Re: [patch,wip] warn on noncontiguous pointers

[Thomas Koenig <tkoenig at netcologne dot de>]

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

> Maybe the ScaTeLib code needs to be updated.

sounds like a good idea to me.

Regards

	Thomas