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

[Luigi Mantellini]

Dear Wolfgang,

2009/1/28 Wolfgang Denk <wd at denx.de>:
> Dear Luigi Mantellini,
>
> In message <b73e93990901281416k766b2bf3qc6429f77545f9191 at mail.gmail.com> you wrote:
>>
>> I think that an audit of the code is important to understand if we
>> have a problem (or not) and how large is the problem.
>
> We (i. e. all of us except you) do not have a problem.

my question is: how can you be sure on this? I haven't used strange
compilers or strange CFLAGS.. I just do "make" on a supported target
(qemu_mips).
I haven't any problem to correct my local source tree. I ask myself
why gcc offers a packed atribute, ms-vc offers pragma packed, ...

>
>> My compiler is not broken...
>
> Well, YMMV...
>

the behavior is clear: in my environment, the default choice is to
align fields on 32bit for speed reasons... and I like this by default
for my applications.
I can ignore the problem using options like -Os (I will try tomorrow)
or -fpack-struct or other global mechanisms or, pay attention on
structures definitions to be sure that the structure size is compiler
and cflags independent.

>> cassini linux # find -name \*.c -o -name \*.h |xargs grep attribute |
>> grep packed | wc -l
>> 3153
>>
>> I see a lot of packed structs...
>
>
>
>> > Here is for example a copy of /usr/include/netinet/ip.h :
> ...
>> This doesn't say anything regarding how kernel guys have resolved the
>> problem (if they are solved...). Checking the kernel headers a lot
>
> This is not a kernel header. This is one of the standard user space
> network headers, and a pretty central one. Feel free to check any
> others.
>

I see. I would like to understand why this structure is
optimization-prof. I will study tomorrow...

>> I think that "struct packing" needs to be understood and a global
>
> I agree on this. This definitely needs to be understood.
>
> Hm... I think I understand it, and it works for me :-)
>

iso C doensn't require packing by default of the structures. To assume
that a structure is packed by default is not a good assumption. All
compilers offer directives to control the behavior of packing... I
believe that there is a reason...

>> mechanism should be used (like -fpack-struct option always defined or
>> a style guideline that requires a tag for each structs). From my point
>
> I think this is not needed. Please read the docuemntation about
> alignment and padding. It is pretty clear.

which documentation should I read? iso c documents? gcc manuals? I
haven't found anything that opposes my affirmations. I want underline
that the structures (IP_t, ... ) don't have field across the
word-machine boundary... but this doesn't exclude an "memory-access"
optimization.
Kindly, can you give me any good links?

Anyway,  I don't want to talk about philosophy. I just noticed an
anomaly and I wanted to share with the ML.

my 2LireItaliane

best regards,

luigi

>
> Best regards,
>
> Wolfgang Denk
>
> --
> DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
> Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
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> knowing  what  the  dictionary  says  it  means,  you  go to jail. No
> exceptions."                     - David Jones @ Megatest Corporation
>



-- 
Luigi 'Comio' Mantellini
R&D - Software
Industrie Dial Face S.p.A.
Via Canzo, 4
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