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

[Hendrik Leppkes]

On Sun, Oct 25, 2015 at 12:30 PM, Matt Oliver <protogonoi at gmail.com> wrote:
> 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.

The "Intel(R) C++ Compiler User and Reference Guide" would care to
disagree with you.

- Hendrik