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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/108889] New: [12/13 Regression] (Re)Allocate in
 assignment shows used uninitialized memory warning with -Wall if LHS is
 unallocated
Date: Wed, 22 Feb 2023 18:04:06 +0000
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X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: fortran
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: diagnostic, wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: burnus at gcc dot gnu.org
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108889

            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: =E2=80=98reference.offset=E2=80=99 is used uninitialized [-Wuninit=
ialized]
Warning: =E2=80=98reference.dim[0].lbound=E2=80=99 is used uninitialized [-=
Wuninitialized]
Warning: =E2=80=98reference.dim[0].ubound=E2=80=99 may be used uninitialized
[-Wmaybe-uninitialized]

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

    D.4310 =3D reference.offset;
    D.4311 =3D reference.dim[0].lbound;
    D.4312 =3D reference.dim[0].ubound;

    D.4313 =3D D.4311 - D.4307;  // D.4307 =3D single.var.dim[0].lbound

But all expressions are actually re-evaluated later:
        D.4317 =3D (real(kind=3D4)[0:] * restrict) reference.data =3D=3D 0B;
        if (D.4317) goto L.4;
...
        L.4:;
...
        reference.dim[0].lbound =3D single.var.dim...
        reference.offset =3D -NON_LVALUE_EXPR <reference.dim[0].lbound>;
        D.4310 =3D reference.offset;
        D.4313 =3D 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] =3D (*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 =E2=80=93 even though there was
uninitialized memory in between. =E2=80=94 But this should be fixed, also t=
o silence
the warning.

* * *=20

! Testcase: Compile with -Wall

program main
  implicit none

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

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

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

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