE_BASE_FIND_ALL, &common)) {
 			clear_commit_marks(array[i], all_flags);
 			clear_commit_marks_many(filled, work, all_flags);
 			commit_list_free(common);

@@ -477,7 +486,8 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit **twos,
 			      struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, n, twos, 1, 0, result);
+	return get_merge_bases_many_0(r, one, n, twos, 1,
+				     MERGE_BASE_FIND_ALL, result);
 }
 
 int repo_get_merge_bases_many_dirty(struct repository *r,

@@ -495,7 +505,8 @@ int repo_get_merge_bases(struct repository *r,
 			 struct commit *two,
 			 struct commit_list **result)
 {
-	return get_merge_bases_many_0(r, one, 1, &two, 1, 0, result);
+	return get_merge_bases_many_0(r, one, 1, &two, 1,
+				     MERGE_BASE_FIND_ALL, result);
 }
 
 /*

@@ -540,7 +551,7 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 	struct commit_list *bases = NULL;
 	int ret = 0, i;
 	timestamp_t generation, max_generation = GENERATION_NUMBER_ZERO;
-	enum merge_base_flags mb_flags = 0;
+	enum merge_base_flags mb_flags = MERGE_BASE_FIND_ALL;
 
 	if (ignore_missing_commits)
 		mb_flags |= MERGE_BASE_IGNORE_MISSING_COMMITS;

index a3f2cd80eb..3f3a563d8a 100644
@@ -19,9 +19,14 @@ int repo_get_merge_bases_many(struct repository *r,
 			      struct commit_list **result);
 enum merge_base_flags {
 	MERGE_BASE_IGNORE_MISSING_COMMITS = (1 << 0),
+	MERGE_BASE_FIND_ALL               = (1 << 1),
 };
 
-/* To be used only when object flags after this call no longer matter */
+/*
+ * To be used only when object flags after this call no longer matter.
+ * Without MERGE_BASE_FIND_ALL and with generation numbers available,
+ * returns after finding the first merge-base, skipping the STALE drain.
+ */
 int repo_get_merge_bases_many_dirty(struct repository *r,
 				    struct commit *one, size_t n,
 				    struct commit **twos,

index dc0421ed2f..51c23b7683 100755
@@ -882,4 +882,44 @@ test_expect_success 'rev-list --maximal-only matches merge-base --independent' '
 	test_cmp expect.sorted actual.sorted
 '
 
+# The following tests verify the early-exit optimisation in
+# paint_down_to_common when merge-base is invoked without --all.
+# Each test checks all four commit-graph configurations.
+
+merge_base_all_modes () {
+	test_when_finished rm -rf .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-full .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-half .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual &&
+	cp commit-graph-no-gdat .git/objects/info/commit-graph &&
+	git merge-base "$@" >actual &&
+	test_cmp expect actual
+}
+
+test_expect_success 'merge-base without --all (unique base)' '
+	git rev-parse commit-5-3 >expect &&
+	merge_base_all_modes commit-5-7 commit-8-3
+'
+
+test_expect_success 'merge-base without --all is one of --all results' '
+	test_when_finished rm -rf .git/objects/info/commit-graph &&
+
+	cp commit-graph-full .git/objects/info/commit-graph &&
+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
+	test_line_count = 1 single &&
+	grep -F -f single all &&
+
+	cp commit-graph-half .git/objects/info/commit-graph &&
+	git merge-base --all commit-5-7 commit-4-8 commit-6-6 commit-8-3 >all &&
+	git merge-base commit-5-7 commit-4-8 commit-6-6 commit-8-3 >single &&
+	test_line_count = 1 single &&
+	grep -F -f single all
+'
+
 test_done

gitgitgadget

"Kristofer Karlsson via GitGitGadget" <[email protected]>

Very nicely done and well described.

Thank you!

As for the difference in variable name, I will attribute it to a mix of oversight and personal preference to keep variable names short if their scope is very small (and longer names for things like fields or larger scope).
In fact, I might have preferred flags instead of mb_flags within paint_down_to_common but it conflicted with the existing flags for the commit so I had to differentiate them.

That said, I think it would also be fair to rename it to mb_flags everywhere.