RE: [PATCH] [aarch64] Implemented reciprocal square root (rsqrt) estimation in -ffast-math

["Kumar, Venkataramanan" <Venkataramanan.Kumar@amd.com>]

Hmm,  Reducing the iterations to "1 step for float" and "2 steps for double"

 I got VE (miscompares) on following benchmarks
416.gamess 
453.povray         
454.calculix   
459.GemsFDTD  

Benedikt , I have ICE for 444.namd with your patch,  not sure if something wrong in my local tree.  

Regards,
Venkat.

> -----Original Message-----
> From: pinskia@gmail.com [mailto:pinskia@gmail.com]
> Sent: Sunday, June 28, 2015 8:35 PM
> To: Kumar, Venkataramanan
> Cc: Dr. Philipp Tomsich; Benedikt Huber; gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH] [aarch64] Implemented reciprocal square root (rsqrt)
> estimation in -ffast-math
> 
> 
> 
> 
> 
> > On Jun 25, 2015, at 9:44 AM, Kumar, Venkataramanan
> <Venkataramanan.Kumar@amd.com> wrote:
> >
> > I got around ~12% gain with -Ofast -mcpu=cortex-a57.
> 
> I get around 11/12% on thunderX with the patch and the decreasing the
> iterations change (1/2) compared to without the patch.
> 
> Thanks,
> Andrew
> 
> 
> >
> > Regards,
> > Venkat.
> >
> >> -----Original Message-----
> >> From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-
> >> owner@gcc.gnu.org] On Behalf Of Dr. Philipp Tomsich
> >> Sent: Thursday, June 25, 2015 9:13 PM
> >> To: Kumar, Venkataramanan
> >> Cc: Benedikt Huber; pinskia@gmail.com; gcc-patches@gcc.gnu.org
> >> Subject: Re: [PATCH] [aarch64] Implemented reciprocal square root
> >> (rsqrt) estimation in -ffast-math
> >>
> >> Kumar,
> >>
> >> what is the relative gain that you see on Cortex-A57?
> >>
> >> Thanks,
> >> Philipp.
> >>
> >>>> On 25 Jun 2015, at 17:35, Kumar, Venkataramanan
> >>> <Venkataramanan.Kumar@amd.com> wrote:
> >>>
> >>> Changing to  "1 step for float" and "2 steps for double" gives
> >>> better gains
> >> now for gromacs on cortex-a57.
> >>>
> >>> Regards,
> >>> Venkat.
> >>>> -----Original Message-----
> >>>> From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-
> >>>> owner@gcc.gnu.org] On Behalf Of Benedikt Huber
> >>>> Sent: Thursday, June 25, 2015 4:09 PM
> >>>> To: pinskia@gmail.com
> >>>> Cc: gcc-patches@gcc.gnu.org; philipp.tomsich@theobroma-
> systems.com
> >>>> Subject: Re: [PATCH] [aarch64] Implemented reciprocal square root
> >>>> (rsqrt) estimation in -ffast-math
> >>>>
> >>>> Andrew,
> >>>>
> >>>>> This is NOT a win on thunderX at least for single precision
> >>>>> because you have
> >>>> to do the divide and sqrt in the same time as it takes 5 multiples
> >>>> (estimate and step are multiplies in the thunderX pipeline).
> >>>> Doubles is 10 multiplies which is just the same as what the patch
> >>>> does (but it is really slightly less than 10, I rounded up). So in
> >>>> the end this is NOT a win at all for thunderX unless we do one less
> >>>> step for both single
> >> and double.
> >>>>
> >>>> Yes, the expected benefit from rsqrt estimation is implementation
> >>>> specific. If one has a better initial rsqrte or an application that
> >>>> can trade precision for execution time, we could offer a command
> >>>> line option to do only 2 steps for doulbe and 1 step for float;
> >>>> similar to -
> >> mrecip-precision for PowerPC.
> >>>> What are your thoughts on that?
> >>>>
> >>>> Best regards,
> >>>> Benedikt
> >