commit-reach: use can_all_from_reach

The is_descendant_of method previously used in_merge_bases() to check if
the commit can reach any of the commits in the provided list. This had
two performance problems:

1. The performance is quadratic in worst-case.

2. A single in_merge_bases() call requires walking beyond the target
   commit in order to find the full set of boundary commits that may be
   merge-bases.

The can_all_from_reach method avoids this quadratic behavior and can
limit the search beyond the target commits using generation numbers. It
requires a small prototype adjustment to stop using commit-date as a
cutoff, as that optimization is no longer appropriate here.

Since in_merge_bases() uses paint_down_to_common(), is_descendant_of()
naturally found cutoffs to avoid walking the entire commit graph. Since
we want to always return the correct result, we cannot use the
min_commit_date cutoff in can_all_from_reach. We then rely on generation
numbers to provide the cutoff.

Since not all repos will have a commit-graph file, nor will we always
have generation numbers computed for a commit-graph file, create a new
method, generation_numbers_enabled(), that checks for a commit-graph
file and sees if the first commit in the file has a non-zero generation
number. In the case that we do not have generation numbers, use the old
logic for is_descendant_of().

Performance was meausured on a copy of the Linux repository using the
'test-tool reach is_descendant_of' command using this input:

A:v4.9
X:v4.10
X:v4.11
X:v4.12
X:v4.13
X:v4.14
X:v4.15
X:v4.16
X:v4.17
X.v3.0

Note that this input is tailored to demonstrate the quadratic nature of
the previous method, as it will compute merge-bases for v4.9 versus all
of the later versions before checking against v4.1.

Before: 0.26 s
 After: 0.21 s

Since we previously used the is_descendant_of method in the ref_newer
method, we also measured performance there using
'test-tool reach ref_newer' with this input:

A:v4.9
B:v3.19

Before: 0.10 s
 After: 0.08 s

By adding a new commit with parent v3.19, we test the non-reachable case
of ref_newer:

Before: 0.09 s
 After: 0.08 s

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/commit-graph.c b/commit-graph.c
index b0a55ad128..e9786fa864 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -233,6 +233,24 @@ static int prepare_commit_graph(struct repository *r)
 	return !!r->objects->commit_graph;
 }
 
+int generation_numbers_enabled(struct repository *r)
+{
+	uint32_t first_generation;
+	struct commit_graph *g;
+	if (!prepare_commit_graph(r))
+	       return 0;
+
+	g = r->objects->commit_graph;
+
+	if (!g->num_commits)
+		return 0;
+
+	first_generation = get_be32(g->chunk_commit_data +
+				    g->hash_len + 8) >> 2;
+
+	return !!first_generation;
+}
+
 static void close_commit_graph(void)
 {
 	free_commit_graph(the_repository->objects->commit_graph);