Re: crc32() optimization

Marc Singer
2002-11-10T18:13:01Z

As it should. I wonder if you'd do better changing the loop slightly.

Check for len == 0 and do a short-circuit return. Then do this

for (++len; len & 0x7; len >>= 3) {
ONCE(); // repeat eight times
...
len >>= 3;
}
while (--len > 0)
ONCE();

This is the implementation I've written for another project which
we've found to be relatively optimal. Note that len *must* be an int
even though contemporary convention is to use the size_t type.

On Sun, Nov 10, 2002 at 04:28:00PM +0100, Joakim Tjernlund wrote:
> Hi David
>
> This patch improves my scan time with 22%( from 2.39 to 1.86 seconds).
> Maybe you want to include it in the 2.4 branch.
>
> I will put this in my next backport of the crc32 stuff from 2.5.
>
> Jocke
>
> Index: fs/jffs2/crc32.h
> ===================================================================
> RCS file: /home/cvs/mtd/fs/jffs2/crc32.h,v
> retrieving revision 1.3
> diff -u -b -r1.3 crc32.h
> --- fs/jffs2/crc32.h 26 Feb 2001 14:44:37 -0000 1.3
> +++ fs/jffs2/crc32.h 10 Nov 2002 15:25:11 -0000
> @@ -13,7 +13,16 @@
> crc32(__u32 val, const void *ss, int len)
> {
> const unsigned char *s = ss;
> - while (--len >= 0)
> + while (len >= 6){
> + val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8);
> + val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8);
> + val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8);
> + val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8);
> + val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8);
> + val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8);
> + len -= 6;
> + }
> + while (len--)
> val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8);
> return val;
> }
>
>
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