[FFmpeg-devel] [PATCH] h264: assembly version of get_cabac for x86_64 with PIC

[Michael Niedermayer]

On Mon, Apr 16, 2012 at 03:50:09PM +0200, Roland Scheidegger wrote:
> Am 16.04.2012 14:06, schrieb Michael Niedermayer:
> > On Mon, Apr 16, 2012 at 12:55:20PM +0200, Roland Scheidegger wrote:
> >> Am 15.04.2012 10:19, schrieb Michael Niedermayer:
> >>> On Sat, Apr 14, 2012 at 01:50:21AM +0200, Roland Scheidegger wrote:
> >>> [...]
> >>>
> >>>> +#ifdef BROKEN_RELOCATIONS
> >>>> +        BRANCHLESS_GET_CABAC("%4", "%q4", "(%1)", "%3", "%w3",
> >>>> +                             "%5", "%q5", "%k0", "%b0",
> >>>> +                             "%a11(%6)", "%a12(%6)", "%13")
> >>>> +#else
> >>>>          BRANCHLESS_GET_CABAC("%4", "(%1)", "%3", "%w3",
> >>>>                               "%5", "%k0", "%b0",
> >>>>                               "%a11(%6)", "%a12(%6)")
> >>>> +#endif
> >>>>  
> >>>>          "test $1, %4                            \n\t"
> >>>>          " jz 4f                                 \n\t"
> >>>>          "add  %10, %1                           \n\t"
> >>>>  
> >>>> +#ifdef BROKEN_RELOCATIONS
> >>>> +        BRANCHLESS_GET_CABAC("%4", "%q4", "(%1)", "%3", "%w3",
> >>>> +                             "%5", "%q5", "%k0", "%b0",
> >>>> +                             "%a11(%6)", "%a12(%6)", "%13")
> >>>> +#else
> >>>>          BRANCHLESS_GET_CABAC("%4", "(%1)", "%3", "%w3",
> >>>>                               "%5", "%k0", "%b0",
> >>>>                               "%a11(%6)", "%a12(%6)")
> >>>> +#endif
> >>>>  
> >>>>          "sub  %10, %1                           \n\t"
> >>>>          "mov  %2, %0                            \n\t"
> >>>
> >>> these 2 ifdefs can be avoided by making the macro arguments between
> >>> the 2 variants match
> >>
> >> Of course but then you have unnecessary arguments for the non-pic case.
> >> In particular also one reg more. I don't think that's a good idea -
> >> didn't test if it even compiles on x86 due to register pressure.
> > 
> > iam talking about the macro arguments, not the asm arguments.
> > The code would be identical after the C preprocessor so no way there
> > could be any issue with anything, you have same C source to the
> > C compiler and same asm to the assembler and binary identical object
> > files except line numbers / debug symbols
> > 
> Maybe I'm missing some macro-foo (which I'm bad at) but I still don't
> see how that should look like, since the macro arguments directly come
> from the operand specification of the inline assembly. Or do you suggest
> passing in some fake immediate (which will be unused) there or something
> similar instead of a reg if we don't need the pic version?

not sure if fake immediate is matching what i meant, but let me provide
an example, i should have done that previously already

instead of

#if this
#define THAT(a,b,c,d)
...
#else
#define THAT(a,c)
...
#endif

asm(
    ...
#if this
THAT(a,b,c,d)
#else
THAT(a,c)
#endif
    ...
)

one can do:
#if this
#define THAT(a,b,c,d)
...
#else
#define THAT(a,b,c,d)
... //no code here uses b or d
#endif

asm(
    ...
THAT(a,b,c,d)
    ...
) // the b and d arguments dont exist in the second case



also keep in mind that code like this:
#define this(a,b,c,d) return a
main(){
    this(0, not exist, "realy not", [{;:/ haha);
}

is perfectly valid, no need for the arguments here to exist of make
any sense as long as they arent used


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The bravest are surely those who have the clearest vision
of what is before them, glory and danger alike, and yet
notwithstanding go out to meet it. -- Thucydides
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