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

[L. E. Segovia]

Hi all,

Yes, the linking step bails out with optimizations disabled under MSVC.
The rest of the compilers perform DCE even at -O0, but under Windows,
the /FORCE:UNRESOLVED linker flag is needed to work around this.

As for the stub generation --  given the amount of places that these
patches touch, and that someone would need to figure out a complete
configuration to test them all, I believe it's very much infeasible.
Conversely, hooking into the macro uses makes testing as simple as
one run with /O2 and another with /O0 -- which we use in development
as well.

best,

amyspark

On 28/07/2023 10:21, Matt Oliver wrote:
> On Fri, 28 Jul 2023 at 20:55, Nicolas George <george at 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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amyspark 🌸 https://www.amyspark.me