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

[Dr. Philipp Tomsich]

Evandro,

We’ve seen a 28% speed-up on gromacs in SPECfp for the (scalar) reciprocal sqrt.

Also, the “reciprocal divide” patches are floating around in various of our git-tree, but 
aren’t ready for public consumption, yet… I’ll leave Benedikt to comment on potential 
timelines for getting that pushed out.

Best,
Philipp.

> On 24 Jun 2015, at 18:42, Evandro Menezes <e.menezes@samsung.com> wrote:
> 
> Benedikt,
> 
> You beat me to it! :-)  Do you have the implementation for dividing using
> the Newton series as well?
> 
> I'm not sure that the series is always for all data types and on all
> processors.  It would be useful to allow each AArch64 processor to enable
> this or not depending on the data type.  BTW, do you have some tests showing
> the speed up?
> 
> Thank you,
> 
> -- 
> Evandro Menezes                              Austin, TX
> 
>> -----Original Message-----
>> From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-owner@gcc.gnu.org]
> On
>> Behalf Of Benedikt Huber
>> Sent: Thursday, June 18, 2015 7:04
>> To: gcc-patches@gcc.gnu.org
>> Cc: benedikt.huber@theobroma-systems.com; philipp.tomsich@theobroma-
>> systems.com
>> Subject: [PATCH] [aarch64] Implemented reciprocal square root (rsqrt)
>> estimation in -ffast-math
>> 
>> 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.
>> 
>> 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
> <Mail Attachment.eml>