[Bug math/5794] New: expm1l(x) overflows when 11356.1768 < x <= 11356.5234

[Bug math/5794] New: expm1l(x) overflows when 11356.1768 < x <= 11356.5234

[petrosyan at gmail dot com]

The value of (e^x - 1) is representable as a long double when x <=
11356.5234, but expm1l(x) overflows to infinity when x > 11356.1768
(approximately).

Steps to Reproduce:
Compile and run the following program:

#include <stdio.h>
#include <math.h>

main ()
{
  long double a, b;
  const long double x = 11356.35L;

  a = expl (x) - 1.0L;
  b = expm1l (x);
  if (a == b) {
    printf ("e^%.2Lf - 1 = %Lg\n", b);
    return 0;
  } else {
    printf ("e^%.2Lf - 1 = %Lg; but expm1l(%.2Lf) returned %Lg\n",
	    x, a, x, b);
    return 1;
  }
}


Actual Results:  e^11356.35 - 1 = 1.00032e+4932; but expm1l(11356.35)
returned inf


Expected Results:  e^11356.35 - 1 = 1.00032e+4932


Additional info:

This bug probably isn't even worth reporting, except for completeness. 
expm1l is designed to compute e^x-1 more accurately when x is negative, so
[at least the i386 version] uses a slightly different algorithm than expl. 
This algorithm overflows for large x where expl would not.  But then, expm1
really isn't intended for large x anyway.

In my own port of the math library, I circumvent the problem by calling
__ieee754_expl when x >= 64, because at that magnitude, -1 is too small to
affect the result.  This is probably overkill, but it works.

-- 
           Summary: expm1l(x) overflows when 11356.1768 < x <= 11356.5234
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: minor
          Priority: P2
         Component: math
        AssignedTo: aj at suse dot de
        ReportedBy: petrosyan at gmail dot com
                CC: glibc-bugs at sources dot redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=5794

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[Bug math/5794] expm1l(x) overflows when 11356.1768 < x <= 11356.5234

[petrosyan at gmail dot com]

------- Additional Comments From petrosyan at gmail dot com  2008-02-26 07:08 -------
original bug filed at https://bugzilla.redhat.com/show_bug.cgi?id=43354

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unspecified                 |2.2.5


http://sourceware.org/bugzilla/show_bug.cgi?id=5794

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[Bug math/5794] expm1l(x) overflows when 11356.1768 < x <= 11356.5234

[petrosyan at gmail dot com]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|2.2.5                       |unspecified


http://sourceware.org/bugzilla/show_bug.cgi?id=5794

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[Bug math/5794] expm1l(x) overflows when 11356.1768 < x <= 11356.5234

[vda dot linux at googlemail dot com]

------- Additional Comments From vda dot linux at googlemail dot com  2008-07-30 10:07 -------
I submitted a fix, see RH bugzilla:
https://bugzilla.redhat.com/show_bug.cgi?id=43354

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http://sourceware.org/bugzilla/show_bug.cgi?id=5794

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[Bug math/5794] expm1l(x) overflows when 11356.1768 < x <= 11356.5234

[drepper at redhat dot com]

------- Additional Comments From drepper at redhat dot com  2008-08-05 22:09 -------
Fixed in cvs.

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http://sourceware.org/bugzilla/show_bug.cgi?id=5794

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[Bug math/5794] expm1l(x) overflows when 11356.1768 < x <= 11356.5234

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=5794

Florian Weimer <fweimer at redhat dot com> changed:

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

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[Bug math/5794] expm1l(x) overflows when 11356.1768 < x <= 11356.5234

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=5794

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

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

--- Comment #4 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-02-29 17:49:28 UTC ---
This was fixed by:

2008-08-05  Ulrich Drepper  <drepper@redhat.com>

        [BZ #5794]
        * sysdeps/i386/fpu/s_expm1l.S: Simply use exp implementation for large
        parameters.
        * sysdeps/x86_64/fpu/s_expm1l.S: Likewise.
        Patch by Denys Vlasenko <dvlasenk@redhat.com>.

A testcase wasn't added to the testsuite at the time.  I've now proposed adding
one: http://sourceware.org/ml/libc-alpha/2012-02/msg00773.html

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