[Bug math/4108] New: erfcf returns incorrectly rounded results compared to erfc

[Bug math/4108] New: erfcf returns incorrectly rounded results compared to erfc

[myles at peakstreaminc dot com]

In the attached program results from erfc are compared with results from erfcf
and large differences are flagged. The errors occur for values near 4.0 and 8.0,
but slightly under.

Two examples:

x: 3.9311595 (407b981e)  float(erfc): 2.7055004e-08 (32e86695)  erfcf: 2.705503e
-08 (32e866a4)  diff: 2.6645353e-14 (45 ULP)

x: 7.9999866 (40ffffe4)  float(erfc): 1.1226714e-29 (f63b46e)  erfcf: 1.1226756e
-29 (f63b4a7)  diff: 4.2882034e-35 (114 ULP)


It is possible the values for erfc are incorrect, as I don't have another source
to check that.

Also happens on x86_64-redhat-linux

-- 
           Summary: erfcf returns incorrectly rounded results compared to
                    erfc
           Product: glibc
           Version: 2.3.4
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: aj at suse dot de
        ReportedBy: myles at peakstreaminc dot com
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: i386-redhat-linux
  GCC host triplet: i386-redhat-linux
GCC target triplet: i386-redhat-linux


http://sourceware.org/bugzilla/show_bug.cgi?id=4108

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[Bug math/4108] erfcf returns incorrectly rounded results compared to erfc

[myles at peakstreaminc dot com]

------- Additional Comments From myles at peakstreaminc dot com  2007-02-28 02:35 -------
Created an attachment (id=1585)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=1585&action=view)
Program (C++) that compares erfcf and erfc results

This program may be compiled with g++, e.g. 
gcc version 3.4.5 20051201 (Red Hat 3.4.5-2)


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http://sourceware.org/bugzilla/show_bug.cgi?id=4108

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[Bug math/4108] erfcf returns incorrectly rounded results compared to erfc

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=4108

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

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

--- Comment #4 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-03-01 21:18:28 UTC ---
Fixed by:

commit 7b1902cb3e64d340247638a8781770c3a4d850ce
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Mar 1 21:15:38 2012 +0000

    Improve erfc accuracy.

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[Bug math/4108] erfcf returns incorrectly rounded results compared to erfc

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=4108

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at sourceware    |jsm28 at gcc dot gnu.org
                   |dot org                     |

--- Comment #3 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-02-26 16:33:28 UTC ---
Patch submitted: http://sourceware.org/ml/libc-alpha/2012-02/msg00640.html

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[Bug math/4108] erfcf returns incorrectly rounded results compared to erfc

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=4108

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|aj at suse dot de           |unassigned at sourceware
                   |                            |dot org

--- Comment #2 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-02-26 14:00:40 UTC ---
Confirmed in essence (the numbers of ULPs given in this report don't seem
right, though).  erfcf (0X1.F7303CP+1) should be 0X1.D0CD2AP-26 but glibc gives
0X1.D0CD46P-26.  erfcf (0X1.FFFFC8P+2) should be 0X1.C768DCP-97 but glibc gives
0X1.C7694EP-97.

Probably same cause as bug 2541.

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