> On Jun 18, 2015, at 5:04 AM, Benedikt Huber
> <benedikt.hu...@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.
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.hu...@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
>
