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@xxxxxxxxxxxxx>
---
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
