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peel_ref: check object type before loading

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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1 parent e6dbffa commit 6c4a060d7d895c90610eb442cb4f910fe43ef13a @peff peff committed with gitster Oct 4, 2012
Showing with 9 additions and 2 deletions.
  1. +9 −2 refs.c
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11 refs.c
@@ -1225,8 +1225,15 @@ int peel_ref(const char *refname, unsigned char *sha1)
}
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);

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