[PATCH] Rename detection: Avoid repeated filespec population

["Björn Steinbrink" <B.Steinbrink@gmx.de>]

In diffcore_rename, we assume that the blob contents in the filespec
aren't required anymore after estimate_similarity has been called and thus
we free it. But estimate_similarity might return early when the file sizes
differ too much. In that case, cnt_data is never set and the next call to
estimate_similarity will populate the filespec again, eventually rereading
the same blob over and over again.

To fix that, we first get the blob sizes and only when the blob contents
are actually required, and when cnt_data will be set, the full filespec is
populated, once.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
---
This actually affects the copy detection way more than the pure
rename detection, due to the larger number of candidates, but it's the
same code, and I only realized that when I reran the stuff to get some
numbers to show off. ;-)

Test with linux-2.6.git and hot caches:
git diff-tree -p db30c705758^2..db30c705758

	| unpatched | patched
------------------------------
plain	|        3s |     3s
-M	|        4s |     3s
-C	|    2m 25s |     9s
-C -C	|    4m 30s |    24s


 diffcore-rename.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/diffcore-rename.c b/diffcore-rename.c
index 168a95b..0b0d6b8 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -153,9 +153,9 @@ static int estimate_similarity(struct diff_filespec *src,
 	 * is a possible size - we really should have a flag to
 	 * say whether the size is valid or not!)
 	 */
-	if (!src->cnt_data && diff_populate_filespec(src, 0))
+	if (!src->cnt_data && diff_populate_filespec(src, 1))
 		return 0;
-	if (!dst->cnt_data && diff_populate_filespec(dst, 0))
+	if (!dst->cnt_data && diff_populate_filespec(dst, 1))
 		return 0;
 
 	max_size = ((src->size > dst->size) ? src->size : dst->size);
@@ -173,6 +173,11 @@ static int estimate_similarity(struct diff_filespec *src,
 	if (base_size * (MAX_SCORE-minimum_score) < delta_size * MAX_SCORE)
 		return 0;
 
+	if (!src->cnt_data && diff_populate_filespec(src, 0))
+		return 0;
+	if (!dst->cnt_data && diff_populate_filespec(dst, 0))
+		return 0;
+
 	delta_limit = (unsigned long)
 		(base_size * (MAX_SCORE-minimum_score) / MAX_SCORE);
 	if (diffcore_count_changes(src, dst,
-- 
1.6.1.135.g3cf3b