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[Bug math/14783] New: Incorrect subnormal fma results in non-default 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: Mon, 29 Oct 2012 23:55:40 +0000
- Subject: [Bug math/14783] New: Incorrect subnormal fma results in non-default rounding modes
- Auto-submitted: auto-generated
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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