[U-Boot] IP_t should be a "packed" struct

[Jerry Van Baren <gerald.vanbaren@ge.com>]

Ben Warren wrote:
> Luigi 'Comio' Mantellini wrote:
>> Hi ML,
>>
>> I'm working on a mips target and I used qemu_mips target to simulate my target 
>> (that I hope to have in the next week...)
>>
>> Following my activities I noticed that IP_t structure is no defined with 
>> attribute "packed". I noticed this issue because using a self-made toolchain 
>> (gcc4.2.4+binutils2.8+uclibc0.9.30) the compiler has aligned all bytes to 
>> 32bit boundary. This is not ok, because the packets IP_t can be non aligned 
>> (see the /net/net.c PingSend function, for an example).
>>
>>   
> Why is your compiler aligning all bytes to 32-bit boundary?  Seems like 
> an awful waste of space.  This struct should pack itself nicely, and 
> does on the small sample of toolchains I've tried (gcc 4.3.2 x86_64 and 
> gcc 4.0.0 ppc_4xx).

The compiler is optimizing for speed and/or execution size at the 
expense of larger data structures either by command (e.g. a -O option) 
or as part of the compiler writer's choice.  CPUs almost always execute 
code significantly faster when the data is properly aligned.  Many CPUs 
require software to deal with the misalignment which costs code space 
and execution time.

Since the compiler wasn't instructed that the IP headers needed to be 
packed, it is within the compiler's right to not pack them.

[snip]

Best regards,
gvb