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 <mich...@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 <mich...@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@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 | tejohn...@google.com | 408-460-2413
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Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
I am the wisest man alive, for I know one thing, and that is that I know
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