[Bug math/16353] New: expm1 missing underflows

[Bug math/16353] New: expm1 missing underflows

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

            Bug ID: 16353
           Summary: expm1 missing underflows
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org

At least some expm1 implementations do not raise the underflow exception for
subnormal arguments, when the result is tiny and inexact.  For example, on
x86_64, expm1 (0x1p-1074).

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[Bug math/16353] expm1 missing underflows

[zimmerma+gcc at loria dot fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

Paul Zimmermann <zimmerma+gcc at loria dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |zimmerma+gcc at loria dot fr

--- Comment #1 from Paul Zimmermann <zimmerma+gcc at loria dot fr> ---
on the other hand in some cases we get a spurious overflow (here for long
double on x86_64 with the current git version):

Testing function expm1 for exponent 0 [seed=8473].
 rounding mode MPFR_RNDN:
      wrong underflow flag: mpfr gives 0, library 16
      x=3.36210314311209350626e-4932
      library gives 3.36210314311209350626e-4932
      mpfr    gives 3.36210314311209350626e-4932

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[Bug math/16353] expm1 missing underflows

[joseph at codesourcery dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=16353

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
Assuming you mean underflow not overflow, this does not seem spurious 
(i.e., the return value and exceptions both appear consistent with an 
infinite-precision value that is only slightly inaccurate).

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[Bug math/16353] expm1 missing underflows

[zimmerma+gcc at loria dot fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

--- Comment #3 from Paul Zimmermann <zimmerma+gcc at loria dot fr> ---
(In reply to joseph@codesourcery.com from comment #2)
> Assuming you mean underflow not overflow, this does not seem spurious 
> (i.e., the return value and exceptions both appear consistent with an 
> infinite-precision value that is only slightly inaccurate).

ok, then I retract Comment 1. (And yes, I meant underflow.)

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[Bug math/16353] expm1 missing/spurious underflows

[zimmerma+gcc at loria dot fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

Paul Zimmermann <zimmerma+gcc at loria dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|expm1 missing underflows    |expm1 missing/spurious
                   |                            |underflows

--- Comment #4 from Paul Zimmermann <zimmerma+gcc at loria dot fr> ---
this example (in extended precision) gives a spurious underflow:

Testing function expm1 for exponent 0 [seed=8344].
 rounding mode MPFR_RNDN:
      wrong underflow flag for x=4.0000000000000028@-4096
      library gives 4.0000000000000028@-4096
      mpfr    gives 4.0000000000000028@-4096
      underflow: mpfr 0, library 16

Note the glibc result is *above* the smallest subnormal 0x1p-16382, thus no
exception should be raised.

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[Bug math/16353] expm1 missing underflows

[zimmerma+gcc at loria dot fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

Paul Zimmermann <zimmerma+gcc at loria dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|expm1 missing/spurious      |expm1 missing underflows
                   |underflows                  |

--- Comment #6 from Paul Zimmermann <zimmerma+gcc at loria dot fr> ---
(In reply to joseph@codesourcery.com from comment #5)
> Missing and spurious underflows are separate bugs, please restore this bug 
> to its original scope and file a separate bug for the spurious underflow.

sorry, done, new bug for spurious underflow is 16539

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[Bug math/16353] expm1 missing underflows

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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[Bug math/16353] expm1 missing underflows

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

--- Comment #7 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
This is an automated email from the git hooks/post-receive script. It was
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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4648909d56c1e9063017bcddd3271dffadef7cb5

commit 4648909d56c1e9063017bcddd3271dffadef7cb5
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Jun 23 20:20:10 2014 +0000

    Fix cosh spurious underflows from expm1 (bug 16354), inaccurate results
near 0 (bug 17061).

