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

[Dr. Philipp Tomsich]

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