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RE: [PATCH] [aarch64] Implemented reciprocal square root (rsqrt) estimation in -ffast-math
- From: "Kumar, Venkataramanan" <Venkataramanan dot Kumar at amd dot com>
- To: "Dr. Philipp Tomsich" <philipp dot tomsich at theobroma-systems dot com>
- Cc: Benedikt Huber <benedikt dot huber at theobroma-systems dot com>, "pinskia at gmail dot com" <pinskia at gmail dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 25 Jun 2015 16:44:44 +0000
- Subject: RE: [PATCH] [aarch64] Implemented reciprocal square root (rsqrt) estimation in -ffast-math
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- References: <1434629045-24650-1-git-send-email-benedikt dot huber at theobroma-systems dot com> <8B73CF78-11D4-4963-A60A-E1C2A3B219E2 at gmail dot com> <F2FF9755-1DF9-4000-8602-77AB12077240 at theobroma-systems dot com> <7794A52CE4D579448B959EED7DD0A4723DD10430 at satlexdag06 dot amd dot com> <1E4680F0-02C8-4999-958C-8B531BC850DA at theobroma-systems dot com>
I got around ~12% gain with -Ofast -mcpu=cortex-a57.
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