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

[Evandro Menezes <e dot menezes at samsung dot com>]

On 12/09/2015 10:52 AM, Kyrill Tkachov wrote:
> Hi Evandro,
>
> On 08/12/15 21:35, Evandro Menezes 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.
> >
> > Is it OK at this point of stage 3?
>
> A comment on the patch itself from me...
>
> diff --git a/gcc/config/aarch64/aarch64-tuning-flags.def 
> b/gcc/config/aarch64/aarch64-tuning-flags.def
> index 6f7dbce..11c6c9a 100644
> --- a/gcc/config/aarch64/aarch64-tuning-flags.def
> +++ b/gcc/config/aarch64/aarch64-tuning-flags.def
> @@ -30,4 +30,4 @@
>
>  AARCH64_EXTRA_TUNING_OPTION ("rename_fma_regs", RENAME_FMA_REGS)
>  AARCH64_EXTRA_TUNING_OPTION ("recip_sqrt", RECIP_SQRT)
> -
> +AARCH64_EXTRA_TUNING_OPTION ("fast_sqrt", FAST_SQRT)
>
> That seems like a misleading name to me.
> If we're doing this, that means that the sqrt instruction is not faster
> than doing the inverse sqrt estimation followed by a multiply.
> I think a name like "synth_sqrt" or "estimate_sqrt" or something along 
> those lines
> is more appropriate.

Unfortunately, this is the case on Exynos M1: the series is faster than 
the instruction. :-(  So, other targets when this is also true, using 
the "fast_sqrt" option might make sense.

Thank you,

--
Evandro Menezes