    This patch fixes bug 16354, spurious underflows from cosh when a tiny
    argument is passed to expm1 and expm1 correctly underflows although
    the final result of cosh should be 1.  As noted in that bug, some
    cases are latent because of expm1 implementations not raising
    underflow (bug 16353), but all the implementations are fixed
    similarly.  They already contained checks for tiny arguments, but the
    checks were too late to avoid underflow from expm1 (although they
    would avoid underflow from subsequent squaring of the result of
    expm1); they are moved before the expm1 calls.

    The thresholds used for considering arguments tiny are not
    particularly consistent in how they relate to the precision of the
    floating-point format in question.  They are, however, all sufficient
    to ensure that the round-to-nearest result of cosh is indeed 1 below
    the threshold (although sometimes they are smaller than necessary).
    But the previous logic did not return 1, but the previously computed 1
    + expm1(abs(x)) value.  And the thresholds in the ldbl-128 and
    ldbl-128ibm code (0x1p-71L - I suspect 0x3f8b was intended in the code
    instead of 0x3fb8 - and (roughly) 0x1p-55L) are not sufficient for
    that value to be 1.  So by moving the test for tiny arguments, and
    consequently returning 1 directly now the expm1 value hasn't been
    computed by that point, this patch also fixes bug 17061, the (large
    number of ulps) inaccuracy for small arguments in those
    implementations.  Tests for that bug are duly added.

    Tested x86_64 and x86 and ulps updated accordingly.  Also tested for
    mips64 and powerpc32 to validate the ldbl-128 and ldbl-128ibm changes.

        [BZ #16354]
        [BZ #17061]
        * sysdeps/ieee754/dbl-64/e_cosh.c (__ieee754_cosh): Check for
        small arguments before calling __expm1.
        * sysdeps/ieee754/flt-32/e_coshf.c (__ieee754_coshf): Check for
        small arguments before calling __expm1f.
        * sysdeps/ieee754/ldbl-128/e_coshl.c (__ieee754_coshl): Check for
        small arguments before calling __expm1l.
        * sysdeps/ieee754/ldbl-128ibm/e_coshl.c (__ieee754_coshl):
        Likewise.
        * sysdeps/ieee754/ldbl-96/e_coshl.c (__ieee754_coshl): Likewise.
        * math/auto-libm-test-in: Add more cosh tests.  Do not allow
        spurious underflow for some cosh tests.
        * math/auto-libm-test-out: Regenerated.
        * sysdeps/i386/fpu/libm-test-ulps: Update.

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

Summary of changes:
 ChangeLog                             |   16 +
 NEWS                                  |   20 +-
 math/auto-libm-test-in                |   13 +-
 math/auto-libm-test-out               |  732 +++++++++++++++++++--------------
 sysdeps/i386/fpu/libm-test-ulps       |    1 +
 sysdeps/ieee754/dbl-64/e_cosh.c       |    4 +-
 sysdeps/ieee754/flt-32/e_coshf.c      |    2 +-
 sysdeps/ieee754/ldbl-128/e_coshl.c    |    4 +-
 sysdeps/ieee754/ldbl-128ibm/e_coshl.c |    2 +-
 sysdeps/ieee754/ldbl-96/e_coshl.c     |    2 +-
 10 files changed, 458 insertions(+), 338 deletions(-)

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[Bug math/16353] expm1 missing underflows

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

--- Comment #8 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4060283decf711ed95a6c4993ffeaba39f8cd40a

commit 4060283decf711ed95a6c4993ffeaba39f8cd40a
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Tue Jun 24 21:00:08 2014 +0000

    Fix x86/x86_64 expm1l spurious underflow exceptions (bug 16539).

    This patch fixes bug 16539, spurious underflow exceptions from x86 /
    x86-64 expm1l.  The problem is that the computation of a base-2
    exponent with extra precision involves spurious underflows for
    arguments that are small but not subnormal, so a check is added to
    just return the argument in those cases.  (If the argument *is*
    subnormal, underflowing is correct and the existing code will always
    underflow, so it suffices to keep using the existing code in that
    case; some expm1 implementations have a bug (bug 16353) with missing
    underflow exceptions, but I don't think there's such a bug in this
    particular version.)

