Re: basic VRP min/max range overflow question

[Robert Dewar <dewar at adacore dot com>]

Paul Schlie wrote:
> > From: "Joseph S. Myers" <joseph@codesourcery.com>
> > "no requirements" means that *any* translation conforms in the case of
> > undefined behavior.  Only those executions not involving undefined
> > behavior have any requirements.
>
>
> What delineates the bounds between undefined and non-undefined behaviors?
>
> (As in the extreme if an undefined behavior may arbitrarily corrupt the
> entire specified program state, and/or modify the languages otherwise
> required semantics governing the translation/execution of a program, it
> would seem that rather than attempting to utilize undefined behaviors as
> a basis of optimizations, the compiler should more properly simply abort
> compilation upon their detection, as the resulting program would be
> otherwise be arguably useless for any likely purpose if the effect of an
> undefined behavior within a program is not bounded?)

But of COURSE you can't detect these situations at compile time. Even
if you had all the input in advance, this would be trivially equivalent
to solving the halting problem. Programming language definitions reserve
this use of undefined PRECISELY for those cases where it cannot be
determined statically whether some rule in the dynamic semantic
definition is or is not met.

When a compiler can determine that a given construct is sure to result
in undefined behavior, e.g. it can prove at compile time that overflow
will always occur, then indeed the best approach is to abort, or raise
some kind of exception (depending on the language), and to generate a
warning at compile time that this is going on. It CAN NOT "abort
compilation", since this is not an error condition, it would be improper
to refuse to compile the program. Besides which it would in practice
be wrong, since the compiler may very well be able to tell that a given
statement IF EXECUTED will cause trouble, but be unable to tell if in
fact it will be executed (my password program is like this, a friendly
compiler would warn that the reference to npasswords_entered (or whatever
I called it) results in undefined behavior, and an attentive programmer
who does not ignore warnings will deal with this warning before the
program causes chaotic results.
>