On Thu, Mar 31, 2011 at 3:38 AM, Dan McGee <dpmcgee@xxxxxxxxx> wrote:
> 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@xxxxxxxxx>
> ---
> 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;
We perfer this style:
if (c < '0' || c > '7')
return NULL;
i.e a line-break and a tab between the if-statement and the conditional code.
> + 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');
Wouldn't this part be cleaner as a constant-length loop? Any
optimizing compiler should end up unrolling this, and we don't get as
much code-duplication...
Oddly enough, this change gave me a drastic (> 20%) performance
increase in my test (isolating get_mode in a separate compilation
unit, and calling it in a loop):
diff --git a/tree-walk.c b/tree-walk.c
index b8d504b..114ad63 100644
--- a/tree-walk.c
+++ b/tree-walk.c
@@ -7,42 +7,30 @@
static const char *get_mode(const char *str, unsigned int *modep)
{
unsigned char c;
- unsigned int mode = 0;
+ unsigned int mode = 0, i;
/*
- * Unroll what looks like a loop since the bounds are
+ * Allow the compiler to unroll the 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');
+ for (i = 0; i < 5; ++i) {
+ 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;
+ if (c < '0' || c > '7')
+ return NULL;
mode = (mode << 3) + (c - '0');
}
- if (*str != ' ') return NULL;
+ if (*str != ' ')
+ return NULL;
*modep = mode;
return str + 1;
--
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