[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 at 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.
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