[FFmpeg-devel] Weird cross platform support in ffmpeg

[FFmpeg-devel] Weird cross platform support in ffmpeg

[Александр]

I tried to build ffmpeg 4.4 library and link with it and I received
multiple unresolved references. I came across on unusual architecture in
ffmpeg for multiple platforms.The library has many places with code like
this:

void foo()
{
    // DO SOMETHING
    if (ARCH_MIPS) // maybe #if ARCH_MIPS (...) #endif should be used instead?
         foo_mips(...);
    if (ARCH_PPC)
        foo_ppc(c);
    if (ARCH_ARM)
         foo_arm(...);
    if (ARCH_AARCH64)
        foo_aarch64(...);
}

This code leads to linker errors because there is no any stub methods for
other platforms. I observed root MakeFile, it optionally includes platform
dependent code for each library (path like $(LIB_SUBDIR)/$(ARCH)/MakeFile)
where each foo_<arch> is defined.

So, how does it work? In my opinion preprocessor directive #if should be
used, otherwise we will receive unresolved reference on any platform. Maybe
I missed something? because this code appears frequently for example:

   1. utils.c : ff_yuv2rgb_init_tables_ppc
   2. swscale_unscaled.c : ff_get_unscaled_swscale_ppc,
   ff_get_unscaled_swscale_arm, ff_get_unscaled_swscale_aarch64
   3. cpu.c : ff_get_cpu_max_align_x86, ff_get_cpu_max_align_mips
   4. float_dsp.c : ff_float_dsp_init_aarch64, ff_float_dsp_init_ppc,
   ff_float_dsp_init_x86
   5. etc.
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Re: [FFmpeg-devel] Weird cross platform support in ffmpeg

[Nicolas George]

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Александр (12022-05-31):
> This code leads to linker errors because there is no any stub methods for
> other platforms. I observed root MakeFile, it optionally includes platform
> dependent code for each library (path like $(LIB_SUBDIR)/$(ARCH)/MakeFile)
> where each foo_<arch> is defined.
> 
> So, how does it work?

Please direct users questions to ffmpeg-users and include the exact
procedure you have used to try to build and the output it produced,
trimmed if necessary.

Regards,

-- 
  Nicolas George

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Re: [FFmpeg-devel] Weird cross platform support in ffmpeg

[Timo Rothenpieler]

On 31.05.2022 12:03, Александр wrote:
> I tried to build ffmpeg 4.4 library and link with it and I received
> multiple unresolved references. I came across on unusual architecture in
> ffmpeg for multiple platforms.The library has many places with code like
> this:
> 
> void foo()
> {
>      // DO SOMETHING
>      if (ARCH_MIPS) // maybe #if ARCH_MIPS (...) #endif should be used instead?
>           foo_mips(...);
>      if (ARCH_PPC)
>          foo_ppc(c);
>      if (ARCH_ARM)
>           foo_arm(...);
>      if (ARCH_AARCH64)
>          foo_aarch64(...);
> }
> 
> This code leads to linker errors because there is no any stub methods for
> other platforms. I observed root MakeFile, it optionally includes platform
> dependent code for each library (path like $(LIB_SUBDIR)/$(ARCH)/MakeFile)
> where each foo_<arch> is defined.
> 
> So, how does it work?

It relies on Compiler-Optimizations, namely Dead-Code-Elimination.
For that reason, FFmpeg does not support being built with -O0, since the 
compiler won't eliminate the dead code, leading to a whole bunch of dead 
references.

Hence, if you run into an issue like that, you need to look at your 
compiler and figure out what the hell it's trying to do.
Or stop trying to build with -O0, which the build system should normally 
never allow you to do in the first place unless you force it.
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Re: [FFmpeg-devel] Weird cross platform support in ffmpeg

[Andreas Rheinhardt]

Timo Rothenpieler:
> On 31.05.2022 12:03, Александр wrote:
>> I tried to build ffmpeg 4.4 library and link with it and I received
>> multiple unresolved references. I came across on unusual architecture in
>> ffmpeg for multiple platforms.The library has many places with code like
>> this:
>>
>> void foo()
>> {
>>      // DO SOMETHING
>>      if (ARCH_MIPS) // maybe #if ARCH_MIPS (...) #endif should be used
>> instead?
>>           foo_mips(...);
>>      if (ARCH_PPC)
>>          foo_ppc(c);
>>      if (ARCH_ARM)
>>           foo_arm(...);
>>      if (ARCH_AARCH64)
>>          foo_aarch64(...);
>> }
>>
>> This code leads to linker errors because there is no any stub methods for
>> other platforms. I observed root MakeFile, it optionally includes
>> platform
>> dependent code for each library (path like
>> $(LIB_SUBDIR)/$(ARCH)/MakeFile)
>> where each foo_<arch> is defined.
>>
>> So, how does it work?
> 
> It relies on Compiler-Optimizations, namely Dead-Code-Elimination.
> For that reason, FFmpeg does not support being built with -O0, since the
> compiler won't eliminate the dead code, leading to a whole bunch of dead
> references.
> 

Incorrect: Both GCC and Clang eliminate dead code at -O0 and FFmpeg can
be built with -O0 with both of them just fine.

