crc32() optimization

[Marc Singer]

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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