[Samba] [RESEND] crc32 optimization

[Andrew Bartlett]

On Tue, 2003-05-27 at 20:42, Joakim Tjernlund wrote:
> Hi Samba
> 
> I noticed that the crc32 function in lib/crc32.c is somewhat
> inefficient. This little patch will reduce the inner loop from 10
> instructions to 8 instructions on x86 with gcc. gcc can't figure
> out this simple optimization by itself on x86.

So, we are saving two instructions...  I think this e-mail consumed more
;-)

> Further optimization is possible by using the impl. in the linux 2.5 kernel, but
> I don't know if it is worth it. Is crc32 common in samba? What would the typical
> buffer len be?

It's reasonably rare.  It's certainly not in any fast-paths that I know
of.

>     Jocke
> 
> Patch against 2_2_8 CVS:
> --- lib/crc32.c	Sat Oct 20 23:23:35 2001
> +++ lib/crc32.c.new	Tue May 13 14:45:31 2003
> @@ -60,7 +60,7 @@
>          uint32 crc=0xffffffff;
>  	int i;
>          for(i=0;i<count;i++)
> -                crc = (crc>>8) ^ CRCTable[(buffer[i] ^ crc) & 0xff];
> +                crc = CRCTable[(buffer[i] ^ crc) & 0xff] ^ (crc>>8);
>          crc^=0xffffffff;
>  	DEBUG(10,("crc32_calc_buffer: %x\n", crc));
>  	dump_data(100, buffer, count);

Looks reasonable, but I'll let somebody else chase it up.  

Andrew Bartlett

-- 
Andrew Bartlett                                 abartlet at pcug.org.au
Manager, Authentication Subsystems, Samba Team  abartlet at samba.org
Student Network Administrator, Hawker College   abartlet at hawkerc.net
http://samba.org     http://build.samba.org     http://hawkerc.net
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