[FFmpeg-devel] Multimedia extensions run-time support check

[Alexander Strange]

On Tue, Jun 24, 2008 at 7:24 PM, Alexander Strange
<astrange at ithinksw.com> wrote:
>
> On Jun 24, 2008, at 4:23 PM, Michael Niedermayer wrote:
>
>> On Tue, Jun 24, 2008 at 05:43:49PM +0200, Laurent Desnogues wrote:
>>>
>>> Hello,
>>>
>>> (I know this is not a very interesting matter, but it's a good way
>>> for me to start messing with FFmpeg code;  if noone cares, let
>>> me know and I'll use some adhoc solution that fits my needs.)
>>>
>>> Currently to know what multimedia extensions are supported,
>>> the code checks the value of mm_flags or the return value of
>>> mm_support() (cf. dsputil.h).
>>>
>>> My understanding is that this is only used to setup things at
>>> initialization time.
>>>
>>> However I find things are inconsistent.  For instance:
>>>
>>> - mm_support is always declared but only defined for MMX,
>>>  IWMMXT (Xscale) and PPC
>>> - mm_flags is declared for ARMV4L but only defined for
>>>  IWMMXT
>>>
>>> This is not a big deal at the moment since it only breaks
>>> the build of some test programs for some configs, such as
>>> dct-test when either MMX is not defined or when building
>>> for ARM without IWMMXT.
>>
>> dct-test.c was broken by
>>   Author: benoit
>>   Date: Tue Mar 25 15:34:27 2008
>>   New Revision: 12580
>>
>>   Log:
>>   Set mm_flags in dct-test to fix floating math.
>>   Patch by Alexander Strange \astrange ithinksw com/
>> and
>>   Author: gpoirier
>>   Date: Sun Mar 30 12:29:28 2008
>>   New Revision: 12628
>>
>>   Log:
>>   check mm_flags for each DCT and skips the ones that can't be run
>>   patch by Alexander Strange %astrange A ithinksw P com%
>>
>> fft-test.c was broken by
>>   Author: diego
>>   Date: Tue Jun 24 11:00:58 2008
>>   New Revision: 13940
>>
>>   Log:
>>   Remove redundant mm_flags declaration, fixes the warning:
>>   libavcodec.a(dsputil_ppc.o): warning: definition of `mm_flags'
>> overriding common
>>   fft-test.o: warning: common is here
>>
>>
>>>
>>> Currently I only see two solutions:
>>>
>>> 1. correctly protect declarations in dsputil.h depending
>>>  on ifdef's
>>> 2. always define fake mm_flags and mm_support that
>>>  are or return zero.
>>
>> I do not belive mm_flags was ever intended to be accessed from outside
>> the specific optimized code. it just happened by mistake as people tried
>> to "fix" gcc warnings without bothering to fully understand the code
>>
>> mm_support() should be a #define 0 when optims are off.
>
> Is this OK?

OK'd by Diego and applied.