Re: Bounds Check Overhead

[Rich Townsend <rhdt@barVOIDtol.udel.edu>]

Nasser Abbasi wrote:
> "Richard Maine" <nospam@see.signature> wrote in message 
> news:1hfxsjh.t88mchrssv9cN%nospam@see.signature...
> 
>>Ludovic Brenta <ludovic@ludovic-brenta.org> wrote:
>>
>>
>>>You seem to imply that Fortran has a similar rule,
>>
>> [that an loop index shall not change within a loop]
>>
>>>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?
>>
> 
>>Others have replied some, but let me make my attempt at clarification
>>because yes, you seem to be confused about multiple related things here.
>>
>>Yes, there is a Fortran language rule that a loop index shall not be
>>changed while a loop is executing. Compilers are not required to enforce
>>that rule, but essentially all compilers make at least some attempt to
>>enforce it and catch the simple cases, which is most cases, but not all.
>>There are cases where the violation of the rule is hard to detect,
>>because the change occurs in some other procedure.
>>
> 
> 
> Hello;
> 
> <about loop counters being changed.>
> 
> I am not sure if there is supposed to compiler flag to enforce this or not, 
> you do not seem to imply that, so I did this very simple test, please see:
> 
> ------------ test for checking on changing loop index----
> $ cat a.f90
> 
> PROGRAM MAIN
> 
>    DO I=1,10
>       CALL foo(I)
>       PRINT *,I
>    END DO
> 
> END PROGRAM
> 
> SUBROUTINE foo(I)
>    I=I+1
> END SUBROUTINE
> -------------  end program ------
> $ g95 a.f90
> $ ./a.exe
>  2
>  4
>  6
>  8
>  10
>  12
>  14
>  16
>  18
>  20
> --------- end run ---------
> 
> I did the same in Ada:
> 
> --------- Ada -------
> procedure Main is
> 
>    PROCEDURE foo(I: in out integer) IS
>       begin
>           I:=I+1;
>    end foo;
> 
>   BEGIN
> 
>      FOR I IN 1..10 LOOP
>          foo(I);
>      END LOOP;
> 
> END Main;
> ------- end ada ----
> 
> The above will not even be allowed to compile since Ada wants 'I' to be an 
> actual variable.
> 
> The compile error I get is "actual for I must be a variable"
> 
> This means I am not even allowed to use "I" in a call.
> This eliminate the problem from accidentally change the loop counter.
> 
> Nasser 
> 
> 

I don't think you're comparing like-for-like. A better comparison:

PROGRAM MAIN

     DO I=1,10
        CALL foo(I)
        PRINT *,I
     END DO

CONTAINS

   SUBROUTINE foo(I)
   INTENT(in) :: i
     I=I+1
   END SUBROUTINE

END PROGRAM MAIN

Also, from looking at your Ada program, it seems you can't pass loop counters to 
procedures. Is this the case?

cheers,

Rich