diffcore-rename: cache file deltas

We find rename candidates by computing a fingerprint hash of
each file, and then comparing those fingerprints. There are
inherently O(n^2) comparisons, so it pays in CPU time to
hoist the (rather expensive) computation of the fingerprint
out of that loop (or to cache it once we have computed it once).

Previously, we didn't keep the filespec information around
because then we had the potential to consume a great deal of
memory. However, instead of keeping all of the filespec
data, we can instead just keep the fingerprint.

This patch implements and uses diff_free_filespec_data_large
to accomplish that goal. We also have to change
estimate_similarity not to needlessly repopulate the
filespec data when we already have the hash.

Practical tests showed 4.5x speedup for a 10% memory usage
increase.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -1675,7 +1675,7 @@ int diff_populate_filespec(struct diff_filespec *s, int size_only)
 	return 0;
 }
 
-void diff_free_filespec_data(struct diff_filespec *s)
+void diff_free_filespec_data_large(struct diff_filespec *s)
 {
 	if (s->should_free)
 		free(s->data);
@@ -1686,6 +1686,11 @@ void diff_free_filespec_data(struct diff_filespec *s)
 		s->should_free = s->should_munmap = 0;
 		s->data = NULL;
 	}
+}
+
+void diff_free_filespec_data(struct diff_filespec *s)
+{
+	diff_free_filespec_data_large(s);
 	free(s->cnt_data);
 	s->cnt_data = NULL;
 }

diffcore-rename.c

@@ -184,7 +184,8 @@ static int estimate_similarity(struct diff_filespec *src,
 	if (base_size * (MAX_SCORE-minimum_score) < delta_size * MAX_SCORE)
 		return 0;
 
-	if (diff_populate_filespec(src, 0) || diff_populate_filespec(dst, 0))
+	if ((!src->cnt_data && diff_populate_filespec(src, 0))
+		|| (!dst->cnt_data && diff_populate_filespec(dst, 0)))
 		return 0; /* error but caught downstream */
 
 
@@ -377,10 +378,10 @@ void diffcore_rename(struct diff_options *options)
 			m->score = estimate_similarity(one, two,
 						       minimum_score);
 			m->name_score = basename_same(one, two);
-			diff_free_filespec_data(one);
+			diff_free_filespec_data_large(one);
 		}
 		/* We do not need the text anymore */
-		diff_free_filespec_data(two);
+		diff_free_filespec_data_large(two);
 		dst_cnt++;
 	}
 	/* cost matrix sorted by most to least similar pair */

diffcore.h

@@ -48,6 +48,7 @@ extern void fill_filespec(struct diff_filespec *, const unsigned char *,
 
 extern int diff_populate_filespec(struct diff_filespec *, int);
 extern void diff_free_filespec_data(struct diff_filespec *);
+extern void diff_free_filespec_data_large(struct diff_filespec *);
 extern int diff_filespec_is_binary(struct diff_filespec *);
 
 struct diff_filepair {