[Bug math/18981] New: i386 scalb*, ldexp return with excess range and precision

[Bug math/18981] New: i386 scalb*, ldexp return with excess range and precision

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18981

            Bug ID: 18981
           Summary: i386 scalb*, ldexp return with excess range and
                    precision
           Product: glibc
           Version: 2.22
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

The i386 scalb* and ldexp functions return values with excess range and
precision.  This causes test failures following Wilco Dijkstra's patches to
make isfinite etc. expand inline, but was unambiguously a bug in any case -
these functions have fully determined results bound to IEEE operations and so
must not have excess range and precision in their results.  (Excess precision
arises for these functions only in the case of subnormal results.)

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[Bug math/18981] i386 scalb*, ldexp return with excess range and precision

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18981

--- Comment #1 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Example failure, for float:

Failure: scalbn (min_value, -MANT_DIG-1): errno set to 0, expected 34 (ERANGE)

I don't see test failures for scalb for some reason, but the issue is clearly
present in the sources.

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[Bug math/18981] i386 scalb*, ldexp return with excess range and precision

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18981

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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=94ced920a951bcab51f2ef955ccd2cc51668e6f3

commit 94ced920a951bcab51f2ef955ccd2cc51668e6f3
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Fri Sep 18 20:34:59 2015 +0000

    Avoid excess range in results from i386 scalb functions (bug 18981).

    i386 scalb / scalbn / scalbln (and thus ldexp) functions for float and
    double can return results with excess range (and consequently excess
    precision for subnormal results).  As the results of these functions
    are fully determined by reference to IEEE 754 operations, this is
    unambiguously a bug, apart from the testsuite failures it causes.

    This patch makes those functions store their results on the stack and
    load them back to eliminate the excess range.  Double rounding is not
    a problem, as the only cases where it could occur are when the result
    overflows or underflows for extended precision, and then the
    double-rounded results are the same as the single-rounded results.

    The new macros will be used for more functions, more such macros
    added, and existing code refactored to use such macros, in subsequent
    patches.

    Tested for x86.  Committed.

        [BZ #18981]
        * sysdeps/i386/fpu/i386-math-asm.h: New file.
        * sysdeps/i386/fpu/e_scalb.S: Include <i386-math-asm.h>.
        (__ieee754_scalb): Use DBL_NARROW_EVAL.
        * sysdeps/i386/fpu/e_scalbf.S: Include <i386-math-asm.h>.
        (__ieee754_scalbf): Use FLT_NARROW_EVAL.
        * sysdeps/i386/fpu/s_scalbn.S: Include <i386-math-asm.h>.
        (__scalbn): Use DBL_NARROW_EVAL.
        * sysdeps/i386/fpu/s_scalbnf.S: Include <i386-math-asm.h>.
        (__scalbnf): Use FLT_NARROW_EVAL.

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

Summary of changes:
 ChangeLog                                          |   13 +++++++++
 NEWS                                               |    2 +-
 sysdeps/i386/fpu/e_scalb.S                         |    2 +
 sysdeps/i386/fpu/e_scalbf.S                        |    2 +
 .../i386/fpu/i386-math-asm.h                       |   29 +++++++++++++------
 sysdeps/i386/fpu/s_scalbn.S                        |    2 +
 sysdeps/i386/fpu/s_scalbnf.S                       |    2 +
 7 files changed, 42 insertions(+), 10 deletions(-)
 copy benchtests/bench-util.h => sysdeps/i386/fpu/i386-math-asm.h (58%)

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[Bug math/18981] i386 scalb*, ldexp return with excess range and precision

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18981

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

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

--- Comment #3 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 2.23.

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[Bug math/18981] i386 scalb*, ldexp return with excess range and precision

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18981

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |2.23

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