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[Bug math/14797] New: fma use of a * b + c on overflow incorrect in directed rounding modes
- From: "jsm28 at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sources dot redhat dot com
- Date: Thu, 01 Nov 2012 23:47:23 +0000
- Subject: [Bug math/14797] New: fma use of a * b + c on overflow incorrect in directed rounding modes
- Auto-submitted: auto-generated
http://sourceware.org/bugzilla/show_bug.cgi?id=14797
Bug #: 14797
Summary: fma use of a * b + c on overflow incorrect in directed
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
Various fma implementations fall back to a * b + c when a * b is known to
overflow so much that the final result would still overflow, whatever the value
of c. But this is incorrect in directed rounding modes where a * b may not be
an infinity. Example using dbl-64/s_fma.c: fma (DBL_MAX, DBL_MAX, -DBL_MIN)
should return DBL_MAX in FE_DOWNWARD mode but returns the next smaller
floating-point number instead.
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