[Bug math/17834] [ldbl-96] scalblnl incorrect for subnormal arguments

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https://sourceware.org/bugzilla/show_bug.cgi?id=17834

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=34e93d6c76b7a6eaf94697a172b958704fd33396

commit 34e93d6c76b7a6eaf94697a172b958704fd33396
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Jan 12 22:34:58 2015 +0000

    Fix ldbl-96 scalblnl for subnormal arguments (bug 17834).

    The ldbl-96 implementation of scalblnl (used for x86_64 and ia64) is
    incorrect for subnormal arguments (this is a separate bug from bug
    17803, which is about underflowing results).  There are two problems
    with the adjustments of subnormal arguments: the "two63" variable
    multiplied by is actually 0x1p52L not 0x1p63L, so is insufficient to
    make values normal, and then GET_LDOUBLE_EXP(es,x), used to extract
    the new exponent, extracts it into a variable that isn't used, while
    the value taken to by the new exponent is wrongly taken from the high
    part of the mantissa before the adjustment (hx).  This patch fixes
    both those problems and adds appropriate tests.

    Tested for x86_64.

        [BZ #17834]
        * sysdeps/ieee754/ldbl-96/s_scalblnl.c (two63): Change value to
        0x1p63L.
        (__scalblnl): Get new exponent of adjusted subnormal value from ES
        not HX.
        * math/libm-test.inc (scalbn_test_data): Add more tests.
        (scalbln_test_data): Likewise.

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

Summary of changes:
 ChangeLog                            |   10 ++++++++++
 NEWS                                 |    2 +-
 math/libm-test.inc                   |   22 ++++++++++++++++++++++
 sysdeps/ieee754/ldbl-96/s_scalblnl.c |    4 ++--
 4 files changed, 35 insertions(+), 3 deletions(-)

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