[Bug fortran/80304] Warn about variable redefinition in do-concurrent

["chinoune.mehdi at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80304

Chinoune <chinoune.mehdi at hotmail dot com> changed:

           What    |Removed                     |Added
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  Attachment #41117|0                           |1
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--- Comment #10 from Chinoune <chinoune.mehdi at hotmail dot com> ---
Created attachment 41129
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41129&action=edit
modified test-program

(In reply to Thomas Koenig from comment #9)
> What you are doing is
> 
>   DO CONCURRENT( i=0:1 ,j=0:1)
>     a = a + add(i,j,abs(i-j))
>     b = b + add2(i,j,abs(i-j))
>   END DO
> 
> This causes the variables a and b to be defined multiple
> times in the loop.
> 
> In the standard, this is given in 8.1.6.7 of J3/10-007:
> 
>  A variable that is referenced in an iteration shall either be previously
> defined during that iteration, or
> shall not be defined or become undefined during any other iteration. A
> variable that is defined or becomes
> undefined by more than one iteration becomes undefined when the loop
> terminates.
> 
> Conceptually, when parallelizing, you cannot depend on a having
> any specific value during the loop execution, so the program
> might read a in one thread, have it changed by another thread,
> then add the value to it and store it back, losing what
> was done by the other thread.
> 
> You cannot do a sum like this with DO CONCURRENT.  In OpenMP terms,
> what you are attempting a reduction.
> 
> It would be nice to warn about this, though.

1- Why 'b' has been calculated correctly while 'a' hasn't ?! .the only
difference between the two functions is ( +1. )
2- I attached a new modified test and it fails.
3- The test-program works well with gfortran-5 gfortran-6 and Intel Fortran
2017.