[Bug math/18956] powf inaccuracy

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https://sourceware.org/bugzilla/show_bug.cgi?id=18956

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=fa752c698146ca3e9f7747d33059fbef9bb02b0e

commit fa752c698146ca3e9f7747d33059fbef9bb02b0e
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Sat Sep 26 00:27:06 2015 +0000

    Fix powf inaccuracy (bug 18956).

    The flt-32 version of powf can be inaccurate because of bugs in the
    extra-precision calculation of (x-1)/(x+1) or (x-1.5)/(x+1.5) as part
    of calculating log(x) with extra precision: a constant used (as part
    of adding 1 or 1.5 through integer arithmetic) is incorrect, and then
    the code fails to mask a computed high part before using it in
    arithmetic that relies on s_h*t_h being exactly representable.  This
    patch fixes these bugs.

    Tested for x86_64 and x86.  x86_64 ulps for powf removed and
    regenerated to reflect reduced ulps from the increased accuracy for
    existing tests.

        [BZ #18956]
        * sysdeps/ieee754/flt-32/e_powf.c (__ieee754_powf): Add 0x00400000
        not 0x0040000 for high bit of mantissa.  Mask with 0xfffff000 when
        extracting high part.
        * math/auto-libm-test-in: Add another test of pow.
        * math/auto-libm-test-out: Regenerated.
        * sysdeps/x86_64/fpu/libm-test-ulps: Update.

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

Summary of changes:
 ChangeLog                         |   10 ++++++++++
 NEWS                              |    4 ++--
 math/auto-libm-test-in            |    1 +
 math/auto-libm-test-out           |   25 +++++++++++++++++++++++++
 sysdeps/ieee754/flt-32/e_powf.c   |    4 +++-
 sysdeps/x86_64/fpu/libm-test-ulps |   16 ++++++++--------
 6 files changed, 49 insertions(+), 11 deletions(-)

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