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
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