diff --git a/diff-tree.c b/diff-tree.c
index c66c787..8bdb1db 100644
--- a/diff-tree.c
+++ b/diff-tree.c
@@ -578,6 +578,9 @@
 	if (!read_stdin)
 		return 0;
 
+	if (detect_rename)
+		diff_setup_opt |= (DIFF_SETUP_USE_SIZE_CACHE |
+				   DIFF_SETUP_USE_CACHE);
 	while (fgets(line, sizeof(line), stdin))
 		diff_tree_stdin(line);
 
diff --git a/diff.c b/diff.c
index ebec71a..357c4ef 100644
--- a/diff.c
+++ b/diff.c
@@ -12,6 +12,7 @@
 static unsigned char null_sha1[20] = { 0, };
 
 static int reverse_diff;
+static int use_size_cache;
 
 static const char *external_diff(void)
 {
@@ -222,12 +223,60 @@
 	return 1;
 }
 
+static struct sha1_size_cache {
+	unsigned char sha1[20];
+	unsigned long size;
+} **sha1_size_cache;
+static int sha1_size_cache_nr, sha1_size_cache_alloc;
+
+static struct sha1_size_cache *locate_size_cache(unsigned char *sha1,
+						 unsigned long size)
+{
+	int first, last;
+	struct sha1_size_cache *e;
+
+	first = 0;
+	last = sha1_size_cache_nr;
+	while (last > first) {
+		int next = (last + first) >> 1;
+		e = sha1_size_cache[next];
+		int cmp = memcmp(e->sha1, sha1, 20);
+		if (!cmp)
+			return e;
+		if (cmp < 0) {
+			last = next;
+			continue;
+		}
+		first = next+1;
+	}
+	/* not found */
+	if (size == UINT_MAX)
+		return NULL;
+	/* insert to make it at "first" */
+	if (sha1_size_cache_alloc <= sha1_size_cache_nr) {
+		sha1_size_cache_alloc = alloc_nr(sha1_size_cache_alloc);
+		sha1_size_cache = xrealloc(sha1_size_cache,
+					   sha1_size_cache_alloc *
+					   sizeof(*sha1_size_cache));
+	}
+	sha1_size_cache_nr++;
+	if (first < sha1_size_cache_nr)
+		memmove(sha1_size_cache + first + 1, sha1_size_cache + first,
+			(sha1_size_cache_nr - first - 1) *
+			sizeof(*sha1_size_cache));
+	e = xmalloc(sizeof(struct sha1_size_cache));
+	sha1_size_cache[first] = e;
+	memcpy(e->sha1, sha1, 20);
+	e->size = size;
+	return e;
+}
+
 /*
  * While doing rename detection and pickaxe operation, we may need to
  * grab the data for the blob (or file) for our own in-core comparison.
  * diff_filespec has data and size fields for this purpose.
  */
-int diff_populate_filespec(struct diff_filespec *s)
+int diff_populate_filespec(struct diff_filespec *s, int size_only)
 {
 	int err = 0;
 	if (!DIFF_FILE_VALID(s))
@@ -235,6 +284,9 @@
 	if (S_ISDIR(s->mode))
 		return -1;
 
+	if (!use_size_cache)
+		size_only = 0;
+
 	if (s->data)
 		return err;
 	if (!s->sha1_valid ||
@@ -254,6 +306,8 @@
 		s->size = st.st_size;
 		if (!s->size)
 			goto empty;
+		if (size_only)
+			return 0;
 		if (S_ISLNK(st.st_mode)) {
 			int ret;
 			s->data = xmalloc(s->size);
@@ -273,9 +327,21 @@
 		close(fd);
 	}
 	else {
+		/* We cannot do size only for SHA1 blobs */
 		char type[20];
+		struct sha1_size_cache *e;
+
+		if (size_only) {
+			e = locate_size_cache(s->sha1, UINT_MAX);
+			if (e) {
+				s->size = e->size;
+				return 0;
+			}
+		}
 		s->data = read_sha1_file(s->sha1, type, &s->size);
 		s->should_free = 1;
+		if (s->data && size_only)
+			locate_size_cache(s->sha1, s->size);
 	}
 	return 0;
 }
@@ -361,7 +427,7 @@
 		return;
 	}
 	else {
-		if (diff_populate_filespec(one))
+		if (diff_populate_filespec(one, 0))
 			die("cannot read data blob for %s", one->path);
 		prep_temp_blob(temp, one->data, one->size,
 			       one->sha1, one->mode);
@@ -496,6 +562,19 @@
 {
 	if (flags & DIFF_SETUP_REVERSE)
 		reverse_diff = 1;
+	if (flags & DIFF_SETUP_USE_CACHE) {
+		if (!active_cache)
+			/* read-cache does not die even when it fails
+			 * so it is safe for us to do this here.  Also
+			 * it does not smudge active_cache or active_nr
+			 * when it fails, so we do not have to worry about
+			 * cleaning it up oufselves either.
+			 */
+			read_cache();
+	}
+	if (flags & DIFF_SETUP_USE_SIZE_CACHE)
+		use_size_cache = 1;
+	
 }
 
