RE: [AArch64] Emit square root using the Newton series

[Evandro Menezes <e.menezes@samsung.com>]

Hi, Marcus.

I've run Geekbench, SPEC CPU2000 and synthetic benchmarks.

I can share these results iterating an array with values between 1 and 1000000 and taking their square root:

Million Operations/s			Juno	
			A53 @850MHz	A57 @1100MHz
X^½	DP	Canon	 31 	 37 
		Newton	 13 	 39 
		%Δ	-57%	6%
	SP	Canon	 48 	 144 
		Newton	 18 	 62 
		%Δ	-63%	-57%
X^-½	DP	Canon	 17 	 16 
		Newton	 14 	 42 
		%Δ	-17%	155%
	SP	Canon	 28 	 70 
		Newton	 20 	 62 
		%Δ	-30%	-11%

As you can see, it's a mixed result for A57 and a definite regression for A53.  In mnost benchmarks overall, this is not a good optimization for A57.  That's why I left it as a target-specific tuning.

Thank you,

-- 
Evandro Menezes                              Austin, TX


> -----Original Message-----
> From: Marcus Shawcroft [mailto:marcus.shawcroft@gmail.com]
> Sent: Wednesday, December 09, 2015 8:06
> To: Evandro Menezes
> Cc: GCC Patches; Marcus Shawcroft; James Greenhalgh; Andrew Pinski; Benedikt
> Huber; philipp.tomsich@theobroma-systems.com
> Subject: Re: [AArch64] Emit square root using the Newton series
> 
> On 8 December 2015 at 21:35, Evandro Menezes <e.menezes@samsung.com> wrote:
> >    Emit square root using the Newton series
> >
> >    2015-12-03  Evandro Menezes  <e.menezes@samsung.com>
> >
> >    gcc/
> >             * config/aarch64/aarch64-protos.h (aarch64_emit_swsqrt):
> >    Declare new
> >             function.
> >             * config/aarch64/aarch64-simd.md (sqrt<mode>2): New
> >    expansion and
> >             insn definitions.
> >             * config/aarch64/aarch64-tuning-flags.def
> >             (AARCH64_EXTRA_TUNE_FAST_SQRT): New tuning macro.
> >             * config/aarch64/aarch64.c (aarch64_emit_swsqrt): Define
> >    new function.
> >             * config/aarch64/aarch64.md (sqrt<mode>2): New expansion
> >    and insn
> >             definitions.
> >             * config/aarch64/aarch64.opt (mlow-precision-recip-sqrt):
> >    Expand option
> >             description.
> >             * doc/invoke.texi (mlow-precision-recip-sqrt): Likewise.
> >
> > This patch extends the patch that added support for implementing
> > x^-1/2 using the Newton series by adding support for x^1/2 as well.
> 
> Hi Evandro, What benchmarking have you done on this patch?
> /M