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From: "anlauf at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/110241] New: Redundant temporaries passing empty array
 constructors
Date: Tue, 13 Jun 2023 19:27:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110241

            Bug ID: 110241
           Summary: Redundant temporaries passing empty array constructors
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: anlauf at gcc dot gnu.org
  Target Milestone: ---

Found while working on pr86277:

program p
  call sub ([real :: 42.])  ! Single temporary
  call sub ([real ::    ])  ! Double temporary
contains
  subroutine sub (arg)
    real, intent(in), optional :: arg(:)
    if (.not. present (arg)) stop 1
  end
end

The dump-tree (at r14-1795-gc1691509e5a887) shows the following for the
first call:

  {
    static real(kind=3D4) A.4[1] =3D {4.2e+1};
    struct array01_real(kind=3D4) parm.5;

    parm.5.span =3D 4;
    parm.5.dtype =3D {.elem_len=3D4, .rank=3D1, .type=3D3};
    parm.5.dim[0].lbound =3D 1;
    parm.5.dim[0].ubound =3D 1;
    parm.5.dim[0].stride =3D 1;
    parm.5.data =3D (void *) &A.4[0];
    parm.5.offset =3D -1;
    sub (&parm.5);
  }

which is as expected, but for the second call:

  {
    struct array01_real(kind=3D4) atmp.6;
    real(kind=3D4) A.7[0];
    struct array01_real(kind=3D4) atmp.9;
    real(kind=3D4) A.10[0];

        typedef real(kind=3D4) [0];
    atmp.6.dtype =3D {.elem_len=3D4, .rank=3D1, .type=3D3};
    atmp.6.dim[0].stride =3D 1;
    atmp.6.dim[0].lbound =3D 0;
    atmp.6.dim[0].ubound =3D -1;
    atmp.6.span =3D 4;
    atmp.6.data =3D (void * restrict) &A.7;
    atmp.6.offset =3D 0;
        typedef real(kind=3D4) [0];
    atmp.9.dtype =3D {.elem_len=3D4, .rank=3D1, .type=3D3};
    atmp.9.dim[0].stride =3D 1;
    atmp.9.dim[0].lbound =3D 0;
    atmp.9.dim[0].ubound =3D -1;
    atmp.9.span =3D 4;
    atmp.9.data =3D (void * restrict) &A.10;
    atmp.9.offset =3D 0;
    {
      integer(kind=3D8) S.11;

      S.11 =3D 0;
      while (1)
        {
          if (S.11 >=3D 0) goto L.2;
          (*(real(kind=3D4)[0] * restrict) atmp.9.data)[S.11] =3D
(*(real(kind=3D4)[0] * restrict) atmp.6.data)[S.11];
          S.11 =3D S.11 + 1;
        }
      L.2:;
    }
    sub (&atmp.9);
  }

While this is at least correct code (as opposed to before the fix for pr862=
77),
it could be as simple as for the non-empty constructor.
(There's a patch attached to the other pr demonstrating that this can be
achieved for this code snippet).=