On Thu, Apr 13, 2017 at 9:20 AM, Aaron Levinson <alevi...@aracnet.com> wrote: > 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. >
We require Dead-Code-Eliminiation to be performed at all times in a FFmpeg build, so your compiler needs to do this for a build to succeed. You can find the reasonings and discussions about this in the Mailing List archives, it comes up once in a while. If you are actually working on some code, I can only recommend to use the pragma in the files you are working on. Its not that bad, really. - Hendrik _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
