[PATCH 3/4] peel_ref: check object type before loading
- From
Jeff King <peff@peff.net>
- Date
- Oct 4, 2012, 08:02 UTC
- Message-ID
- <20121004080253.GC31325@sigill.intra.peff.net>
- In-Reply-To
- <20121004075609.GA1355@sigill.intra.peff.net>
The point of peel_ref is to dereference tags; if the base object is not a tag, then we can return early without even loading the object into memory.
This patch accomplishes that by checking sha1_object_info for the type. For a packed object, we can get away with just looking in the pack index. For a loose object, we only need to inflate the first couple of header bytes.
This is a bit of a gamble; if we do find a tag object, then we will end up loading the content anyway, and the extra lookup will have been wasteful. However, if it is not a tag object, then we save loading the object entirely. Depending on the ratio of non-tags to tags in the input, this can be a minor win or minor loss.
However, it does give us one potential major win: if a ref points to a large blob (e.g., via an unannotated tag), then we can avoid looking at it entirely.
Signed-off-by: Jeff King <peff@peff.net> --- This optimization is the one that gave me the most pause. While upload-pack does call peel_ref on everything, the other callers all constrain themselves to refs/tags/. So for many projects, we will be calling it mostly on annotated tags, and it may be a very small net loss. But in practice, it will not matter for most projects with a sane number of normal tags, and saving even one accidental giant blob load can have a huge impact.
refs.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/refs.c b/refs.c index f672ad9..02e47b1 100644 --- a/refs.c +++ b/refs.c @@ -1225,8 +1225,15 @@ fallback: } fallback: - o = parse_object(base); - if (o && o->type == OBJ_TAG) { + o = lookup_unknown_object(base); + if (o->type == OBJ_NONE) { + int type = sha1_object_info(base, NULL); + if (type < 0) + return -1; + o->type = type; + } + + if (o->type == OBJ_TAG) { o = deref_tag_noverify(o); if (o) { hashcpy(sha1, o->sha1);
-- 1.8.0.rc0.10.g8dd2a92