[Bug math/20660] [arm] Use VSQRT

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https://sourceware.org/bugzilla/show_bug.cgi?id=20660

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=0f04fc07f6a36e761ec76100043798ea5c7a590c

commit 0f04fc07f6a36e761ec76100043798ea5c7a590c
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Oct 20 23:24:44 2016 +0000

    Use VSQRT instruction for ARM sqrt (bug 20660).

    This patch makes ARM sqrt and sqrtf use the VSQRT VFP square root
    instruction when available, instead of much larger generic code for
    computing square roots.

    Now, GCC will normally inline sqrt calls except for negative arguments
    where errno needs to be set, and because the benchtests fail to use
    -fno-builtin that means no significant difference in benchmark results
    for sqrt (note, however, there are lots of __ieee754_sqrt calls
    internally in libm, which are *not* inlined - although some
    architectures define __ieee754_sqrt in their math_private.h for that
    purpose, ARM doesn't - so improving out-of-line sqrt performance is
    still relevant to those other functions, if not for most ordinary
    direct users of sqrt).  With the benchtests changed to use
    -fno-builtin for sqrt tests, typical performance results before the
    change are ("max" is wildly varying in any case):

        "duration": 9.88358e+09,
        "iterations": 4.8783e+07,
        "max": 457.764,
        "min": 183.105,
        "mean": 202.603

    and after it are:

        "duration": 9.45663e+09,
        "iterations": 2.24385e+08,
        "max": 274.659,
        "min": 30.517,
        "mean": 42.1447

    Tested for ARM (hard-float and soft-float).

        [BZ #20660]
        * sysdeps/arm/e_sqrt.c: New file.
        * sysdeps/arm/e_sqrtf.c: Likewise.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                               |    6 +++++
 sysdeps/{sparc/sparc32 => arm}/e_sqrt.c |   32 +++++++++++++++++++++---------
 sysdeps/{x86_64/fpu => arm}/e_sqrtf.c   |   32 ++++++++++++++++++++++--------
 3 files changed, 51 insertions(+), 19 deletions(-)
 copy sysdeps/{sparc/sparc32 => arm}/e_sqrt.c (59%)
 copy sysdeps/{x86_64/fpu => arm}/e_sqrtf.c (59%)

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