[PATCH 4/5] tree-walk: unroll get_mode since loop boundaries are well-known

[Dan McGee <dpmcgee@gmail.com>]

We know our mode entry in our tree objects should be 5 or 6 characters
long. This change both enforces this fact and also unrolls the parsing
of the information giving the compiler more room for optimization of the
operations.

Signed-off-by: Dan McGee <dpmcgee@gmail.com>
---
 tree-walk.c |   41 ++++++++++++++++++++++++++++++++++-------
 1 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/tree-walk.c b/tree-walk.c
index f386151..41383b0 100644
--- a/tree-walk.c
+++ b/tree-walk.c
@@ -9,16 +9,43 @@ static const char *get_mode(const char *str, unsigned int *modep)
 	unsigned char c;
 	unsigned int mode = 0;
 
-	if (*str == ' ')
-		return NULL;
-
-	while ((c = *str++) != ' ') {
-		if (c < '0' || c > '7')
-			return NULL;
+	/*
+	 * Unroll what looks like a loop since the bounds are
+	 * well-known. There should be at least 5 and at most 6
+	 * characters available in any valid mode, as '40000' is the
+	 * shortest while '160000' (S_IFGITLINK) is the longest.
+	 */
+	/* char 1 */
+	c = *str++;
+	if (c < '0' || c > '7') return NULL;
+	mode = (mode << 3) + (c - '0');
+	/* char 2 */
+	c = *str++;
+	if (c < '0' || c > '7') return NULL;
+	mode = (mode << 3) + (c - '0');
+	/* char 3 */
+	c = *str++;
+	if (c < '0' || c > '7') return NULL;
+	mode = (mode << 3) + (c - '0');
+	/* char 4 */
+	c = *str++;
+	if (c < '0' || c > '7') return NULL;
+	mode = (mode << 3) + (c - '0');
+	/* char 5 */
+	c = *str++;
+	if (c < '0' || c > '7') return NULL;
+	mode = (mode << 3) + (c - '0');
+	/* char 6, optional */
+	if (*str != ' ') {
+		c = *str++;
+		if (c < '0' || c > '7') return NULL;
 		mode = (mode << 3) + (c - '0');
 	}
+
+	if (*str != ' ') return NULL;
+
 	*modep = mode;
-	return str;
+	return str + 1;
 }
 
 static void decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned long size)
-- 
1.7.4.2