Re: RE: [lwip-users] IP Address Display Functions

[Simon Goldschmidt]

> > Also, I see a *huge* difference in performance with packing by 2 versus
> 1
> > which we do in many if not all ports.
> 
> I find it interesting that this makes a difference.

Why? I think that's pretty good to explain: All the network protocols we use 
packing for would require 2-byte-packing only, not 1-byte-packing (unless I'm 
wrong here). The result of 2-byte-packing is that the compiler may read 16-bit 
words instead of bytes which results in half the number of read accesses on 
16-bit-or-more platforms.

Of course, this only works if we never include (unaligned) byte members in 
structs - we're pretty much. But as most of the protocol headers are multiples 
of 2 or 4, this might work.

> I.e. it sounds to me like your compiler is
> reading more into packed directives than it needs to (perhaps to make
> things simpler for it): it is affecting both the packing of the
> structure and the way it accesses the fields in that structure.

How would it differ between the two? It would have to know how the data is 
aligned at compile time, which it can't??

Simon
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