[FFmpeg-devel] [PATCH] x86/intmath: allow the source operand for icc/icl ff_ctzll to be a memory location

[Matt Oliver]

On 25 October 2015 at 22:16, Hendrik Leppkes <h.leppkes at gmail.com> wrote:

> On Sun, Oct 25, 2015 at 11:26 AM, Matt Oliver <protogonoi at gmail.com>
> wrote:
> > On 25 October 2015 at 12:22, Ganesh Ajjanagadde <gajjanag at mit.edu>
> wrote:
> >
> >> On Sat, Oct 24, 2015 at 7:03 PM, James Almer <jamrial at gmail.com> wrote:
> >> > On 10/24/2015 7:48 PM, Ganesh Ajjanagadde wrote:
> >> >> On Sat, Oct 24, 2015 at 6:08 PM, James Almer <jamrial at gmail.com>
> wrote:
> >> >>> This gives the compiler some flexibility
> >> >>>
> >> >>> Signed-off-by: James Almer <jamrial at gmail.com>
> >> >>> ---
> >> >>>  libavutil/x86/intmath.h | 2 +-
> >> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> >>>
> >> >>> diff --git a/libavutil/x86/intmath.h b/libavutil/x86/intmath.h
> >> >>> index 7881e3c..10d3e7f 100644
> >> >>> --- a/libavutil/x86/intmath.h
> >> >>> +++ b/libavutil/x86/intmath.h
> >> >>> @@ -36,7 +36,7 @@ static av_always_inline av_const int
> >> ff_ctzll_x86(long long v)
> >> >>>  {
> >> >>>  #   if ARCH_X86_64
> >> >>>      uint64_t c;
> >> >>> -    __asm__("bsfq %1,%0" : "=r" (c) : "r" (v));
> >> >>> +    __asm__ ("bsfq %1,%0" : "=r" (c) : "rm" (v));
> >> >>>      return c;
> >> >>>  #   else
> >> >>>      return ((uint32_t)v == 0) ? _bit_scan_forward((uint32_t)(v >>
> >> 32)) + 32 : _bit_scan_forward((uint32_t)v);
> >> >>> --
> >> >>> 2.6.2
> >> >>
> >> >> This question may be silly, but can you clarify when this asm code is
> >> >> used instead of __builtin_ctzll? For instance, I think on GNU/Linux,
> >> >> x86-64, sufficiently modern GCC (even with asm enabled), we should
> >> >> always use the builtin: the compiler knows best what to do with its
> >> >> builtin.
> >> >
> >> > This is ICC/ICL, not GCC.
> >>
> >> Ah, now I recall that _bit_scan_forward64 was not always available on
> >> ICC. Anyway, this is just a heads up to whoever uses ICC/ICL and the
> >> like: you may want to find out to which versions this built in
> >> applies.
> >
> >
> > bit_scan_forward64 isnt available on ICL at all, hence the use of asm.
> >
>
> Intels intrinsic guide lists _BitScanForward64 as the intrinsic to use,
> however.
>
> https://software.intel.com/sites/landingpage/IntrinsicsGuide/#techs=Other&expand=373


 _BitScanForward64 is a msvc intrinsic only available on windows platforms
so not usable with ICC. The native asm implementation also performs better
with ICL hence its use.