[Bug math/14783] New: Incorrect subnormal fma results in non-default rounding modes

[Bug math/14783] New: Incorrect subnormal fma results in non-default rounding modes

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14783

             Bug #: 14783
           Summary: Incorrect subnormal fma results in non-default
                    rounding modes
           Product: glibc
           Version: 2.16
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jsm28@gcc.gnu.org
    Classification: Unclassified


The various fma implementations have logic for scaling down results, in the
case of subnormal results that are at least half the smallest positive normal
value in magnitude, that assumes round-to-nearest and so is incorrect in other
rounding modes.  (The code at fault is the same as in bug 14152, but from the
user perspective they are separate bugs, missing exceptions (any rounding mode)
versus incorrect results (non-default rounding modes).)

For example, on x86, fmal (0x1.4p-16382L, 0x1.0000000000000004p-1L,
0x1p-16384L) returns 0x7.000000000000001p-16385L independent of rounding mode,
but this should be 0x7.000000000000002p-16385L in FE_UPWARD mode.

Working on a patch.

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[Bug math/14783] Incorrect subnormal fma results in non-default rounding modes

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14783

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

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

--- Comment #1 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-10-30 13:56:59 UTC ---
Fixed for 2.17 by:

commit 8627a2329c5e6fc09bf9f0f070a21aca41f2b122
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Tue Oct 30 13:54:50 2012 +0000

    Fix fma missing underflows and bad results for some subnormal results (bugs
14152, 14783).

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[Bug math/14783] Incorrect subnormal fma results in non-default rounding modes

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14783

Florian Weimer <fweimer at redhat dot com> changed:

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

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