[New-bugs-announce] [issue2937] Incorrect rounding in floating-point operations with gcc/x87
report at bugs.python.org
Wed May 21 22:36:57 CEST 2008
New submission from Mark Dickinson <dickinsm at gmail.com>:
On some older Intel 32-bit hardware, under Linux, floating-point
operations don't always give correctly rounded results. Here's an
example involving addition, on SuSE Linux 10.2/Xeon.
Python 2.6a3+ (trunk:63521, May 21 2008, 15:40:39)
[GCC 4.1.2 20061115 (prerelease) (SUSE Linux)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 1e16 + 2.999
>>> 1e16 + 2.9999
The second result should really be 1e16+2., not 1e16+4. This appears to
be related to this GCC issue:
Various fixes are possible. One possible fix is to add the -ffloat-
store flag to the gcc options. Another is to use the information in
fpu_control.h, if available, to set the precision control. Yet another
is to sprinkle some 'volatile' modifiers throughout floatobject.c.
It's not clear to me that this *should* be fixed, but I think the
problem should at least be documented. Hence this bug report.
components: Interpreter Core
title: Incorrect rounding in floating-point operations with gcc/x87
versions: Python 2.6, Python 3.0
Tracker <report at bugs.python.org>
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