commit: use generation numbers for in_merge_bases()

The containment algorithm for 'git branch --contains' is different
from that for 'git tag --contains' in that it uses is_descendant_of()
instead of contains_tag_algo(). The expensive portion of the branch
algorithm is computing merge bases.

When a commit-graph file exists with generation numbers computed,
we can avoid this merge-base calculation when the target commit has
a larger generation number than the initial commits.

Performance tests were run on a copy of the Linux repository where
HEAD is contained in v4.13 but no earlier tag. Also, all tags were
copied to branches and 'git branch --contains' was tested:

Before: 60.0s
After:   0.4s
Rel %: -99.3%

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/commit.c b/commit.c
index 39a3749abd71536649e91dac2eacbfcb9f00eadd..3ecdc1335639783f1a9a262ccf28cb2d500792ad 100644 (file)
--- a/commit.c
+++ b/commit.c
@@ -1056,12 +1056,19 @@ int in_merge_bases_many(struct commit *commit, int nr_reference, struct commit *
 {
        struct commit_list *bases;
        int ret = 0, i;
+       uint32_t min_generation = GENERATION_NUMBER_INFINITY;
 
        if (parse_commit(commit))
                return ret;
-       for (i = 0; i < nr_reference; i++)
+       for (i = 0; i < nr_reference; i++) {
                if (parse_commit(reference[i]))
                        return ret;
+               if (reference[i]->generation < min_generation)
+                       min_generation = reference[i]->generation;
+       }
+
+       if (commit->generation > min_generation)
+               return ret;
 
        bases = paint_down_to_common(commit, nr_reference, reference);
        if (commit->object.flags & PARENT2)