> Hence, if you run into an issue like that, you need to look at your
> compiler and figure out what the hell it's trying to do.
> Or stop trying to build with -O0, which the build system should normally
> never allow you to do in the first place unless you force it.
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Re: [FFmpeg-devel] Weird cross platform support in ffmpeg

[Александр]

I analyzed problem a little bit. Only gcc/clang allows to use this trick.
Msvc compiler forbidens such code (even with different enabled
optimizations like remove unused references, whole program optimization
etc) I have made patch, which uses preprocessor #if directive instead. If
it will be helpful, I can share the patch.

вт, 31 мая 2022 г. в 17:24, Andreas Rheinhardt <
andreas.rheinhardt@outlook.com>:

> Timo Rothenpieler:
> > On 31.05.2022 12:03, Александр wrote:
> >> I tried to build ffmpeg 4.4 library and link with it and I received
> >> multiple unresolved references. I came across on unusual architecture in
> >> ffmpeg for multiple platforms.The library has many places with code like
> >> this:
> >>
> >> void foo()
> >> {
> >>      // DO SOMETHING
> >>      if (ARCH_MIPS) // maybe #if ARCH_MIPS (...) #endif should be used
> >> instead?
> >>           foo_mips(...);
> >>      if (ARCH_PPC)
> >>          foo_ppc(c);
> >>      if (ARCH_ARM)
> >>           foo_arm(...);
> >>      if (ARCH_AARCH64)
> >>          foo_aarch64(...);
> >> }
> >>
> >> This code leads to linker errors because there is no any stub methods
> for
> >> other platforms. I observed root MakeFile, it optionally includes
> >> platform
> >> dependent code for each library (path like
> >> $(LIB_SUBDIR)/$(ARCH)/MakeFile)
> >> where each foo_<arch> is defined.
> >>
> >> So, how does it work?
> >
> > It relies on Compiler-Optimizations, namely Dead-Code-Elimination.
> > For that reason, FFmpeg does not support being built with -O0, since the
> > compiler won't eliminate the dead code, leading to a whole bunch of dead
> > references.
> >
>
> Incorrect: Both GCC and Clang eliminate dead code at -O0 and FFmpeg can
> be built with -O0 with both of them just fine.
>
> > Hence, if you run into an issue like that, you need to look at your
> > compiler and figure out what the hell it's trying to do.
> > Or stop trying to build with -O0, which the build system should normally
> > never allow you to do in the first place unless you force it.
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
>
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Re: [FFmpeg-devel] Weird cross platform support in ffmpeg

[Nicolas George]

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Александр (12022-05-31):
> I analyzed problem a little bit. Only gcc/clang allows to use this trick.
> Msvc compiler forbidens such code (even with different enabled
> optimizations like remove unused references, whole program optimization
> etc) I have made patch, which uses preprocessor #if directive instead. If
> it will be helpful, I can share the patch.

FATE shows FFmpeg builds fine on several MSVC instances, so no, we do
not need such a patch at this time.

I have asked you to show on ffmpeg-users the exact thing you are
attempting to do, you have neglected to do that. It is obvious to me
that you are trying to tweak the build process in a way that is not
supported: stop.

> вт, 31 мая 2022 г. в 17:24, Andreas Rheinhardt <
> andreas.rheinhardt@outlook.com>:

Top-posting is forbidden on all FFmpeg mailing-lists. If you do not know
what it means look it up. You are likely to receive no useful reply if
you continue.

Regards,

-- 
  Nicolas George

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Re: [FFmpeg-devel] Weird cross platform support in ffmpeg

[Александр]

I duplicated question at another channel (users). I wrote here because I
get answers on my question here. On another channel I am ready to provide
any detailed information.

вт, 31 мая 2022 г. в 17:40, Nicolas George <george@nsup.org>:

> Александр (12022-05-31):
> > I analyzed problem a little bit. Only gcc/clang allows to use this trick.
> > Msvc compiler forbidens such code (even with different enabled
> > optimizations like remove unused references, whole program optimization
> > etc) I have made patch, which uses preprocessor #if directive instead. If
> > it will be helpful, I can share the patch.
>
> FATE shows FFmpeg builds fine on several MSVC instances, so no, we do
> not need such a patch at this time.
>
> I have asked you to show on ffmpeg-users the exact thing you are
> attempting to do, you have neglected to do that. It is obvious to me
> that you are trying to tweak the build process in a way that is not
> supported: stop.
>
> > вт, 31 мая 2022 г. в 17:24, Andreas Rheinhardt <
> > andreas.rheinhardt@outlook.com>:
>
> Top-posting is forbidden on all FFmpeg mailing-lists. If you do not know
> what it means look it up. You are likely to receive no useful reply if
> you continue.
>
> Regards,
>
> --
>   Nicolas George
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
>
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Re: [FFmpeg-devel] Weird cross platform support in ffmpeg

[Wang Bin]

Nicolas George <george@nsup.org> 于2022年5月31日周二 22:40写道：

> Александр (12022-05-31):
> > I analyzed problem a little bit. Only gcc/clang allows to use this trick.
> > Msvc compiler forbidens such code (even with different enabled
> > optimizations like remove unused references, whole program optimization
> > etc) I have made patch, which uses preprocessor #if directive instead. If
> > it will be helpful, I can share the patch.
>
> FATE shows FFmpeg builds fine on several MSVC instances, so no, we do
> not need such a patch at this time.
>
> I have asked you to show on ffmpeg-users the exact thing you are
> attempting to do, you have neglected to do that. It is obvious to me
> that you are trying to tweak the build process in a way that is not
> supported: stop.
>
>
If MSVC whole program optimization is enabled (compiler flag /GL), DCE will
not work.
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