Author christian.heimes
Recipients christian.heimes, gvanrossum, nascheme, noam, tim.peters
Date 2007-12-11.15:47:22
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Message-id <475EB109.1040506@cheimes.de>
In-reply-to <1197386480.04.0.438061160697.issue1580@psf.upfronthosting.co.za>
Content
Guido van Rossum wrote:
> (1a) Perhaps it's better to only do this for Python 3.0, which has a
> smaller set of platforms to support.

+1
Does Python depend on a working, valid and non-broken IEEE 754 floating
point arithmetic? Could we state the Python's float type depends on
IEEE754-1985 conform 64bit double precision format. Developers for a
platform with no IEEE754 conform double must role their own floatobject
implementation.

> (2) Have you two (Christian and Noam) figured out yet why repr(1e5) is
> '1.0' on some platforms?  If it's not byte order (and I no longer
> believe it is) what could it be?  An uninitialized variable?  It doesn't
> seem to vary with the processor, but it could vary with the C runtime or OS.

I wasn't able to reproduce the problem on my work stations. Can you give
us more information about the computer with failing tests? CPU, OS,
Kernel, libc etc.

> (3) Detecting NaNs and infs in a platform-independent way is tricky,
> probably beyond you (I know it's beyond me).  (Of course, producing
> standardized output for them and recognizing these as input is easy, and
> that should definitely be done.)

In fact detecting NaNs and Infs is platform-independent is dead easy IFF
 the platform has IEEE 754 support. A double uses 64bit to store the
data, 1 bit signum, 11 bit exponent and 53bit fraction part (aka
mantissa). When all bits of the exponent are set (0x7ff) the number is
either a NaN or +/-Inf. For Infs the fraction part has no bits set (0)
and the signum bit signals +Inf (sig: 0) or -Inf (sig: 1). If one to all
bits of the mantissa is set, it's a NaN.

The form (normalized or denormalized) is regardless for the detection of
NaN and Inf. An autoconf macro can easily detect the endianness of the
double implementation.

We can also use the much slower version and check if x > DBL_MAX for
+Inf, x < -DBL_MAX for -Inf and x != x for NaN.

Christian
History
Date User Action Args
2007-12-11 15:47:23christian.heimessetspambayes_score: 0.00361846 -> 0.00361846
recipients: + christian.heimes, gvanrossum, tim.peters, nascheme, noam
2007-12-11 15:47:22christian.heimeslinkissue1580 messages
2007-12-11 15:47:22christian.heimescreate