Re: [Jack-Devel] packed data structures in jack2

Le 19 juil. 2012 à 19:32, Basil Nutmeg a écrit :

> On Thu, Jul 19, 2012 at 11:57:42AM -0400, Paul Davis wrote:
>> On Thu, Jul 19, 2012 at 11:41 AM, Basil Nutmeg <
>> [hidden]> wrote:
>> 
>>> 
>>> /* ... this type has the same layout between 32-bit and 64-bit. */
>>> struct A_with_alignment {
>>>  aligned_char x;
>>>  aligned_int64_t i;
>>> };
>>> 
>> 
>> this translates to:
>> 
>>     struct A_with_alignment {
>>          char     x    __attribute__((aligned(sizeof(char));
>>          int64_t i     __attribute__((aligned(sizeof(int64_t));
>>      };
>> 
>> which i think is not the same as what "packed" does at all. it could be
>> enough though, and arguably is better because it preserves aligned access.
> 
> Right, the goal isn't to emulate "packed" exactly; the goal is to solve a
> specific problem without incurring the undesirable side effects :-).
> 
>> 
>> however, as stephane noted: far easier to just change the macros that do
>> this stuff so that they are no-ops.
> 
> This is easy to do, and it works for my project today. However, I was
> thinking about what would be best for Jack in the long term.
> 
> I think it would be preferable to have a solution which could support
> mixed 64/32 mode even on platforms which don't have misaligned accesses
> (64-bit ARM chips are coming...), which would avoid the performance
> issues, and which would avoid "breaking the rules" even on platforms
> where it works, because clever compiler writers can still theoretically 
> find ways to take advantage of alignment assumptions there. And it would
> undo what is technically an ABI break in the recent addition of "packed"
> to <jack/types.h>.
> 
> I think it's even worthwhile to make moderately more invasive source
> changes to achieve this. And, I'm willing to do some work to help make it
> happen. But if the maintainers disagree, I'll move on.

Use a git branch.

> 
>> 
>>> As another idea would be to have a source file which simply includes all
>>> the important headers and is compiled with -Wpadded and maybe even
>>> -Werror, to catch any mistakes.
>>> 
>> 
>> i don't this is portable to non-gnu compilers. i could be wrong.
> 
> Perhaps so, or perhaps other compilers have analogous features; I don't
> know offhand. I'd be willing to do some research here. It might not be a
> complete solution, but it might be a nice sanity check, regardless.
> 
> -- Basil

It has to work on Windows with VC++ and Mingw kid of compiler at least.

Stéphane 

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