[Bug math/17964] [powerpc] sqrt inaccurate

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https://sourceware.org/bugzilla/show_bug.cgi?id=17964

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=e8bd5286c68bc35be3b41e94c15c4387dcb3bec9

commit e8bd5286c68bc35be3b41e94c15c4387dcb3bec9
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Feb 12 23:05:37 2015 +0000

    Fix powerpc software sqrt (bug 17964).

    As Adhemerval noted in
    <https://sourceware.org/ml/libc-alpha/2015-01/msg00451.html>, the
    powerpc sqrt implementation for when _ARCH_PPCSQ is not defined is
    inaccurate in some cases.

    The problem is that this code relies on fused multiply-add, and relies
    on the compiler contracting a * b + c to get a fused operation.  But
    sysdeps/ieee754/dbl-64/Makefile disables contraction for e_sqrt.c,
    because the implementation in that directory relies on *not* having
    contracted operations.

    While it would be possible to arrange makefiles so that an earlier
    sysdeps directory can disable the setting in
    sysdeps/ieee754/dbl-64/Makefile, it seems a lot cleaner to make the
    dependence on fused operations explicit in the .c file.  GCC 4.6
    introduced support for __builtin_fma on powerpc and other
    architectures with such instructions, so we can rely on that; this
    patch duly makes the code use __builtin_fma for all such fused
    operations.

    Tested for powerpc32 (hard float).

    2015-02-12  Joseph Myers  <joseph@codesourcery.com>

        [BZ #17964]
        * sysdeps/powerpc/fpu/e_sqrt.c (__slow_ieee754_sqrt): Use
        __builtin_fma instead of relying on contraction of a * b + c.

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

Summary of changes:
 ChangeLog                    |    6 ++++++
 NEWS                         |    2 +-
 sysdeps/powerpc/fpu/e_sqrt.c |   33 ++++++++++++++++++---------------
 3 files changed, 25 insertions(+), 16 deletions(-)

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