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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada,comp.lang.fortran
Subject: Re: Bounds Check Overhead
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Date: Fri, 26 May 2006 12:16:33 +0200
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Date: 2006-05-26T12:16:33+02:00
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Brooks Moses <bmoses-nospam@cits1.stanford.edu> writes:

> Simon Wright wrote:
>> In Ada one should where possible use the 'Range attribute:
>>    for I in Some_Array'Range loop
>>       Process (Some_Array (I));
>>    end loop;
>> where I _can't_ exceeed the bounds, so it would be surprising if a
>> compiler inserted bounds checks. Is there a Fortran equivalent? I seem
>> to remember something like that in VAX F77, but it's been a while...
>
> The Fortran equivalent is:
>
>     do i = lbound(Some_Array), ubound(Some_Array)
>       call Process(Some_Array(i))
>     end do
>
> I don't know that compilers are smart enough to omit the range check,
> though -- because, in the general case, it shouldn't be omitted.
> There could be something in the body of the loop that modified the
> loop variable -- which would be illegal code, but the whole point of
> range-checking is that it works even if the code is illegal, so unless
> the compiler is otherwise able to guarantee that the loop variable is
> not modified by a statement in the loop, it still needs to do the
> check.

And that's why Ada specifies that I cannot change inside the loop, and
is undefined outside the loop; see ARM 5.5 (9, 10).

You seem to imply that Fortran has a similar rule, but that compilers
do not enforce that rule, and therefore have to perform range checking
to enforce a non-existent language rule about array access.  I am
confused.  Could you clarify?

-- 
Ludovic Brenta.