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

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

On 09/12/15 16:59, Evandro Menezes wrote:
> 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.

Sure, but the way your patch is written, we will emit the series when "fast_sqrt" is set, rather
than the other way around, unless I'm misreading the logic in:

diff --git a/gcc/config/aarch64/aarch64-simd.md b/gcc/config/aarch64/aarch64-simd.md
index 030a101..f6d2da4 100644
--- a/gcc/config/aarch64/aarch64-simd.md
+++ b/gcc/config/aarch64/aarch64-simd.md
@@ -4280,7 +4280,23 @@
 
 ;; sqrt
 
-(define_insn "sqrt<mode>2"
+(define_expand "sqrt<mode>2"
+  [(set (match_operand:VDQF 0 "register_operand")
+	(sqrt:VDQF (match_operand:VDQF 1 "register_operand")))]
+  "TARGET_SIMD"
+{
+  if ((AARCH64_EXTRA_TUNE_FAST_SQRT & aarch64_tune_params.extra_tuning_flags)
+      && !optimize_function_for_size_p (cfun)
+      && flag_finite_math_only
+      && !flag_trapping_math
+      && flag_unsafe_math_optimizations)
+    {
+      aarch64_emit_swsqrt (operands[0], operands[1]);
+      DONE;
+    }
+})


Thanks,
Kyrill