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[Bug math/14796] New: fma use of Dekker multiplication in directed rounding modes unsafe
- 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 23:43:33 +0000
- Subject: [Bug math/14796] New: fma use of Dekker multiplication in directed rounding modes unsafe
- Auto-submitted: auto-generated
http://sourceware.org/bugzilla/show_bug.cgi?id=14796
Bug #: 14796
Summary: fma use of Dekker multiplication in directed rounding
modes unsafe
Product: glibc
Version: 2.16
Status: NEW
Severity: normal
Priority: P2
Component: math
AssignedTo: unassigned@sourceware.org
ReportedBy: jsm28@gcc.gnu.org
Classification: Unclassified
fma uses Dekker multiplication, but in directed rounding modes this is unsafe
(at least for odd mantissa lengths); the rounding mode should be set to
FE_TONEAREST before using Dekker multiplication / Knuth addition. Example,
when using dbl-64/s_fma.c: fma (0x0.fffffffffffff8p0, 0x0.fffffffffffff8p0,
-0x0.fffffffffffffp0) returns 0x1p-1074 in FE_DOWNWARD mode; it should return
0x1p-106 in all rounding modes.
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