Re: Ada front-end depends on signed overflow

[Robert Dewar <dewar at adacore dot com>]

Paul Schlie wrote:
> > From: Dave Korn <dave.korn@artimi.com>
> > ----Original Message----
> >
> > > From: Paul Schlie
> > > Sent: 08 June 2005 
> >
> >
> >
> > > - Can you give an example of an operation which may yield an undefined
> > >  non-deterministic result which is reliably useful for anything?
> >
> >  Random number generation?

randomness has nothing whatever to do with non-determinisn. They
are completely different concepts.

But there are of course many examples.

THere are many examples in concurrent programming where non-determinism
is useful, and in set programming, arbitrary non-deterministic selection
from a set is fundamental.

But this is a complete red herring in this discussion

The reason that for example in Ada we say that
a+b means non-determinisitically either compute a then b, or
b then a, is not that it is useful for these results to be
different, but precisely because we expect NO reasonable
program to ever write an expression a+b in which the two
semantic meanings that are possible are different, and
we want the compiler to take advantage of this to generate
better code.

For example

  a + f(b)

we typically expect f(b) to be called first, even though
formally it might be the case that f(b) modifies a, so
this choice could have some effect from the formal
non-determinism of the semantics. Our actual attitude
is that if anyone writes code like this in which f(b)
modifies a, they are highly incompetent and we don't
care what happens.