[FFmpeg-devel] Debug builds and if (ARCH_...) linking issues

[Aaron Levinson]

I wanted to build a debug build of ffmpeg using Visual C++ today, one 
without any optimizations.  This implies the use of the -Od compiler 
option.  Unfortunately, I quickly discovered that the build fails soon 
after it starts because it can't find certain architecture-specific 
references.  For example, in libavutil/cpu.c, there is the following:

     if (ARCH_AARCH64)
         return ff_get_cpu_flags_aarch64();

The linker couldn't find ff_get_cpu_flags_aarch64 (and other similar 
references) and failed.  This isn't an issue when optimizations are 
turned on because the compiler notices that ARCH_AARCH64 is defined as 0 
and eliminates the relevant code.

Effectively, successful builds of ffmpeg depend on this compiler 
optimization.  This appears to have been the standard practice in the 
ffmpeg code base for at least the last few years, but it is unclear to 
me why this approach is being used, since, in addition to depending on 
specific compiler behavior, it prevents fully debug builds from 
succeeding, at least with Visual C++.

If people like the if (ARCH_...) syntax, while it wouldn't look quite as 
nice, what's wrong with doing the following:

#if ARCH_AARCH64
     if (ARCH_AARCH64)
         return ff_get_cpu_flags_aarch64();
#endif

Another, much less desirable option is to use #pragma optimize for the 
relevant functions in ffmpeg to turn optimizations on for specific 
functions.

A third option would be to build only the relevant files with 
optimizations turned on, but this will end up making the Makefiles more 
complicated, and the relative simplicity of the Makefiles is appealing.

For now, I'm using both -Od and -Og with Visual C++ (-Og turns on some 
optimizations, but not as much as -O1 or -O2), but this isn't the same 
as a true debug build.

Thanks,
Aaron Levinson