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

[Michael Niedermayer]

Hi

On Tue, Oct 31, 2017 at 04:29:18PM +0000, Teresa Johnson wrote:
> It's needed for the same reason the used attribute was already added to the
> "static const" variables. For those, when building with just -O2, they
> could be removed by optimization since they had local (file) scope, and we
> couldn't see the uses in the inline assembly (without the used attribute).
> With ThinLTO we have whole program scope, so even though they are
> non-static and have global scope we can do dead code elimination on them if
> we don't see that they are used.

currently we add "used" to DECLARE_ASM_CONST()
which is specific to inline asm use.

DECLARE_ALIGNED() is not specific to use in asm.

For DECLARE_ASM_CONST() the "used" is unneeded only in the subset of
inline asm cases where it is accessed through the asm operands like:
    __asm__ volatile(
        "movq          %0, %%mm7    \n\t"
        "movq          %1, %%mm6    \n\t"
        ::"m"(red_16mask),"m"(green_16mask));

The compiler has full vissibility here of the access and if it removes
it its a  compiler bug.

this is compared to code like:
 "pand "MANGLE(mask24l)", %%mm0\n\t"

Here the compiler has no easy vissibility of the use, it is part of
the asm string.

and comparing this to DECLARE_ALIGNED(), the big difference is
that DECLARE_ALIGNED() is used by plain C code which never should need
"used". So adding "used" to DECLARE_ALIGNED() would add alot more
"unused detection overriding" than what is needed



> 
> Teresa
> 
> On Tue, Oct 31, 2017 at 4:19 PM, Michael Niedermayer <michael at niedermayer.cc
> > wrote:
> 
> > 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?
> > > _______________________________________________
> > > ffmpeg-devel mailing list
> > > ffmpeg-devel at ffmpeg.org
> > > http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> >
> > --
> > Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> >
> > Avoid a single point of failure, be that a person or equipment.
> >
> 
> 
> 
> -- 
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I am the wisest man alive, for I know one thing, and that is that I know
nothing. -- Socrates
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