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

[Curt McDowell]

On Sat, Apr 22, 2006 at 01:27:44PM +0200, Timmy Brolin wrote:
> 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.

ip_addr2 is a gross hack, whereas memcpy is clean.

As you pointed out, this code is not in a critical path.  If we cared
about efficiency (which here we don't), we could add an arch/cc macro
that copies 4 bytes regardless of alignment.

The ip_addr2 hack doesn't work on machines with 4-byte alignment
restrictions (ARM, MIPS); hence multiple instances of people having to
replace it with memcpy.

Regards,
Curt McDowell
Broadcom Corp.