 struct diff_queue_struct diff_queued_diff;
diff --git a/diff.h b/diff.h
index 40a6757..a07ee9f 100644
--- a/diff.h
+++ b/diff.h
@@ -29,6 +29,8 @@
 extern int diff_scoreopt_parse(const char *opt);
 
 #define DIFF_SETUP_REVERSE      	1
+#define DIFF_SETUP_USE_CACHE		2
+#define DIFF_SETUP_USE_SIZE_CACHE	4
 extern void diff_setup(int flags);
 
 #define DIFF_DETECT_RENAME	1
diff --git a/diffcore-pickaxe.c b/diffcore-pickaxe.c
index 9cf3a50..ef9c5c1 100644
--- a/diffcore-pickaxe.c
+++ b/diffcore-pickaxe.c
@@ -11,7 +11,7 @@
 {
 	unsigned long offset, sz;
 	const char *data;
-	if (diff_populate_filespec(one))
+	if (diff_populate_filespec(one, 0))
 		return 0;
 	sz = one->size;
 	data = one->data;
diff --git a/diffcore-rename.c b/diffcore-rename.c
index 6389ded..035d4eb 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -99,8 +99,11 @@
 	if (src->sha1_valid && dst->sha1_valid &&
 	    !memcmp(src->sha1, dst->sha1, 20))
 		return 1;
-	if (diff_populate_filespec(src) || diff_populate_filespec(dst))
-		/* this is an error but will be caught downstream */
+	if (diff_populate_filespec(src, 1) || diff_populate_filespec(dst, 1))
+		return 0;
+	if (src->size != dst->size)
+		return 0;
+	if (diff_populate_filespec(src, 0) || diff_populate_filespec(dst, 0))
 		return 0;
 	if (src->size == dst->size &&
 	    !memcmp(src->data, dst->data, src->size))
@@ -125,9 +128,11 @@
 	 * dst, and then some edit has been applied to dst.
 	 *
 	 * Compare them and return how similar they are, representing
-	 * the score as an integer between 0 and 10000, except
-	 * where they match exactly it is considered better than anything
-	 * else.
+	 * the score as an integer between 0 and MAX_SCORE.
+	 *
+	 * When there is an exact match, it is considered a better
+	 * match than anything else; the destination does not even
+	 * call into this function in that case.
 	 */
 	void *delta;
 	unsigned long delta_size, base_size;
@@ -147,6 +152,7 @@
 	/* We would not consider edits that change the file size so
 	 * drastically.  delta_size must be smaller than
 	 * (MAX_SCORE-minimum_score)/MAX_SCORE * min(src->size, dst->size).
+	 *
 	 * Note that base_size == 0 case is handled here already
 	 * and the final score computation below would not have a
 	 * divide-by-zero issue.
@@ -154,6 +160,9 @@
 	if (base_size * (MAX_SCORE-minimum_score) < delta_size * MAX_SCORE)
 		return 0;
 
+	if (diff_populate_filespec(src, 0) || diff_populate_filespec(dst, 0))
+		return 0; /* error but caught downstream */
+
 	delta = diff_delta(src->data, src->size,
 			   dst->data, dst->size,
 			   &delta_size);
diff --git a/diffcore.h b/diffcore.h
index 462014b..60ee775 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -33,7 +33,7 @@
 extern void fill_filespec(struct diff_filespec *, const unsigned char *,
 			  unsigned short);
 
-extern int diff_populate_filespec(struct diff_filespec *);
+extern int diff_populate_filespec(struct diff_filespec *, int);
 extern void diff_free_filespec_data(struct diff_filespec *);
 
 struct diff_filepair {