Re: [PATCH] [aarch64] Implemented reciprocal square root (rsqrt) estimation in -ffast-math

[Ramana Radhakrishnan <ramana dot radhakrishnan at foss dot arm dot com>]

Benedikt,

On 25/06/15 08:01, pinskia@gmail.com wrote:
> > On Jun 18, 2015, at 5:04 AM, Benedikt Huber <benedikt.huber@theobroma-systems.com> wrote:
> >
> > arch64 offers the instructions frsqrte and frsqrts, for rsqrt estimation and
> > a Newton-Raphson step, respectively.
> > There are ARMv8 implementations where this is faster than using fdiv and rsqrt.
> > It runs three steps for double and two steps for float to achieve the needed precision.
>
> This is NOT a win on thunderX at least for single precision because you have to do the divide and sqrt in the same time as it takes 5 multiples (estimate and step are multiplies in the thunderX pipeline).  Doubles is 10 multiplies which is just the same as what the patch does (but it is really slightly less than 10, I rounded up). So in the end this is NOT a win at all for thunderX unless we do one less step for both single and double.


Have you seen this 
https://gcc.gnu.org/ml/gcc-patches/2015-03/msg00164.html ? Really this 
is something that should be gated by the costs infrastructure .


regards
Ramana





> Thanks,
> Andrew
>
>
> > There is one caveat and open question.
> > Since -ffast-math enables flush to zero intermediate values between approximation steps
> > will be flushed to zero if they are denormal.
> > E.g. This happens in the case of rsqrt (DBL_MAX) and rsqrtf (FLT_MAX).
> > The test cases pass, but it is unclear to me whether this is expected behavior with -ffast-math.
> >
> > The patch applies to commit:
> > svn+ssh://gcc.gnu.org/svn/gcc/trunk@224470
> >
> > Please consider including this patch.
> > Thank you and best regards,
> > Benedikt Huber
> >
> > Benedikt Huber (1):
> >   2015-06-15  Benedikt Huber  <benedikt.huber@theobroma-systems.com>
> >
> > gcc/ChangeLog                            |   9 +++
> > gcc/config/aarch64/aarch64-builtins.c    |  60 ++++++++++++++++
> > gcc/config/aarch64/aarch64-protos.h      |   2 +
> > gcc/config/aarch64/aarch64-simd.md       |  27 ++++++++
> > gcc/config/aarch64/aarch64.c             |  63 +++++++++++++++++
> > gcc/config/aarch64/aarch64.md            |   3 +
> > gcc/testsuite/gcc.target/aarch64/rsqrt.c | 113 +++++++++++++++++++++++++++++++
> > 7 files changed, 277 insertions(+)
> > create mode 100644 gcc/testsuite/gcc.target/aarch64/rsqrt.c
> >
> > --
> > 1.9.1