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-ow...@gcc.gnu.org [mailto:gcc-patches- > ow...@gcc.gnu.org] On Behalf Of Benedikt Huber > Sent: Thursday, June 25, 2015 4:09 PM > To: pins...@gmail.com > Cc: gcc-patches@gcc.gnu.org; philipp.toms...@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
