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, 7 insertions, 4 deletions

diff --git a/walker.c b/walker.c
index b470d43e54..c6d0e20c72 100644
--- a/walker.c
+++ b/walker.c
@@ -14,6 +14,7 @@
 #include "blob.h"
 #include "refs.h"
 #include "progress.h"
+#include "prio-queue.h"
 
 static struct object_id current_commit_oid;
 
@@ -78,7 +79,7 @@ static int process_tree(struct walker *walker, struct tree *tree)
 #define SEEN		(1U << 1)
 #define TO_SCAN		(1U << 2)
 
-static struct commit_list *complete = NULL;
+static struct prio_queue complete = { compare_commits_by_commit_date };
 
 static int process_commit(struct walker *walker, struct commit *commit)
 {
@@ -87,7 +88,10 @@ static int process_commit(struct walker *walker, struct commit *commit)
 	if (repo_parse_commit(the_repository, commit))
 		return -1;
 
-	while (complete && complete->item->date >= commit->date) {
+	while (complete.nr) {
+		struct commit *item = prio_queue_peek(&complete);
+		if (item->date < commit->date)
+			break;
 		pop_most_recent_commit(&complete, COMPLETE);
 	}
 
@@ -233,7 +237,7 @@ static int mark_complete(const char *path UNUSED,
 
 	if (commit) {
 		commit->object.flags |= COMPLETE;
-		commit_list_insert(commit, &complete);
+		prio_queue_put(&complete, commit);
 	}
 	return 0;
 }
@@ -302,7 +306,6 @@ int walker_fetch(struct walker *walker, int targets, char **target,
 	if (!walker->get_recover) {
 		refs_for_each_ref(get_main_ref_store(the_repository),
 				  mark_complete, NULL);
-		commit_list_sort_by_date(&complete);
 	}
 
 	for (i = 0; i < targets; i++) {