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Date: Mon, 22 Oct 2007 17:35:45 -0700 (PDT) From: David Miller <davem@...emloft.net> To: joe@...ches.com Cc: acme@...stprotocols.net, netdev@...r.kernel.org Subject: Re: [PATCH] - trivial - Improve appletalk checksum calculation From: Joe Perches <joe@...ches.com> Date: Mon, 22 Oct 2007 12:36:19 -0700 > It's a bit after 2.6.1 now... > > Removes unnecessary if, uses 16 bit rotate left. > Performance improves ~30% > > Signed-off-by: Joe Perches <joe@...ches.com> I'm not sure your transformation is equivalent. Did you actually test this with appletalk traffic to make sure the sum is computed correctly? Or more simply, did you write a test program to send some test data through the old and new functions? Here is what I think the problem is: > static unsigned long atalk_sum_partial(const unsigned char *data, > int len, unsigned long sum) > { > - /* This ought to be unwrapped neatly. I'll trust gcc for now */ > while (len--) { > - sum += *data; > - sum <<= 1; > - if (sum & 0x10000) { > - sum++; > - sum &= 0xffff; > - } > - data++; > + sum += *data++; > + sum = ((sum & 0x8000)>>15) | ((sum & 0x7fff)<<1); > } > return sum; > } The old code is handling overflow, so that when bit 16 gets set by and addition, that overflow bit gets cleared and then added back into the sum. Your code is rotating bit 15 down by one bit and bits 0-14 up by one bit, my remedial math knowledge tell me that's likely not the same thing. :-) - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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