On 25 October 2015 at 22:16, Hendrik Leppkes <h.lepp...@gmail.com> wrote:
> On Sun, Oct 25, 2015 at 11:26 AM, Matt Oliver <protogo...@gmail.com> > wrote: > > On 25 October 2015 at 12:22, Ganesh Ajjanagadde <gajja...@mit.edu> > wrote: > > > >> On Sat, Oct 24, 2015 at 7:03 PM, James Almer <jamr...@gmail.com> wrote: > >> > On 10/24/2015 7:48 PM, Ganesh Ajjanagadde wrote: > >> >> On Sat, Oct 24, 2015 at 6:08 PM, James Almer <jamr...@gmail.com> > wrote: > >> >>> This gives the compiler some flexibility > >> >>> > >> >>> Signed-off-by: James Almer <jamr...@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. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel