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

[Matt Oliver]

On 14 April 2017 at 08:05, Hendrik Leppkes <h.leppkes at gmail.com> wrote:

> On Fri, Apr 14, 2017 at 12:00 AM, Ronald S. Bultje <rsbultje at gmail.com>
> wrote:
> > Hi Aaron,
> >
> > On Thu, Apr 13, 2017 at 4:47 PM, Aaron Levinson <alevinsn at aracnet.com>
> > wrote:
> >
> >> #if ARCH_AARCH64
> >>     if (ARCH_AARCH64)
> >>         return ff_get_cpu_flags_aarch64();
> >> #endif
> >>
> >
> > If you add #if, at least replace the if with it. #if + if is redundant.
> >
> >
> >> Such a script could be reviewed and checked into the source base, then
> >> applied to the source base, and possibly reused in the future as
> necessary.
> >
> >
> > Why not use the script as a preprocessor during compilation? We did that
> > for pre-2013 MSVC compilation and that worked """fine""".
> >
> > Or why not ask Microsoft to add a CLI option to cl.exe for enabling DCE
> but
> > otherwise disabling optimizations? All other compilers seem capable of
> > this, so it's odd to see that big Microsoft is incompetent whereas a
> bunch
> > of opensource hippies could do the same 10+ yrs ago.
> >
>
> Its not a MSVC-exclusive problem. Many compilers don't perform DCE in
> full debug builds, which are as such not possible with ffmpeg, making
> debuging sometimes a bit annoying when a bunch of variables are
> optimized out and stuff gets inlined.


Unfortunately writing a script is a rather complex task as there are many
DCE blocks that are actually generated by macro expansion and various
pre-processor trickery (swresample is full of this sort of stuff). Combine
that with DCE blocks being nested within each other and it becomes a rather
complex task to find them all. Much more complex than what a simple regex
script can handle.

If people object to changing the current code base then the only solution
would be a program/script that can generate some empty definitions for all
the functions/variables that are used in DCE blocks so as to avoid the
errors about them not existing. These empty definitions would have to be
maintained in a set of external files or generated at compile time.

There are actually a considerable number of missing funcs/vars used in DCE
blocks. As an example i have attached a list of empty funcs/vars that I
have been able to programmatically detect (there are a lot! and i may have
missed some).
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