[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.