[AArch64] Emit square root using the Newton series

[Kyrill Tkachov]

On 09/12/15 17:02, Kyrill Tkachov wrote:
>
> 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:
>

Sorry, what I meant to say is it would be clearer, IMO, to describe the compiler action that is being taken
(e.g. the rename_fma_regs tuning flag), in this case it's estimating sqrt using a series.

Kyrill

> 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
>
>