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Re: [patch,wip] warn on noncontiguous pointers
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