[Bug math/17803] scalblnl differs from scabnl and ldexpl (with radix 2)

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https://sourceware.org/bugzilla/show_bug.cgi?id=17803

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=5a9e4c09a2601a8100ea9a1f7bc0360782cd1625

commit 5a9e4c09a2601a8100ea9a1f7bc0360782cd1625
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Jan 12 23:02:14 2015 +0000

    Fix ldbl-96 scalblnl underflowing results (bug 17803).

    The ldbl-96 implementation of scalblnl (used for x86_64 and ia64) uses
    a condition k <= -63 to determine when a standard underflowing result
    tiny*__copysignl(tiny,x) should be returned.  However, that condition
    corresponds to values with exponent -16446 or less, and in the case of
    -16446, the correct result for round-to-nearest depends on whether the
    value is exactly 0x1p-16446 (half the least subnormal) or more than
    that.  This patch fixes the bug by changing the condition to k <= -64
    and accordingly adjusting the exponent by 64 not 63 when converting to
    a normal value.

    Tested for x86_64.

        [BZ #17803]
        * sysdeps/ieee754/ldbl-96/s_scalblnl.c (twom63): Rename to
        twom64.  Adjust value to 0x1p-64L.
        (__scalblnl): Only return standard underflowing result for K <=
        -64 not K <= -63; adjust exponent for underflowing result by 64
        not 63.
        * math/libm-test.inc (scalbn_test_data): Add more tests.
        (scalbln_test_data): Likewise.

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

Summary of changes:
 ChangeLog                            |    9 ++++++++
 NEWS                                 |    2 +-
 math/libm-test.inc                   |   38 ++++++++++++++++++++++++++++++++++
 sysdeps/ieee754/ldbl-96/s_scalblnl.c |    8 +++---
 4 files changed, 52 insertions(+), 5 deletions(-)

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