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Re: As-if Infinitely Ranged Integer Model


On Mon, 2009-07-27 at 15:32 +0200, Paolo Bonzini wrote:
> On 07/27/2009 12:25 PM, Robert Dewar wrote:
> > Laurent GUERBY wrote:
> >
> >>>> if Dynamic_N >= T'First and Dynamic_N > T'Last then
> >>> Huh? I can't understand the first comparison.
> >>> Actually Ada is not Fortran-66 and allows empty loops, no?
> >>
> >> Ada for loop over "A .. B" will be empty if "A > B" and we obviously
> >> must not raise an exception if the loop is empty hence the first
> >> comparison is equivalent to "not (T'First > Dynamic_N)" which is true
> >> when the loop is not empty.
> >
> > GNAT does not do this kind of hoisting, though this particular case
> > could be fiddled in the front end.
> 
> I suppose though that in Ada Dynamic_N will often be a subtype of T, at 
> which point the hoisting is pointless because you know that the check 
> passes.

In most Ada code array T index type will likely be "Natural range <>"
and so the type system will not give useful bounds for optimizations.

Laurent




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