Re: Concatenation and Characters

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jeffrey Carter <jrcarter@acm.org> writes:

> Robert A Duff wrote:
> > The Ada 83 optimizer could reason as follows:
> >     I is of subtype Positive.
> >     So either the value of I is in Positive, or else I is uninitialized.
> >     If I is in Positive, we can leave out the array bounds check,
> >     because the index subtype of String is also Positive.
> >     If I is uninitialized, we can leave out the check because
> >     the program execution will be erroneous (unpredictable).
> 
> Well, if you say this was so, I guess it probably was so.

Not true.  I wasn't thinking clearly.  For the example given, you are
correct that a bounds check is needed, at least on the upper bound.  I
suppose the compiler could optimize away the check on the lower bound.

In any case, although I put my foot in my mount about *that* example, I
think my more general point is correct: there are cases where an Ada 83
compiler could optimize away checks (based on the erroneousness of
uninit vars), where an Ada 95 compiler cannot (or at least has to prove
more).

>... Having used
> Ada 83 for over a decade, I never encountered a compiler that wouldn't
> check that I was in the range of S (not the index subtype of
> String). Clearly the range of S could be anything from null through
> Positive. I'm surprised that Ada 83 would allow a variable to index an
> array of unknown range without a bounds check.

- Bob