[FFmpeg-devel] [PATCH] Fix missing used attribute for inline assembly variables

[Michael Niedermayer]

On Tue, Oct 31, 2017 at 03:52:14PM +0000, Thomas Köppe wrote:
> +Teresa, who drafted the patch initially.
> 
> On 31 October 2017 at 15:38, Michael Niedermayer <michael at niedermayer.cc>
> wrote:
> 
> > On Tue, Oct 31, 2017 at 12:16:18PM +0000, Thomas Köppe wrote:
> > > Variables used in inline assembly need to be marked with
> > attribute((used)).
> >
> > This should not be the case.
> > Variables referenced through in/out operands should not need that.
> > Only ones accessed directly bypassing operands should need this
> >
> 
> I've added Teresa to the thread, who initially analyzed the problem. (For
> background on ThinLTO, see here cppcon talk:
> https://www.youtube.com/watch?v=p9nH2vZ2mNo.)
> 
> [...]
> > > -    #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (n)))
> > v
> > > +    #define DECLARE_ALIGNED(n,t,v)      t av_used __attribute__
> > ((aligned (n))) v
> >
> > which variables actually are affected/ need this ?
> >
> 
> Without this annotation, and when compiling and linking with ThinLTO, I get
> an undefined reference to "ff_h264_cabac_tables", and also to
> "ff_pw_96", "ff_pw_53",
> "ff_pw_42", "ff_w1111" and many more.

i guess these are all accessed directly, like through MANGLE()


> 
> 
> > Marking all aligned variables as used makes it harder to spot unused
> > variables leading to accumulation of cruft
> >
> 
> I see. Maybe we can annotate only the affected variables more granularly?
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-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Avoid a single point of failure, be that a person or equipment.
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