Re: [lwip-users] lwIP Checksum routine

[Sathya Thammanur]

Thanks for the reply Jim. 

Sathya

On 11/14/05, Jim Gibbons <address@hidden> wrote:

We did an optimization for one port (NiosII).  This is very CPU dependent.  In our particular case, we did better with 16-bit accesses owing to a slow shifter.  We did the best by handling 8 half-words in one pass of an outer loop.  This allowed us to use small constant offsets that could be encoded in the load instructions, e.g., acc += data[0]; acc += data[1]; etc.  The loop overheads and the pointer update (data += 8) became a much smaller fraction of the CPU time taken.

But, as I said, this stuff is very CPU dependent.  Considering that, I think that the core code is as it should be. 

It's a simple thing to change for your particular CPU, so I would urge you to do so.  I would also urge you to try a couple of different things and measure your results.  We were surprised when we found that full word accesses weren't good for us, and you may find some surprising things with your CPU.

You might also want to check your ethernet chip.  Some of the newer ones can assist you at the time of transmission.

Good luck!

Sathya Thammanur wrote:

Hi all,
The lwip_chksum() function in lwip/src/core/inet.c seems to be unoptimized. This is doing halfword reads and additions. Wouldnt it be better to do word accesses and hence additions? There would be some prologue and epilogue code to checks for bringing the buffer to halfword->word boundaries. HAs anyone tried doing the same for any of their ports? Or am I missing something out here?

Thanks,
Sathya

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