[RFC PATCH 13/13] commit-reach: use can_all_from_reach

[Derrick Stolee <dstolee@microsoft.com>]

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.

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: 0.31 s
 After: 0.27 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':

Before: 0.12 s
 After: 0.11 s

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---

One thing I know is missing from this commit is a special-case to use
the old logic when there is no commit-graph present. The
can_all_from_reach() algorithm can be worse when we do not have good
generation number cutoffs. In the previous case of
can_all_from_reach_with_flags(), we already had an established pattern
of using commit date as a cutoff, so the generation number is only a
second cutoff and the algorithm cannot walk more commits than before.

 commit-reach.c        | 34 +++++++++++++++++-----------------
 commit-reach.h        |  3 ++-
 t/helper/test-reach.c |  2 +-
 3 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/commit-reach.c b/commit-reach.c
index 249e9a4fac..a823d6965c 100644
--- a/commit-reach.c
+++ b/commit-reach.c
@@ -273,17 +273,15 @@ struct commit_list *get_merge_bases(struct commit *one, struct commit *two)
  */
 int is_descendant_of(struct commit *commit, struct commit_list *with_commit)
 {
+	struct commit_list *from_list = NULL;
+	int result;
 	if (!with_commit)
 		return 1;
-	while (with_commit) {
-		struct commit *other;
 
-		other = with_commit->item;
-		with_commit = with_commit->next;
-		if (in_merge_bases(other, commit))
-			return 1;
-	}
-	return 0;
+	commit_list_insert(commit, &from_list);
+	result = can_all_from_reach(from_list, with_commit, 0);
+	free_commit_list(from_list);
+	return result;
 }
 
 /*
@@ -605,10 +603,11 @@ int can_all_from_reach_with_flag(struct object_array *from,
 	return result;
 }
 
-int can_all_from_reach(struct commit_list *from, struct commit_list *to)
+int can_all_from_reach(struct commit_list *from, struct commit_list *to,
+		       int cutoff_by_min_date)
 {
 	struct object_array from_objs = OBJECT_ARRAY_INIT;
-	time_t min_commit_date = from->item->date;
+	time_t min_commit_date = cutoff_by_min_date ? from->item->date : 0;
 	struct commit_list *from_iter = from;
 	struct commit_list *to_iter = to;
 	int result;
@@ -617,20 +616,21 @@ int can_all_from_reach(struct commit_list *from, struct commit_list *to)
 	while (from_iter) {
 		add_object_array(&from_iter->item->object, NULL, &from_objs);
 
-		if (from_iter->item->date < min_commit_date)
+		if (!parse_commit(from_iter->item) &&
+		    from_iter->item->date < min_commit_date)
 			min_commit_date = from_iter->item->date;
 
 		from_iter = from_iter->next;
 	}
 
 	while (to_iter) {
-		parse_commit(to_iter->item);
-
-		if (to_iter->item->date < min_commit_date)
-			min_commit_date = to_iter->item->date;
+		if (!parse_commit(to_iter->item)) {
+			if (to_iter->item->date < min_commit_date)
+				min_commit_date = to_iter->item->date;
 
-		if (to_iter->item->generation < min_generation)
-			min_generation = to_iter->item->generation;
+			if (to_iter->item->generation < min_generation)
+				min_generation = to_iter->item->generation;
+		}
 
 		to_iter->item->object.flags |= PARENT2;
 
diff --git a/commit-reach.h b/commit-reach.h
index 3eb4c057e6..180f865d7d 100644
--- a/commit-reach.h
+++ b/commit-reach.h
@@ -31,6 +31,7 @@ int can_all_from_reach_with_flag(struct object_array *from, int with_flag,
 				 int assign_flag, time_t min_commit_date,
 				 uint32_t min_generation);
 
-int can_all_from_reach(struct commit_list *from, struct commit_list *to);
+int can_all_from_reach(struct commit_list *from, struct commit_list *to,
+		       int cutoff_by_min_date);
 
 #endif
diff --git a/t/helper/test-reach.c b/t/helper/test-reach.c
index 14aaef5bff..c05137c9f3 100644
--- a/t/helper/test-reach.c
+++ b/t/helper/test-reach.c
@@ -120,7 +120,7 @@ int cmd__reach(int ac, const char **av)
 			list = list->next;
 		}
 	} else if (!strcmp(av[1], "can_all_from_reach")) {
-		int result = can_all_from_reach(list_X, list_Y);
+		int result = can_all_from_reach(list_X, list_Y, 1);
 		printf("%s(X,Y):%d\n", av[1], result);
 	}
 
-- 
2.18.0.118.gd4f65b8d14