Re: [patch,wip] warn on noncontiguous pointers

[Cesar Philippidis <cesar at codesourcery dot com>]

On 09/26/2018 01:49 PM, Thomas Koenig wrote:
> Hi Cesar,
> 
>> As of GCC 8, gfortran now errors when a pointer with a contiguous
>> attribute is set to point to a target without a contiguous attribute. I
>> think this is overly strict, and should probably be demoted to a
>> pedantic warning as I've done in the attached patch.
> 
> We had a lengthy discussion on that one. Still, we can dig into the
> standard for that one.
> 
> J3/10-007 says in 7.2.2.3  Data pointer assignment
> 
> # 7 If the pointer object has the CONTIGUOUS attribute, the pointer
> # target shall be contiguous.
> 
> # 9 If bounds-remapping-list is specified, the pointer target shall
> # be simply contiguous (6.5.4) or of rank one
> 
> program test
>    implicit none
>    real,pointer :: fptr1(:)
>    real,pointer,contiguous :: fptr3(:,:,:)
> 
>    allocate(fptr1(12))
>    call random_number(fptr1)
> 
>    !Test pointer reshape II
> 
>    fptr3(1:2,1:2,1:2) => fptr1(4:)
> 
> end program
> 
> So, by paragraph 9, this would be OK. Let's see what paragraph 7
> means when it says "contiguous". 5.3.7 says
> 
> An object is contiguous if it is
> 
> # (1) an object with the CONTIGUOUS attribute,
> # (2) a nonpointer whole array that is not assumed-shape,
> # (3) an assumed-shape array that is argument associated with an
>      array that is contiguous,
> # (4) an array allocated by an ALLOCATE statement,
> # (5) a pointer associated with a contiguous target, or
> # (6) a nonzero-sized array section (6.5.3) provided that
> #   (a) its base object is contiguous,
> #   (b) it does not have a vector subscript,
> #   (c) the elements of the section, in array element order, are a
> #       subset of the base object elements that are consecutive in
> #       array element order,
> #   (d) if the array is of type character and a substring-range appears,
> #       the substring-range specifies all of the characters of the
> #       parent string (6.4.1),
> #   (e) only its final part-ref has nonzero rank, and
> #   (f) it is not the real or imaginary part (6.4.4) of an array of type
> #       complex.
> 
> An object is not contiguous if it is an array subobject, and
> 
> [conditions not relevant elided]
> 
> # It is processor dependent whether any other object is contiguous.
> 
> If we go down the list, we see that fptr1(4:) is not contiguous; it
> is not an array (it is a pointer), so (4) also does not apply.
> 
> So, we are in the realm of processor dependent behavior, so we can
> chose what to do.
> 
> The last time we discussed this, we agreed on a hard error.  One
> important argument is that a mistakenly applied contiguous
> attribute will lead to wrong code, and that it is quite easy
> to check this, as we do now.
> 
> So, I think we should leave the behavior as it is now, and

Thank you for the explanation. That all seems reasonable.

>> Maybe the ScaTeLib code needs to be updated.
> 
> sounds like a good idea to me.

ACK.

Thanks,
Cesar