commit: convert pop_most_recent_commit() to prio_queue

pop_most_recent_commit() calls commit_list_insert_by_date() for parent
commits, which is itself called in a loop.  This can lead to quadratic
complexity if there are many merges.  Replace the commit_list with a
prio_queue to ensure logarithmic worst case complexity and convert all
three users.

Add a performance test that exercises one of them using a pathological
history that consists of 50% merges and 50% root commits to demonstrate
the speedup:

Test                          v2.50.1           HEAD
----------------------------------------------------------------------
1501.2: rev-parse ':/65535'   2.48(2.47+0.00)   0.20(0.19+0.00) -91.9%

Alas, sane histories don't benefit from the conversion much, and
traversing Git's own history takes a 1% performance hit on my machine:

   $ hyperfine -w3 -L git ./git_2.50.1,./git '{git} rev-parse :/^Initial.revision'
   Benchmark 1: ./git_2.50.1 rev-parse :/^Initial.revision
     Time (mean ± σ):      1.071 s ±  0.004 s    [User: 1.052 s, System: 0.017 s]
     Range (min … max):    1.067 s …  1.078 s    10 runs

   Benchmark 2: ./git rev-parse :/^Initial.revision
     Time (mean ± σ):      1.079 s ±  0.003 s    [User: 1.060 s, System: 0.017 s]
     Range (min … max):    1.074 s …  1.083 s    10 runs

   Summary
     ./git_2.50.1 rev-parse :/^Initial.revision ran
       1.01 ± 0.00 times faster than ./git rev-parse :/^Initial.revision

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/object-name.c b/object-name.c
index 9288b2dd24..8a787b3ff3 100644
--- a/object-name.c
+++ b/object-name.c
@@ -28,6 +28,7 @@
 #include "commit-reach.h"
 #include "date.h"
 #include "object-file-convert.h"
+#include "prio-queue.h"
 
 static int get_oid_oneline(struct repository *r, const char *, struct object_id *,
 			   const struct commit_list *);
@@ -1457,7 +1458,7 @@ static int get_oid_oneline(struct repository *r,
 			   const char *prefix, struct object_id *oid,
 			   const struct commit_list *list)
 {
-	struct commit_list *copy = NULL, **copy_tail = ©
+	struct prio_queue copy = { compare_commits_by_commit_date };
 	const struct commit_list *l;
 	int found = 0;
 	int negative = 0;
@@ -1479,9 +1480,9 @@ static int get_oid_oneline(struct repository *r,
 
 	for (l = list; l; l = l->next) {
 		l->item->object.flags |= ONELINE_SEEN;
-		copy_tail = &commit_list_insert(l->item, copy_tail)->next;
+		prio_queue_put(&copy, l->item);
 	}
-	while (copy) {
+	while (copy.nr) {
 		const char *p, *buf;
 		struct commit *commit;
 		int matches;
@@ -1503,7 +1504,7 @@ static int get_oid_oneline(struct repository *r,
 	regfree(&regex);
 	for (l = list; l; l = l->next)
 		clear_commit_marks(l->item, ONELINE_SEEN);
-	free_commit_list(copy);
+	clear_prio_queue(&copy);
 	return found ? 0 : -1;
 }
 
@@ -2057,7 +2058,6 @@ static enum get_oid_result get_oid_with_context_1(struct repository *repo,
 			cb.list = &list;
 			refs_for_each_ref(get_main_ref_store(repo), handle_one_ref, &cb);
 			refs_head_ref(get_main_ref_store(repo), handle_one_ref, &cb);
-			commit_list_sort_by_date(&list);
 			ret = get_oid_oneline(repo, name + 2, oid, list);
 
 			free_commit_list(list);