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

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

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