[Bug middle-end/66459] bogus warning 'w.offset' may be used uninitialized in this function [-Wmaybe-uninitialized]

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66459

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2015-06-08 00:00:00         |2019-2-3
                 CC|                            |tkoenig at gcc dot gnu.org
          Component|fortran                     |middle-end

--- Comment #4 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
The code that gfortran generates is correct, it is just that
the middle-end does not quite understand it and generates a
warning for it.

The -fdump-tree-original dump shows

      if (*l)
        {
          {
[...]
            w.dtype = {.elem_len=4, .rank=2, .type=1};
            D.3881 = (integer(kind=8)) MAX_EXPR <*m, 0>;
            D.3882 = NON_LVALUE_EXPR <__builtin_expect ((integer(kind=8))
(D.3881 == 0), 0, 46) ? 0 : __builtin_expect ((integer(kind=8))
(9223372036854775807 / D.3881 <= 0), 0, 41) ? 1 : 0>;
            D.3883 = NON_LVALUE_EXPR <D.3881>;
            D.3884 = (integer(kind=8)) MAX_EXPR <*n, 0>;
            D.3885 = (__builtin_expect ((integer(kind=8)) (D.3884 == 0), 0, 46)
? 0 : __builtin_expect ((integer(kind=8)) (9223372036854775807 / D.3884 <
D.3883), 0, 41) ? 1 : 0) + D.3882;
            D.3886 = D.3883 * D.3884;
            D.3887 = D.3886;
            D.3888 = (__builtin_expect ((integer(kind=8)) ((unsigned long)
D.3886 > 4611686018427387903), 0, 41) ? 1 : 0) + D.3885;
            D.3889 = ~NON_LVALUE_EXPR <D.3883>;

            if (D.3891)
              {
                size.0 = 0;
              }
            else
              {
                size.0 = (unsigned long) D.3886 * 4;
              }
            overflow.1 = D.3888;
            if (__builtin_expect ((integer(kind=8)) (overflow.1 != 0), 0, 41))
              {
                _gfortran_runtime_error (&"Ganzzahl\xc3\xbcberlauf bei der
Berechnung des zu reservierenden Speichers"[1]{lb: 1 sz: 1});
              }
            else
              {
                if (__builtin_expect ((integer(kind=8)) (w.data != 0B), 0, 43))
                  {
                    _gfortran_runtime_error_at (&"At line 9 of file b.f"[1]{lb:
1 sz: 1}, &"Versuch, bereits reservierte Variable \xc2\xbb%s\xc2\xab zu
reservieren"[1]{lb: 1 sz: 1}, &"w"[1]{lb: 1 sz: 1});
                  }
                else
                  {
                    w.data = (void * restrict) __builtin_malloc (MAX_EXPR
<size.0, 1>);
                    if (__builtin_expect ((integer(kind=8)) (w.data == 0B), 0,
42))
                      {
                        _gfortran_os_error (&"Reservierung w\xc3\xbcrde
Speichergrenze \xc3\xbcberschreiten"[1]{lb: 1 sz: 1});
                      }
                  }
              }
            w.dim[0].lbound = 1;
            w.dim[0].ubound = (integer(kind=8)) *m;
            w.dim[0].stride = 1;
            w.dim[1].lbound = 1;
            w.dim[1].ubound = (integer(kind=8)) *n;
            w.dim[1].stride = D.3883;
            w.offset = D.3889;
          }
        }

and then

                      if ((integer(kind=4)[0:] * restrict) w.data != 0B)
                        {
                          (*(integer(kind=4)[0:] * restrict) w.data)[(w.offset
+ (integer(kind=8)) i * w.dim[1].stride) + (integer(kind=8)) j] = 0;

Now, the thing is that, on leaving the if (*l) block, data has to be non-zero,
otherwise _gfortran_os_error would have been called (which is marked
with TREE_THIS_VOLATILE, hence noreturn). So, the variables 

Is there something the Fortran front end can do better? If not, I think
this is better classified as a middle-end bug.