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[Bug math/14785] New: Incorrect results from fma in round-toward-zero mode when a * b is small and of opposite sign to c
- From: "jsm28 at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sources dot redhat dot com
- Date: Tue, 30 Oct 2012 01:08:05 +0000
- Subject: [Bug math/14785] New: Incorrect results from fma in round-toward-zero mode when a * b is small and of opposite sign to c
- Auto-submitted: auto-generated
http://sourceware.org/bugzilla/show_bug.cgi?id=14785
Bug #: 14785
Summary: Incorrect results from fma in round-toward-zero mode
when a * b is small and of opposite sign to c
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 (a, b, c) gives incorrect results in round-to-zero mode in some cases when
a * b is sufficiently small that the implementation uses a * b + c for the
computation, but a * b has opposite sign to c. Example, on x86: fmal
(0x1p-16382L, -0x1p-16382L, 0x1p16383L). In such a case, a * b rounds to zero,
and so the final result returned is equal to c, but correct rounding towards
zero should return the next value after c in the direction of 0. Using a * b +
c would give the correct results in the other three rounding modes, but not in
FE_TOWARDZERO mode when the signs differ like this.
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