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[Bug math/14793] New: Further spurious underflows from fma when a * b is small
- 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 00:53:00 +0000
- Subject: [Bug math/14793] New: Further spurious underflows from fma when a * b is small
- Auto-submitted: auto-generated
http://sourceware.org/bugzilla/show_bug.cgi?id=14793
Bug #: 14793
Summary: Further spurious underflows from fma when a * b is
small
Product: glibc
Version: 2.16
Status: NEW
Severity: normal
Priority: P2
Component: math
AssignedTo: unassigned@sourceware.org
ReportedBy: jsm28@gcc.gnu.org
Classification: Unclassified
Given my patch http://sourceware.org/ml/libc-alpha/2012-11/msg00001.html for
bug 14784 there are still some underflows from fma (a, b, c) when a * b is
small, because the logic has a gap of two exponents between the cases when a *
b is so small that its exact value doesn't matter, and the cases where the
scaling-up for small a * b is enough to avoid underflow when computing the
Dekker multiplication. Example, on x86 or x86_64: fmal
(0x1.0000000000000002p-16382L, 0x1.0000000000000002p-66L, 0x1p16383L) raises a
spurious underflow exception.
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