[Bug tree-optimization/66459] bogus warning 'w.offset' may be used uninitialized in this function [-Wmaybe-uninitialized] caused by loop invariant motion (LIM)

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

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2019-02-03 00:00:00         |2022-8-30
            Summary|bogus warning 'w.offset'    |bogus warning 'w.offset'
                   |may be used uninitialized   |may be used uninitialized
                   |in this function            |in this function
                   |[-Wmaybe-uninitialized]     |[-Wmaybe-uninitialized]
                   |                            |caused by loop invariant
                   |                            |motion (LIM)

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
what we warn about in the end is the path when l == 0 && n > 0.  that's also
visible in the initial gfortran IL:

  try
    {
      w.data = 0B;
      w.dtype = {.elem_len=4, .rank=2, .type=1};
      if (*l)
        {
...
        }
      L.1:;
      {
        integer(kind=4) D.4258;

        D.4258 = *n;
        i = 1;
        while (1)
          {
            {
              logical(kind=4) D.4261;

              D.4261 = i > D.4258;
              if (D.4261) goto L.3;
              {
                integer(kind=4) D.4262;

                D.4262 = *m;
                j = 1;
                while (1)
                  {
                    {
                      logical(kind=4) D.4265;

                      D.4265 = j > D.4262;
                      if (D.4265) goto L.5;
                      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;
                        }
                      L.6:;
                      L.4:;
                      j = j + 1;
                    }
                  }
                L.5:;
              }
              L.2:;
              i = i + 1;
            }
          }
        L.3:;

the n > 0 check is simplified from D.4258 = *n; i = 1; D.4261 = i > D.4258;
There's some extra guard, m > j, but that's not related so what uninit
analysis would need to see is the w.data != 0 guard but as Manu said, the
LIM pass hoists the loop invariant uses out of this conditional.

IIRC there's a related bug somewhere for LIM exposing uninitialized uses
to code paths that are not known to execute but here it's not trivial to
see the uninitializedness.

The bogus diagnostic goes away with -fno-tree-loop-im (but you usually do
not want to do that for performance reasons).  At -O3 we unswitch on
w.data != 0 but LIM already moved the access then.  One old idea is to
make sure to disable diagnostics on stmts we move for the sake of
introducing false negatives ...