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

[Michael Niedermayer]

On Tue, Jun 24, 2008 at 10:51:40PM -0400, Alexander Strange wrote:
>
> On Jun 24, 2008, at 9:46 PM, Michael Niedermayer wrote:
>
>> On Tue, Jun 24, 2008 at 07:24:52PM -0400, Alexander Strange 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?
>>
>> no, just use a variable that does not conflict with mm_flags. like a local
>> var or a different name.
>> Besides i do not think fft-test.c should be linked with dsputil*
>
> Oops, there was a miscommunication.
>
> patch 1 - rename mm_flags in dct-test
> patch 2 - just remove mm_flags setting from fft-test

both ok

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Freedom in capitalist society always remains about the same as it was in
ancient Greek republics: Freedom for slave owners. -- Vladimir Lenin
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