[Bug math/18197] New: atan inaccuracy

[Bug math/18197] New: atan inaccuracy

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18197

            Bug ID: 18197
           Summary: atan inaccuracy
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org

The dbl-64 version of atan is inaccurate in non-default rounding modes, with
errors exceeding the 9ulp maximum considered acceptable by libm-test.inc.  For
example (on x86_64):

Failure: Test: atan_upward (-0x3.b02d84p-4)
Result:
 is:         -2.26554758731449840114e-01  -0x1.cffbf0fc09b910000000p-3
 should be:  -2.26554758731449507047e-01  -0x1.cffbf0fc09b850000000p-3
 difference:  3.33066907387546962128e-16   0x1.80000000000000000000p-52
 ulp       :  12.0000
 max.ulp   :  1.0000

Failure: Test: atan_towardzero (-0x3.3fb708p-4)
Result:
 is:         -2.00331721927011846551e-01  -0x1.9a478490391a30000000p-3
 should be:  -2.00331721927011541239e-01  -0x1.9a478490391980000000p-3
 difference:  3.05311331771918048616e-16   0x1.60000000000000000000p-52
 ulp       :  11.0000
 max.ulp   :  1.0000

Failure: Test: atan_downward (-0x3.3fb708p-4)
Result:
 is:         -2.00331721927011846552e-01  -0x1.9a478490391a30000000p-3
 should be:  -2.00331721927011568996e-01  -0x1.9a478490391990000000p-3
 difference:  2.77555756156289135105e-16   0x1.40000000000000000000p-52
 ulp       :  10.0000
 max.ulp   :  1.0000

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[Bug math/18197] atan inaccuracy

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18197

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
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The branch, master has been updated
       via  ae63c7ebedcaa57017df4cb8ff0494a4705321ee (commit)
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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=ae63c7ebedcaa57017df4cb8ff0494a4705321ee

commit ae63c7ebedcaa57017df4cb8ff0494a4705321ee
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Apr 8 17:14:12 2015 +0000

    Fix dbl-64 atan in non-default rounding modes (bug 18197).

    The dbl-64 implementation of atan does computations that expect to run
    in round-to-nearest mode, and in other modes the errors can accumulate
    to more than the maximum accepted 9ulp.  This patch makes it use
    FE_TONEAREST internally, similar to other functions with such issues.

    Tested for x86_64 and x86; no ulps updates needed.

        [BZ #18197]
        * sysdeps/ieee754/dbl-64/s_atan.c: Include <fenv.h>.
        (atan): Set FE_TONEAREST mode for internal computations.
        * math/auto-libm-test-in: Add more tests of atan.
        * math/auto-libm-test-out: Regenerated.

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

Summary of changes:
 ChangeLog                       |    8 ++++++
 NEWS                            |    2 +-
 math/auto-libm-test-in          |    2 +
 math/auto-libm-test-out         |   50 +++++++++++++++++++++++++++++++++++++++
 sysdeps/ieee754/dbl-64/s_atan.c |    2 +
 5 files changed, 63 insertions(+), 1 deletions(-)

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[Bug math/18197] atan inaccuracy

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18197

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #2 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 2.22.

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