[Bug tree-optimization/68823] [6/7 Regression][graphite] tramp3d-v4 compiled with -floop-nest-optimize crashes

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68823

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Ok, fails with:

g++ -floop-nest-optimize -Ofast -w tramp3d-v4.cpp -fdump-tree-graphite-details
-$ fdbg-cnt=graphite_scop:4

Problem are following side-by-side loops:

[scheduler] original ast:
{
  for (int c0 = 0; c0 <= 2; c0 += 1)
    S_3(c0);
  for (int c0 = 0; c0 <= 2; c0 += 1)
    S_6(c0);
}

[scheduler] AST generated by isl:
{
  for (int c0 = 0; c0 <= 2; c0 += 1)
    S_6(c0);
  for (int c0 = 0; c0 <= 2; c0 += 1)
    S_3(c0);
}

which are interchanged. But due to fact that there's a data dependency:

...
  <bb 3> [75.00%]:
  # i_38 = PHI <i_11(4), 0(2)>
  _34 = (sizetype) i_38;
  _12 = _34 * 8;
  _7 = &retval + _12;
  _8 = s1_4(D) + _12;
  _9 = MEM[(int *)_8];
  MEM[(Element_t[2] &)_7][0] = _9; <----- store to retval
  _10 = MEM[(int *)_8 + 4B];
  MEM[(Element_t[2] &)_7][1] = _10;
  i_11 = i_38 + 1;
  if (i_11 == 3)
    goto <bb 5>; [33.33%]
  else
    goto <bb 4>; [66.67%]

  <bb 4> [50.00%]:
  goto <bb 3>; [100.00%]

  <bb 5> [25.00%]:

  <bb 10> [25.00%]:

  <bb 6> [75.00%]:
  # i_39 = PHI <i_17(7), 0(10)>
  _37 = (sizetype) i_39;
  _35 = _37 * 8;
  _13 = _3(D) + _35;
  _14 = &retval + _35;
  _15 = MEM[(int *)_14];
  MEM[(Element_t[2] &)_13][0] = _15; <------ load from retval, stored to _3+x
  _16 = MEM[(int *)_14 + 4B];
  MEM[(Element_t[2] &)_13][1] = _16;
  i_17 = i_39 + 1;
  if (i_17 == 3)
    goto <bb 8>; [33.33%]
  else
    goto <bb 7>; [66.67%]

  <bb 7> [50.00%]:
  goto <bb 6>; [100.00%]
...

return _3(D);

I can work on that, but any hint where such dependency should be caught?