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[Bug math/16740] IBM long double frexpl wrong when value slightly smaller than a power of two


https://sourceware.org/bugzilla/show_bug.cgi?id=16740

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=380bd0fd2418f8988217de950f8b8ff18af0cb2b

commit 380bd0fd2418f8988217de950f8b8ff18af0cb2b
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Feb 26 15:13:22 2015 +0000

    Fix ldbl-128ibm logbl near powers of 2 (bug 18030).

    The ldbl-128ibm implementation of logbl produces incorrect results
    when the high part of the argument is a power of 2 and the low part a
    nonzero number with the opposite sign (and so the returned exponent
    should be 1 less than that of the high part).  For example, logbl
    (0x1.ffffffffffffffp1L) returns 2 but should return 1.  (This is
    similar to (fixed) bug 16740 for frexpl, and (fixed) bug 18029 for
    ilogbl.)  This patch adds checks for that case.

    Tested for powerpc.

        [BZ #18030]
        * sysdeps/ieee754/ldbl-128ibm/s_logbl.c (__logbl): Adjust exponent
        of power of 2 down when low part has opposite sign.
        * math/libm-test.inc (logb_test_data): Add more tests.

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

Summary of changes:
 ChangeLog                             |    7 +++++++
 NEWS                                  |    2 +-
 math/libm-test.inc                    |    5 +++++
 sysdeps/ieee754/ldbl-128ibm/s_logbl.c |   17 ++++++++++++++---
 4 files changed, 27 insertions(+), 4 deletions(-)

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