[PATCH v3 7/9] commit: add short-circuit to paint_down_to_common()

[Derrick Stolee]

When running 'git branch --contains', the in_merge_bases_many()
method calls paint_down_to_common() to discover if a specific
commit is reachable from a set of branches. Commits with lower
generation number are not needed to correctly answer the
containment query of in_merge_bases_many().

Add a new parameter, min_generation, to paint_down_to_common() that
prevents walking commits with generation number strictly less than
min_generation. If 0 is given, then there is no functional change.

For in_merge_bases_many(), we can pass commit->generation as the
cutoff, and this saves time during 'git branch --contains' queries
that would otherwise walk "around" the commit we are inspecting.

For a copy of the Linux repository, where HEAD is checked out at
v4.13~100, we get the following performance improvement for
'git branch --contains' over the previous commit:

Before: 0.21s
After:  0.13s
Rel %: -38%

Signed-off-by: Derrick Stolee <dsto...@microsoft.com>
---
 commit.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/commit.c b/commit.c
index bceb79c419..a70f120878 100644
--- a/commit.c
+++ b/commit.c
@@ -805,11 +805,14 @@ static int queue_has_nonstale(struct prio_queue *queue)
 }
 
 /* all input commits in one and twos[] must have been parsed! */
-static struct commit_list *paint_down_to_common(struct commit *one, int n, 
struct commit **twos)
+static struct commit_list *paint_down_to_common(struct commit *one, int n,
+                                               struct commit **twos,
+                                               int min_generation)
 {
        struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
        struct commit_list *result = NULL;
        int i;
+       uint32_t last_gen = GENERATION_NUMBER_INFINITY;
 
        one->object.flags |= PARENT1;
        if (!n) {
@@ -828,6 +831,13 @@ static struct commit_list *paint_down_to_common(struct 
commit *one, int n, struc
                struct commit_list *parents;
                int flags;
 
+               if (commit->generation > last_gen)
+                       BUG("bad generation skip");
+               last_gen = commit->generation;
+
+               if (commit->generation < min_generation)
+                       break;
+
                flags = commit->object.flags & (PARENT1 | PARENT2 | STALE);
                if (flags == (PARENT1 | PARENT2)) {
                        if (!(commit->object.flags & RESULT)) {
@@ -876,7 +886,7 @@ static struct commit_list *merge_bases_many(struct commit 
*one, int n, struct co
                        return NULL;
        }
 
-       list = paint_down_to_common(one, n, twos);
+       list = paint_down_to_common(one, n, twos, 0);
 
        while (list) {
                struct commit *commit = pop_commit(&list);
@@ -943,7 +953,7 @@ static int remove_redundant(struct commit **array, int cnt)
                        filled_index[filled] = j;
                        work[filled++] = array[j];
                }
-               common = paint_down_to_common(array[i], filled, work);
+               common = paint_down_to_common(array[i], filled, work, 0);
                if (array[i]->object.flags & PARENT2)
                        redundant[i] = 1;
                for (j = 0; j < filled; j++)
@@ -1067,7 +1077,7 @@ int in_merge_bases_many(struct commit *commit, int 
nr_reference, struct commit *
        if (commit->generation > min_generation)
                return 0;
 
-       bases = paint_down_to_common(commit, nr_reference, reference);
+       bases = paint_down_to_common(commit, nr_reference, reference, 
commit->generation);
        if (commit->object.flags & PARENT2)
                ret = 1;
        clear_commit_marks(commit, all_flags);
-- 
2.17.0.39.g685157f7fb