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

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

On 02/26/16 17:42, Evandro Menezes wrote:
> On 02/26/16 08:59, James Greenhalgh wrote:
>> On Mon, Feb 22, 2016 at 06:50:44PM -0600, Evandro Menezes wrote:
>>> In preparation for the patch adding the Newton series also for
>>> square root, I'd like to propose this patch changing the name of the
>>> existing tuning flag for the reciprocal square root.
>> This is fine, other names like sw_rsqrt, expand_rsqrt, nr_rsqrt would 
>> also
>> be OK. Pick your favourite!
>>
>> One comment on the replacement invoke.texi text below, otherwise this is
>> OK to apply now.
>>
>>> diff --git a/gcc/config/aarch64/aarch64.opt 
>>> b/gcc/config/aarch64/aarch64.opt
>>> index 5cbd4cd..155d2bd 100644
>>> --- a/gcc/config/aarch64/aarch64.opt
>>> +++ b/gcc/config/aarch64/aarch64.opt
>>> @@ -151,5 +151,5 @@ PC relative literal loads.
>>>     mlow-precision-recip-sqrt
>>>   Common Var(flag_mrecip_low_precision_sqrt) Optimization
>>> -When calculating a sqrt approximation, run fewer steps.
>>> +Calculate the reciprocal square-root approximation in fewer steps.
>>>   This reduces precision, but can result in faster computation.
>>> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
>>> index 490df93..eeff24d 100644
>>> --- a/gcc/doc/invoke.texi
>>> +++ b/gcc/doc/invoke.texi
>>> @@ -12879,12 +12879,10 @@ corresponding flag to the linker.
>>>   @item -mno-low-precision-recip-sqrt
>>>   @opindex -mlow-precision-recip-sqrt
>>>   @opindex -mno-low-precision-recip-sqrt
>>> -The square root estimate uses two steps instead of three for 
>>> double-precision,
>>> -and one step instead of two for single-precision.
>>> -Thus reducing latency and precision.
>>> -This is only relevant if @option{-ffast-math} activates
>>> -reciprocal square root estimate instructions.
>>> -Which in turn depends on the target processor.
>>> +The reciprocal square root approximation uses one step less than 
>>> otherwise,
>>> +thus reducing latency and precision.
>> When calculating the reciprocal square root approximation, use one less
>> step than otherwise, thus reducing latency and precision.
>>
>
> Checked in as r233772.

But not without some log hiccups, sorry...

-- 
Evandro Menezes