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From: Paul A Bristow (pbristow_at_[hidden])
Date: 2005-11-15 10:21:54


| -----Original Message-----
| From: boost-bounces_at_[hidden]
| [mailto:boost-bounces_at_[hidden]] On Behalf Of Ian McCulloch
| Sent: 13 November 2005 22:11
| To: boost_at_[hidden]
| Subject: Re: [boost] [Test] BOOST_CHECK_CLOSE Feature request
| John Maddock wrote:
| >> Related to this, there is a very interesting article on
| comparing IEEE
| >> floats at
| >>
| gfloats.htm
| >>
| >> This makes use of the property that IEEE floating point formats are
| >> lexicographically ordered when reinterpreted as an
| appropriate length
| >> signed integer. It appears you can use this to define a closeness
| >> measure based on how many units in the last place the two numbers
| >> differ by (equivalently, if you enumerated all possible floats and
| >> counted how far
| >> away one number is from the other in the enumeration). I only just
| >> came across it now so I havn't tried playing with it yet, but it
| >> looks like it would make a useful closeness measure.
| >
| > I must admit I've been looking for a way to measure errors as ULP.
| > However, I have my doubts about this: are integer types and
| floating point
| > types
| > always of the same endianness? I suspect not, but can't be
| 100% sure.
| > There are also problems with long doubles to consider:
| padding bits on
| > some implementations, and strange "a long double is
| actually a pair of
| > doubles" on Darwin.
| It is frustrating, that the article shows that it is quite
| easy to do a
| ULP-based comparison given the right hardware (even doing
| some bit-banging
| on eg, Darwin would be not so hard), but a portable version
| to use as a
| fallback seems really hard. Maybe it is possible using
| frexp(), ldexp()
| etc. The problem is if you don't assume IEEE arithmetic,
| then there isn't
| much you can guarantee about floating point behaviour. Not to mention
| handling FLT_RADIX != 2 ... Scalbn() might be useful there,
| but that is
| C99 only...

But TR1 pulls in C99, so we should be able to start relying on it?

| There is also the problem of platforms that are not quite IEEE. For
| example, IIRC on Alpha by default doesn't generate subnormals.

This is correct, but I am not sure that this matters in making the

Expectations of total portability seem misguided in any case IMO.
So can't we simply test for IEEE fp layout with
and give up if not?

As a specific example, I have recently been using NTL quad_float type
(simillar to Darwin long double) to check that the Math functions can use a
UDT. This worked fine with Boost Test and I think just adding a version for
this and other UDTs as the need arises is all we can hope for.


Paul A Bristow
Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB
Phone and SMS text +44 1539 561830, Mobile and SMS text +44 7714 330204
mailto: pbristow_at_[hidden]

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