Re: The disturbing myth of Eiffel portability

[fred@genesis.demon.co.uk (Lawrence Kirby)]

In article <3294e64b.74799475@news2.ibm.net>
           cjames3@ibm.net "The Rt Rev'd Colin James III, KOTM 1/96" writes:

>It is common knowledge that the same ANSI C source code when compiled by
>different ANSI C compilers produces executables which when run yield
>different, inconsistent floating point results.  

They should not be inconsistent for correctly written code. What can differ
is the accuracy of the results. That results aren't exact in general is a
fact of life with floating point.

>This is due to the fact that the runtime floating point packages of ANSI C
>compilers vary as to word size and are not required to conform to strict
>standards, other than weasel words, as are Ada compilers.

ANSI C does define minimum precision and other requirement for floating
point types. The only question is whether they are adequate for your needs.
ANSI C compilers on many platforms conform to the relevant IEEE floating
point formats so if this is a big issue for you simply specify that your
code is only certified for, say, an ANSI + IEEE 754 compliant platform.

>Eiffel emits ANSI C code.
>
>Therefore Eiffel is in fact not truly portable and can in fact yield
>different numerical results according to platform.

It depends what the actual specs for floating point in Eiffel are. If they
are stricter than those in ANSI C then not all implementations of Eiffel
may be compliant with its specs. You don't certify code for such
platforms.

>That begs the following question:  how can the largest Eiffel project of
>over 700,000 lines of code (the CALfp bank's derivatives' program) expect
>to be taken seriously if the source code when compiled on different
>platforms could produce inconsistent and erroneous results.  

You have yet to establish that it does.

>In other words, the reason that banks generally have learned to avoid C due
>to floating point problems is not solved at all but rather propagated and
>worsened by the questionable strategic decision to adopt Eiffel.  That fact
>may of course explain why no other famous banks with both US _and_ European
>computer operations have jumped on the Eiffel bandwagon and predictably
>would not, such as the perfect example of CitiCorp which had the first GE
>bank computers about 1962.

Can you think of a platform that a bank is likely to adopt these days
for hosting Eiffel that doesn't support suitable floating point formats?

>Don Knuth tried to get around the floating point problem in the 1960's by
>suggesting in Volumes 1 and 2 of his opus to use scaled integers.  It's too
>bad that the immature project weenies choose not to read that classic
>anymore these days and thus don't know the meaning of integer math.

5-10 years ago I might have agreed with you. However IEEE formats have
been generally adopted and do guarantee exact results for calculations
on integers that can be represented exactly.

-- 
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Lawrence Kirby | fred@genesis.demon.co.uk
Wilts, England | 70734.126@compuserve.com
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