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

["Kumar, Venkataramanan" <Venkataramanan dot Kumar at amd dot com>]

> -----Original Message-----
> From: Dr. Philipp Tomsich [mailto:philipp.tomsich@theobroma-systems.com]
> Sent: Monday, June 29, 2015 2:17 PM
> To: Kumar, Venkataramanan
> Cc: pinskia@gmail.com; Benedikt Huber; gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH] [aarch64] Implemented reciprocal square root (rsqrt)
> estimation in -ffast-math
> 
> Kumar,
> 
> This does not come unexpected, as the initial estimation and each iteration
> will add an architecturally-defined number of bits of precision (ARMv8
> guarantuees only a minimum number of bits provided per operationâ the
> exact number is specific to each micro-arch, though).
> Depending on your architecture and on the required number of precise bits
> by any given benchmark, one may see miscompares.

True.  
 

> 
> Do you know the exact number of bits that the initial estimate and the
> subsequent refinement steps add for your micro-arch?

I am not sure on this. I  need to check for cortex-a57 case.  

But best thing for "cortex-a57" case is not to use this optimization by default. 
If we get an -mrecip-sqrt command line , then we can add it for "gromacs" kind application to get gains.

Any thoughts on this?
    
Regards,
Venkat.
 

> 
> Thanks,
> Philipp.
> 
> > On 29 Jun 2015, at 10:17, Kumar, Venkataramanan
> <Venkataramanan.Kumar@amd.com> wrote:
> >
> >
> > 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
> >>>