Fix quadratic performance in rewrite_one.

Parent commits are usually older than their children. Thus,
on each iteration of the loop in rewrite_one, add_parents_to_list
traverses all commits previously processed by the loop.
It performs very poorly in case of very long rewrite chains.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/revision.c b/revision.c
index fc667552592daa3c894d0df4e97469d1809dbf27..d464f14d500500b22fea74723a0fe69819f2adc7 100644 (file)
--- a/revision.c
+++ b/revision.c
@@ -412,10 +412,26 @@ static void try_to_simplify_commit(struct rev_info *revs, struct commit *commit)
        commit->object.flags |= TREESAME;
 }
 
-static int add_parents_to_list(struct rev_info *revs, struct commit *commit, struct commit_list **list)
+static void insert_by_date_cached(struct commit *p, struct commit_list **head,
+                   struct commit_list *cached_base, struct commit_list **cache)
+{
+       struct commit_list *new_entry;
+
+       if (cached_base && p->date < cached_base->item->date)
+               new_entry = insert_by_date(p, &cached_base->next);
+       else
+               new_entry = insert_by_date(p, head);
+
+       if (cache && (!*cache || p->date < (*cache)->item->date))
+               *cache = new_entry;
+}
+
+static int add_parents_to_list(struct rev_info *revs, struct commit *commit,
+                   struct commit_list **list, struct commit_list **cache_ptr)
 {
        struct commit_list *parent = commit->parents;
        unsigned left_flag;
+       struct commit_list *cached_base = cache_ptr ? *cache_ptr : NULL;
 
        if (commit->object.flags & ADDED)
                return 0;
@@ -445,7 +461,7 @@ static int add_parents_to_list(struct rev_info *revs, struct commit *commit, str
                        if (p->object.flags & SEEN)
                                continue;
                        p->object.flags |= SEEN;
-                       insert_by_date(p, list);
+                       insert_by_date_cached(p, list, cached_base, cache_ptr);
                }
                return 0;
        }
@@ -470,7 +486,7 @@ static int add_parents_to_list(struct rev_info *revs, struct commit *commit, str
                p->object.flags |= left_flag;
                if (!(p->object.flags & SEEN)) {
                        p->object.flags |= SEEN;
-                       insert_by_date(p, list);
+                       insert_by_date_cached(p, list, cached_base, cache_ptr);
                }
                if(revs->first_parent_only)
                        break;
@@ -611,7 +627,7 @@ static int limit_list(struct rev_info *revs)
 
                if (revs->max_age != -1 && (commit->date < revs->max_age))
                        obj->flags |= UNINTERESTING;
-               if (add_parents_to_list(revs, commit, &list) < 0)
+               if (add_parents_to_list(revs, commit, &list, NULL) < 0)
                        return -1;
                if (obj->flags & UNINTERESTING) {
                        mark_parents_uninteresting(commit);
@@ -1458,10 +1474,12 @@ enum rewrite_result {
 
 static enum rewrite_result rewrite_one(struct rev_info *revs, struct commit **pp)
 {
+       struct commit_list *cache = NULL;
+
        for (;;) {
                struct commit *p = *pp;
                if (!revs->limited)
-                       if (add_parents_to_list(revs, p, &revs->commits) < 0)
+                       if (add_parents_to_list(revs, p, &revs->commits, &cache) < 0)
                                return rewrite_one_error;
                if (p->parents && p->parents->next)
                        return rewrite_one_ok;
@@ -1580,7 +1598,7 @@ static struct commit *get_revision_1(struct rev_info *revs)
                        if (revs->max_age != -1 &&
                            (commit->date < revs->max_age))
                                continue;
-                       if (add_parents_to_list(revs, commit, &revs->commits) < 0)
+                       if (add_parents_to_list(revs, commit, &revs->commits, NULL) < 0)
                                return NULL;
                }