[Bug target/96793] __builtin_floor produces wrong result when rounding direction is FE_DOWNWARD

ubizjak at gmail dot com gcc-bugzilla@gcc.gnu.org
Tue Dec 22 17:20:26 GMT 2020


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96793

--- Comment #19 from Uroš Bizjak <ubizjak at gmail dot com> ---


https://gcc.gnu.org/g:337ed0eb490b14899f4049bc4c8922eb1d8a2e67

commit r11-6303-g337ed0eb490b14899f4049bc4c8922eb1d8a2e67
Author: Uros Bizjak <ubizjak@gmail.com>
Date:   Tue Dec 22 18:13:24 2020 +0100

    i386: Fix __builtin_floor with FE_DOWNWARD rounding direction [PR96793]

    x86_expand_floorceil expander uses x86_sse_copysign_to_positive, which
    is unable to change the sign from - to +.  When FE_DOWNWARD rounding
    direction is in effect, the expanded sequence that involves subtraction
    can trigger x - x = -0.0 special rule.  x86_sse_copysign_to_positive
    fails to change the sign of the intermediate value, assumed to always
    be positive, back to positive.

    The patch adds one extra fabs that strips the sign from the intermediate
    value when flag_rounding_math is in effect.

    2020-12-22  Uroš Bizjak  <ubizjak@gmail.com>

    gcc/
            PR target/96793
            * config/i386/i386-expand.c (ix86_expand_floorceil):
            Remove the sign of the intermediate value for flag_rounding_math.
            (ix86_expand_floorceildf_32): Ditto.

    gcc/testsuite/
            PR target/96793
            * gcc.target/i386/pr96793.c: New test.


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