    Tested x86_64 and x86; no ulps updates needed.

    (auto-libm-test-out diffs omitted below.)

        [BZ #16539]
        * sysdeps/i386/fpu/e_expl.S (IEEE754_EXPL) [USE_AS_EXPM1L]: Just
        return the argument for normal arguments with exponent below -64.
        * sysdeps/x86_64/fpu/e_expl.S (IEEE754_EXPL) [USE_AS_EXPM1L]:
        Likewise.
        * math/auto-libm-test-in: Add another test of expm1.
        * math/auto-libm-test-out: Regenerated.

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

Summary of changes:
 ChangeLog                   |    8 ++++
 NEWS                        |   22 ++++++------
 math/auto-libm-test-in      |    2 +
 math/auto-libm-test-out     |   81 +++++++++++++++++++++++++++++++++++++++++++
 sysdeps/i386/fpu/e_expl.S   |   10 +++++
 sysdeps/x86_64/fpu/e_expl.S |   10 +++++
 6 files changed, 122 insertions(+), 11 deletions(-)

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[Bug math/16353] expm1 missing underflows

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

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

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

--- Comment #10 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 2.22.

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[Bug math/16353] expm1 missing underflows

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16353

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=554edb23ffc7a953ca86309cc5f02dbd1a63abe0

commit 554edb23ffc7a953ca86309cc5f02dbd1a63abe0
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Jun 22 21:06:19 2015 +0000

    Fix expm1 missing underflows (bug 16353).

    Similar to various other bugs in this area, some expm1 implementations
    do not raise the underflow exception for subnormal arguments, when the
    result is tiny and inexact.  This patch forces the exception in a
    similar way to previous fixes.

    (The issue does not apply to the ldbl-* implementations or to those
    for x86 / x86_64 long double.  The change to
    sysdeps/ieee754/dbl-64/wordsize-64/e_cosh.c is one I missed when
    previously fixing bug 16354; the bug in that implementation was
    previously latent, but the expm1 fixes stopped it being latent and so
    required it to be fixed to avoid spurious underflows from cosh.)

    Tested for x86_64 and x86.

        [BZ #16353]
        * sysdeps/i386/fpu/s_expm1.S (dbl_min): New object.
        (__expm1): Force underflow exception for arguments with small
        absolute value.
        * sysdeps/i386/fpu/s_expm1f.S (flt_min): New object.
        (__expm1f): Force underflow exception for arguments with small
        absolute value.
        * sysdeps/ieee754/dbl-64/s_expm1.c: Include <float.h>.
        (__expm1): Force underflow exception for arguments with small
        absolute value.
        * sysdeps/ieee754/flt-32/s_expm1f.c: Include <float.h>.
        (__expm1f): Force underflow exception for arguments with small
        absolute value.
        * sysdeps/ieee754/dbl-64/wordsize-64/e_cosh.c (__ieee754_cosh):
        Check for small arguments before calling __expm1.
        * math/auto-libm-test-in: Do not mark underflow exceptions as
        possibly missing for bug 16353.
        * math/auto-libm-test-out: Regenerated.

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

Summary of changes:
 ChangeLog                                   |   21 +++
 NEWS                                        |   28 ++--
 math/auto-libm-test-in                      |   11 +-
 math/auto-libm-test-out                     |  226 +++++++++++++-------------
 sysdeps/i386/fpu/s_expm1.S                  |   22 +++
 sysdeps/i386/fpu/s_expm1f.S                 |   22 +++
 sysdeps/ieee754/dbl-64/s_expm1.c            |    6 +
 sysdeps/ieee754/dbl-64/wordsize-64/e_cosh.c |    3 +-
 sysdeps/ieee754/flt-32/s_expm1f.c           |    6 +
 9 files changed, 211 insertions(+), 134 deletions(-)

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