Re: [lwip-users] [PATCH] fix warning for gcc and possible unaligned access

[Timmy Brolin]

Curt McDowell wrote:

> Pedro Alves wrote
>  
>
> > Seems like the right thing to do.
> > If the packing is removed from struct ip_addr, and struct 
> > ip_addr2 is removed, then there is no need to memcpy anymore.
> >
> > A simple:
> >
> > sipaddr = hdr->sipaddr;
> >
> > will do.
> >    
>
> memcpy may still be needed because hdr->sipaddr is misaligned at +2 due to the
> format of the ARP packet.  My theory is that struct ip_addr2 was originally
> invented because the 4-byte struct/uint32 copy was crashing on someone's 2-byte
> aligned CPU!
>
> Regards,
> Curt McDowell
> Broadcom Corp.
Either ip_addr2 or memcpy is needed.
The code "sipaddr = hdr->sipaddr;" would crash on any CPU which do not 
support unaligned accesses.
The current ip_addr2 solution is technically more efficient than memcpy 
since it copies the ip_addr with just two 16bit reads+writes.
The only solution which would be slightly more efficient is to copy 
hdr->sipaddr to a 32bit register using two 16bit accesses, and then 
write the 32bit register to sipaddr. But since this is not a critical 
code path, it is more important to keep the code as clean as possible 
rather than trying to save one single memory access.
The ip_addr2 solution makes the code nice and clean.

Regards,
Timmy Brolin