[PATCH] delta micro optimization

[PATCH] delta micro optimization

[Nicolas Pitre]

My kernel work habit made me look at the generated assembly for the 
delta code, and one obvious albeit small improvement is this patch.

Signed-off-by: Nicolas Pitre <nico@cam.org>

---

diff --git a/delta.h b/delta.h
index 31d1820..a15350d 100644
--- a/delta.h
+++ b/delta.h
@@ -19,14 +19,14 @@ extern void *patch_delta(void *src_buf, 
 static inline unsigned long get_delta_hdr_size(const unsigned char **datap)
 {
 	const unsigned char *data = *datap;
-	unsigned char cmd = *data++;
-	unsigned long size = cmd & ~0x80;
-	int i = 7;
-	while (cmd & 0x80) {
+	unsigned char cmd;
+	unsigned long size = 0;
+	int i = 0;
+	do {
 		cmd = *data++;
 		size |= (cmd & ~0x80) << i;
 		i += 7;
-	}
+	} while (cmd & 0x80);
 	*datap = data;
 	return size;
 }

Re: [PATCH] delta micro optimization

[Junio C Hamano]

That looks obviously correct but it really is micro.  Have you
benched it?  On which architectures?  Does it help everywhere or
register starved ones benefit more than others?

Re: [PATCH] delta micro optimization

[Nicolas Pitre]

On Fri, 10 Feb 2006, Junio C Hamano wrote:

> That looks obviously correct but it really is micro.  Have you
> benched it?  On which architectures?  Does it help everywhere or
> register starved ones benefit more than others?

I doubt the performance difference would be measurable on i386 although 
the code is a bit smaller.

On ARM it causes a code reduction of 12.5% in count-delta.o.


Nicolas