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

[Kyrill Tkachov <kyrylo.tkachov@arm.com>]

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.

Thanks,
Kyrill