[Bug fortran/108889] New: [12/13 Regression] (Re)Allocate in assignment shows used uninitialized memory warning with -Wall if LHS is unallocated

["burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108889

            Bug ID: 108889
           Summary: [12/13 Regression] (Re)Allocate in assignment shows
                    used uninitialized memory warning with -Wall if LHS is
                    unallocated
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: diagnostic, wrong-code
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
  Target Milestone: ---

The following testcase shows the following -Wall warnings:

Warning: ‘reference.offset’ is used uninitialized [-Wuninitialized]
Warning: ‘reference.dim[0].lbound’ is used uninitialized [-Wuninitialized]
Warning: ‘reference.dim[0].ubound’ may be used uninitialized
[-Wmaybe-uninitialized]

The warning (but not the issue) is new since GCC 12. The dump shows:

    D.4310 = reference.offset;
    D.4311 = reference.dim[0].lbound;
    D.4312 = reference.dim[0].ubound;

    D.4313 = D.4311 - D.4307;  // D.4307 = single.var.dim[0].lbound

But all expressions are actually re-evaluated later:
        D.4317 = (real(kind=4)[0:] * restrict) reference.data == 0B;
        if (D.4317) goto L.4;
...
        L.4:;
...
        reference.dim[0].lbound = single.var.dim...
        reference.offset = -NON_LVALUE_EXPR <reference.dim[0].lbound>;
        D.4310 = reference.offset;
        D.4313 = reference.dim[0].lbound - D.4307;
...
        L.5:;  /// If the shape was correct, there is a jump to here / L.5
      while (1)
        {
          if (S.2 > D.4308) goto L.6;
          (*D.4309)[(S.2 + D.4313) + D.4310] = (*D.4305)[S.2 + D.4306];

Thus, D.4313 + D.4310 needs to be evaluated in the no-(re)alloc case and in the
needs-to-be allocated case.


Thus, the produced code is fine at the end – even though there was
uninitialized memory in between. — But this should be fixed, also to silence
the warning.

* * * 

! Testcase: Compile with -Wall

program main
  implicit none

  type :: struct
    real, allocatable :: var(:)
  end type struct

  type(struct) :: single
  real, allocatable :: reference(:)

  single%var = [1,2,3,4,5]
  reference = single%var

  if (size(reference) /= size(single%var)) stop 1
  if (lbound(reference, 1) /= 1) stop 3
  if (any (reference /= single%var)) stop 3
end