[PATCH 1/3] ref-filter: avoid parsing tagged objects in match_points_at()

From: Jeff King <peff@peff.net>
Date: Jul 2, 2023, 22:35 UTC
Message-ID: <20230702223540.GA3494279@coredump.intra.peff.net>
In-Reply-To: <20230702223342.GA1598765@coredump.intra.peff.net>

When we peel tags to check if they match a --points-at oid, we recursively parse the tagged object to see if it is also a tag. But since the tag itself tells us the type of the object it points to (and even gives us the appropriate object struct via its "tagged" member), we can use that directly.

We do still have to make sure to call parse_tag() before looking at each tag. This is redundant for the outermost tag (since we did call parse_object() to find its type), but that's OK; parse_tag() is smart enough to make this a noop when the tag has already been parsed.

In my clone of linux.git, with 782 tags (and only 3 non-tags), this yields a significant speedup (bringing us back where we were before the commit before this one started recursively dereferencing tags):

  Benchmark 1: ./git.old for-each-ref --points-at=HEAD --format="%(refname)"
    Time (mean ± σ):      20.3 ms ±   0.5 ms    [User: 11.1 ms, System: 9.1 ms]
    Range (min … max):    19.6 ms …  21.5 ms    141 runs

  Benchmark 2: ./git.new for-each-ref --points-at=HEAD --format="%(refname)"
    Time (mean ± σ):      11.4 ms ±   0.2 ms    [User: 6.3 ms, System: 5.0 ms]
    Range (min … max):    11.0 ms …  12.2 ms    250 runs

  Summary
    './git.new for-each-ref --points-at=HEAD --format="%(refname)"' ran
      1.79 ± 0.05 times faster than './git.old for-each-ref --points-at=HEAD --format="%(refname)"'

Signed-off-by: Jeff King <peff@peff.net>
---
This could optionally be squashed into the original. If we leave it as
its own patch, it might be worth replacing "the commit before this one"
with the actual oid (once Junio has picked it up and it's stable).

 ref-filter.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/ref-filter.c b/ref-filter.c
index ad7f244414..e091f056ab 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -2225,10 +2225,18 @@ static const struct object_id *match_points_at(struct oid_array *points_at,
 		return oid;
 	obj = parse_object(the_repository, oid);
 	while (obj && obj->type == OBJ_TAG) {
-		oid = get_tagged_oid((struct tag *)obj);
+		struct tag *tag = (struct tag *)obj;
+
+		if (parse_tag(tag) < 0) {
+			obj = NULL;
+			break;
+		}
+
+		oid = get_tagged_oid(tag);
 		if (oid_array_lookup(points_at, oid) >= 0)
 			return oid;
-		obj = parse_object(the_repository, oid);
+
+		obj = tag->tagged;
 	}
 	if (!obj)
 		die(_("malformed object at '%s'"), refname);

-- 
2.41.0.586.g3c0cc15bc7