[FFmpeg-devel] Stack variables alignment check in libavcodec/dsputil.c

[Ramiro Polla]

Michael Niedermayer wrote:
> Hi
>
> On Sat, Aug 11, 2007 at 08:14:45PM -0300, Ramiro Polla wrote:
> [...]
>   
>>> why is the realignment not always done?
>>>   
>>>       
>> If ffmpeg.c is compiled with gcc, it will align main() to 16 bytes and 
>> maintain alignment everywhere. If libavcodec is called from MSVC++, 
>> these functions will be called with 4-byte alignment. The same happens 
>> for threads (both in w32threads and pthreads-win32).
>>     
>
> how do you know during compilation how libavcodec will be called?
>
>   

You don't, unless you're building it for a specific purpose. In that 
case, you should know about this option (it should print out somewhere).

>   
>>> what effect does it have on object size?
>>>
>>>   
>>>       
>> .text:    unpatched patched
>> dsputil.o 0004e270 0004e280
>> pthread.o 000003c0 000003d0
>> utils.o   000029d0 00002a30
>>     
>
> 128 bytes difference
>
> please add the realignment unconditionally! 128 bytes arent worth the
> time users might end up spending due to some random crash (as well as the
> number of bugreports we might get) and as someone else already said 
> the ABI does not gurantee 16byte alignment so the realignment is needed
>
>   

Ok. Attached patch sets it unconditionally for every platform and OS. 
Should we make a list in configure only for the platforms that need it, 
or is this ok?
Patch also adds avcodec_open, which I missed yesterday, and a note in 
ff_check_alignment.

Ramiro Polla
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