Re: [lwip-devel] Checksum optimizations

[Jonathan Larmour]

David Empson wrote:
> Bill Auerbach wrote:
> 
>> In the inet_chksum.c code, there are 6 occurrences of:
>>
>>    while ((acc >> 16) != 0) {
>>      acc = (acc & 0xffffUL) + (acc >> 16);
>>    }
>>
>> Is it not true that the loop can run a maximum of once?
[David explains when it can run more]
>  
> The following code is correct in all situations:
>  
> if ((acc >> 16) != 0) {
>  acc = (acc & 0xffffUL) + (acc >> 16);
>  if ((acc >> 16) != 0) {
>   acc = (acc & 0xffffUL) + 1;
>  }
> }
>  
> The first 'if' could be omitted if there is a high probability that it
> will be true.

As Andrey Volkov hinted, the 'if' tests are probably expensive enough (and
addition/shift/mask cheap enough) that it would probably be better to just
have:

acc = (acc & 0xffffUL) + (acc >> 16);
acc = (acc & 0xffffUL) + (acc >> 16);

I'm not sure this will go into 1.3.0 at this point (even though it's
simple) so it's probably best to post a patch at
https://savannah.nongnu.org/patch/?func=additem&group=lwip so this is
picked up after 1.3.0.

Bill, care to do this?

Jifl
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