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

[Hendrik Leppkes]

On Thu, Apr 13, 2017 at 7:16 PM, Matt Oliver <protogonoi at gmail.com> wrote:
> On 14 April 2017 at 02:11, Rostislav Pehlivanov <atomnuker at gmail.com> wrote:
>
>>
>>
>> On 13 April 2017 at 16:51, wm4 <nfxjfg at googlemail.com> wrote:
>>
>>> On Thu, 13 Apr 2017 17:39:57 +1000
>>> Matt Oliver <protogonoi at gmail.com> wrote:
>>>
>>> > On 13 April 2017 at 17:20, Aaron Levinson <alevinsn at 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