[gcc r17-3254] fortran: Fix element size in collective subroutines implementation [PR126799]

https://gcc.gnu.org/g:fbc358491eac26f6680d738c59c9ce62564988f6

commit r17-3254-gfbc358491eac26f6680d738c59c9ce62564988f6
Author: Mikael Morin <[email protected]>
Date:   Thu Aug 13 09:40:14 2026 +0200

    fortran: Fix element size in collective subroutines implementation [PR126799]
    
    The shared memory implementation of collective subroutines uses the array
    descriptor span as element size.  This gives a bigger value than the true
    size in some cases, as demonstrated in the testcase with a pointer pointing
    to an array subreference.  The bigger size estimate doesn't only cause
    over allocation of memory.  As the packing and unpacking functions don't
    have the same problem, they initialize and copy back only a part of the
    allocated buffer, and their element indexing in the buffer doesn't match
    that of the collective subroutine, causing wrong values to be produced.
    
            PR fortran/126799
    
    libgfortran/ChangeLog:
    
            * caf/shmem/collective_subroutine.c (collsub_reduce_array): Use
            the element length field instead of the span field as element size.
    
    gcc/testsuite/ChangeLog:
    
            * gfortran.dg/coarray/collectives_5.f90: New test.

Diff:
---
 .../gfortran.dg/coarray/collectives_5.f90          | 34 ++++++++++++++++++++++
 libgfortran/caf/shmem/collective_subroutine.c      |  2 +-
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/gcc/testsuite/gfortran.dg/coarray/collectives_5.f90 b/gcc/testsuite/gfortran.dg/coarray/collectives_5.f90
new file mode 100644
index 000000000000..5906abff4ed8
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/coarray/collectives_5.f90
@@ -0,0 +1,34 @@
+! { dg-do run }
+!
+! PR fortran/126799
+! Check that the collective subroutines code uses span and element length
+! correctly when they are different (in the non-contiguous pointer case).
+! The shared memory implementation used to use the span incorrectly as element
+! length, causing wrong values to be produced.
+
+program prog
+  implicit none
+  integer, parameter :: k = 2, n = 5
+  type t
+    integer(kind=k) :: c1, c2
+  end type
+  type(t), target :: x(n)
+  integer(kind=k), pointer :: p(:)
+  integer :: i, icount
+  icount = num_images()
+  !print *, icount
+  x = [ (t(i*this_image(),i+this_image()), i=1,n) ]
+  p => x%c1
+  call summation(p)
+  !print '(i4,":",*(" ", i5))', this_image(), x%c1
+  if (any(x%c1 /= [ ((icount * (icount + 1) / 2) * i, i=1,n) ])) error stop 1
+  p => x%c2
+  call summation(p)
+  !print '(i4,":",*(" ", i5))', this_image(), x%c2
+  if (any(x%c2 /= [ (icount * i + icount * (icount + 1) / 2, i=1,n) ])) error stop 2
+contains
+  subroutine summation(a)
+    integer(kind=k), pointer, intent(in) :: a(:)
+    call co_sum(a)
+  end subroutine
+end program
diff --git a/libgfortran/caf/shmem/collective_subroutine.c b/libgfortran/caf/shmem/collective_subroutine.c
index b498ad2802f5..fd5d56e88b2a 100644
--- a/libgfortran/caf/shmem/collective_subroutine.c
+++ b/libgfortran/caf/shmem/collective_subroutine.c
@@ -335,7 +335,7 @@ collsub_reduce_array (gfc_descriptor_t *desc, int result_image,
   if (pi.num_elem == 0)
     return;
 
-  elem_size = GFC_DESCRIPTOR_SPAN (desc);
+  elem_size = GFC_DESCRIPTOR_SIZE (desc);
   this_image_size_bytes = elem_size * pi.num_elem;
 
   buffer = get_collsub_buf (