netdev - Re: [PATCH] - trivial - Improve appletalk checksum calculation

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Message-ID: <ygfk5p7vvq7.fsf@janus.isnogud.escape.de>
Date:	28 Oct 2007 21:18:40 +0100
From:	Urs Thuermann <urs.thuermann@....de>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	joe@...ches.com (Joe Perches), davem@...emloft.net,
	acme@...stprotocols.net, netdev@...r.kernel.org
Subject: Re: [PATCH] - trivial - Improve appletalk checksum calculation

Herbert Xu <herbert@...dor.apana.org.au> writes:

> But your code differs significantly from Stephen's version.
> However, if it is correct it does look like a good improvement.
> 
> So please write a simple test program.  It can't that bad since
> there are only 65536 values to test :)

I think the code is simple enough to see immediately that it should
calculate the same checksum.  But I have written a short test program
to show this and tested it on i686 (gcc-4.2.2) and powerpc (gcc-3.4.5)
without optimization and with -O6.

BTW, shouldn't unsigned long be replaced by unsigned int to avoid
64-bit operations one some platforms?

urs


#include <stdio.h>

unsigned long f1(unsigned long sum)
{
    sum <<= 1;
    if (sum & 0x10000) {
	sum++;
	sum &= 0xffff;
    }
    return sum;
}

unsigned long f2(unsigned long sum)
{
    sum = ((sum & 0x8000)>>15) | ((sum & 0x7fff)<<1);
    return sum;
}

int main()
{
    unsigned long s;

    for (s = 0; s < 65536; s++) {
	if (f1(s) != f2(s) || f1(s) > 65535)
	    printf("%ld\n", s);
    }
    return 0;
}
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