On Thu, Apr 13, 2017 at 7:16 PM, Matt Oliver <protogo...@gmail.com> wrote: > On 14 April 2017 at 02:11, Rostislav Pehlivanov <atomnu...@gmail.com> wrote: > >> >> >> On 13 April 2017 at 16:51, wm4 <nfx...@googlemail.com> wrote: >> >>> On Thu, 13 Apr 2017 17:39:57 +1000 >>> Matt Oliver <protogo...@gmail.com> wrote: >>> >>> > On 13 April 2017 at 17:20, 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. >>> > > >>> > >>> > Similar patches have been submitted before. This is an issue with Dead >>> Code >>> > Elimination (DCE) within FFmpeg and the fact the MSVC doesn't support >>> > removing it in debug builds. >>> > >>> > There have been some discussions on the mailing list in the past about >>> > resolving this but nothing was ever decided. >>> > >>> > As a quick and dirty work around I have a tool that i wrote that scans >>> in >>> > the configure/makefile from a ffmpeg distro and generates a native >>> Visual >>> > Studio project file that can be used to just compile within Visual >>> Studio >>> > itself. You just pass it the desired configure options that you want to >>> use >>> > to build the project and it will make one for you. The main thing is >>> that >>> > it scans the source and automatically generates the missing DCE sections >>> > and adds them so that everything will compile correctly with Debug >>> builds >>> > and you can debug directly in VS. You can find the tool here >>> > http://shiftmediaproject.github.io/1-projects/ (normally i wouldn't put >>> > external references on the mailing list but this may be of use to you). >>> >>> Any chance you could revive your old patches to remove the DCE >>> requirement? (Not sure if there were any patches.) >>> >>> Before you do any real work, make a thread on the ML requesting >>> comments on this. Although I would very much welcome such patches, I'm >>> not fully sure about others. >>> >>> This DCE requirement is pretty painful, and affects debugging on Linux >>> as well. >>> >> > I put up a general discussion a while ago ( > http://ffmpeg.org/pipermail/ffmpeg-devel/2016-December/204530.html) but > there were a few people who opposed a direct removal of DCE and no one > really came up with a consensus as to what the acceptable approach would be. >
I wouldn't like any weird hacks in the source just to work-around the lack of DCE in debug builds, so we should decide to either keep using it or get rid of it. - Hendrik _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
