[Bug math/23961] powf can overflow to inf without setting errno in non-nearest rounding mode

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https://sourceware.org/bugzilla/show_bug.cgi?id=23961

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=505b5b292293a5d6bd4046a6bc7f8c2381a33da4

commit 505b5b292293a5d6bd4046a6bc7f8c2381a33da4
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Mon Dec 10 11:08:36 2018 +0000

    Fix powf overflow handling in non-nearest rounding mode [BZ #23961]

    The threshold value at which powf overflows depends on the rounding mode
    and the current check did not take this into account. So when the result
    was rounded away from zero it could become infinity without setting
    errno to ERANGE.

    Example: pow(0x1.7ac7cp+5, 23) is 0x1.fffffep+127 + 0.1633ulp

    If the result goes above 0x1.fffffep+127 + 0.5ulp then errno is set,
    which is fine in nearest rounding mode, but

      powf(0x1.7ac7cp+5, 23) is inf in upward rounding mode
      powf(-0x1.7ac7cp+5, 23) is -inf in downward rounding mode

    and the previous implementation did not set errno in these cases.

    The fix tries to avoid affecting the common code path or calling a
    function that may introduce a stack frame, so float arithmetics is used
    to check the rounding mode and the threshold is selected accordingly.

        [BZ #23961]
        * math/auto-libm-test-in: Add new test case.
        * math/auto-libm-test-out-pow: Regenerated.
        * sysdeps/ieee754/flt-32/e_powf.c (__powf): Fix overflow check.

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

Summary of changes:
 ChangeLog                       |    7 +++++
 math/auto-libm-test-in          |    2 +
 math/auto-libm-test-out-pow     |   50 +++++++++++++++++++++++++++++++++++++++
 sysdeps/ieee754/flt-32/e_powf.c |   12 +++++++++
 4 files changed, 71 insertions(+), 0 deletions(-)

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