Re: [FFmpeg-devel] [PATCH v4 0/4] Fix MSVC build without optimizations

[Matt Oliver <protogonoi@gmail.com>]

On Fri, 28 Jul 2023 at 20:55, Nicolas George <george@nsup.org> wrote:

> Reimar Döffinger (12023-07-28):
> > I assume the issue is missing symbols during linking?
> > If you really want this, why not create a file that provides dummy
> > symbols for all that are missing, concentrating the #if mess in
> > a single place and avoiding affecting any of the regular code, and thus
> > having no impact at all when compiling with optimizations.
> > Yes, it's likely to be a good bit of maintenance effort for those who
> > want to use it, but at least anyone not caring about this feature can
> > ignore it, so at least I would not have a reason to be against it.
>
> This is an interesting idea. It would even be possible to include a tool
> that generate these stubs directly from the linker's errors, reducing
> the maintenance.
>
> Maybe even make the stubs static inline functions rather than actual
> linking symbols.
>

The issue with dead code elimination and msvc has been raised many times
over the years and the general response has been to not support it. The
last discussion was back in 2016 (
https://ffmpeg.org/pipermail/ffmpeg-devel/2016-December/204530.html) which
apparently was by me.
There was an attempt previously by someone to try and semi-manually add all
the missing symbols using dummy functions but it obviously didnt go
anywhere as if I remember correctly it was difficult to track them all down
for all possible configuration options and maintaining it was just too hard
and it never got completed. I dont have the link but if you crawl through
old mailing list posts it is in there somewhere.

About a decade ago I went the auto generation route and created a tool that
generated Visual Studio projects by scanning ffmpegs make/configure files
and as part of that the tool creates dummy files with all the missing
symbols for the requested configuration options (
https://github.com/ShiftMediaProject/FFVS-Project-Generator). Its not the
most readable code but it does work. Whats potentially more interesting is
over all the years that project has been used the number of dummy dce stubs
has decreased from what used to be there when some of those early attempts
I mentioned were made. However just checking it now shows that there are
still a good 1000 dummy functions created for avcodec alone, although
admittedly theres only about a dozen for all the other libs. So a
pre-processing tool can work nicely, the trick is to be able to get it to
work with different compilers as the command line options and output format
is slightly different and that only gives you the function name you still
need to then scan the code to work out the correct types for the functions
parameters and return types in order to make a dummy stub.
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