Subject: [PATCH 1/7] commit: simplify fastpath of merge-base computation

From: Junio C Hamano <[email-masked]>
Date: Tue, 18 Oct 2016 21:23:39 -0700

The get_merge_bases_many*() family of functions first call
merge_bases_many() to find all possible merge bases between a single
commit "one" and a set of other commits "twos[]".  Because this
typically involves traversing the commit graph, which uses the
object flags on the commits involved, the function needs to clear
the flags before it returns.

Except for one special case.  If "one" is exactly one of the "twos",
"one" is the merge base and merge_bases_many() returns a list of
merge bases with a single element, "one", on it, without smudging
the object flags of the commits at all.

We used to use a loop over "twos[]" array to see if this fast-path
would have kicked in in merge_bases_many(), in which case we can
return without cleaning the bits at all.  We can do the same by
inspecting the result in a more direct way, which is conceptually
cleaner.

Signed-off-by: Junio C Hamano <[email-masked]>
---
 commit.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/commit.c b/commit.c
index aada266f9a..6266c0380c 100644
--- a/commit.c
+++ b/commit.c
@@ -955,10 +955,17 @@ static struct commit_list *get_merge_bases_many_0(struct commit *one,
 	int cnt, i;
 
 	result = merge_bases_many(one, n, twos);
-	for (i = 0; i < n; i++) {
-		if (one == twos[i])
-			return result;
-	}
+
+	/*
+	 * The fast-path of 'one' being the merge-base; there is no
+	 * need to clean the object flags in this case.
+	 */
+	if (result && !result->next &&
+	    result->item == one &&
+	    !(one->object.flags & RESULT))
+		return result;
+
+	/* If we didn't get any, or there is only one, we are done */
 	if (!result || !result->next) {
 		if (cleanup) {
 			clear_commit_marks(one, all_flags);
-- 
2.10.1-631-gb2c64dcf30