[Bug tree-optimization/105198] [11/12 Regression] Wrong code for C loop (GCC 12 -O2, GCC 11 -O3)

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105198

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
So before pcom we have

  <bb 2> [local count: 114863530]:
  j_29 = k_28(D) + -1;
  _1 = (long unsigned int) j_29;
  _2 = _1 * 4;
  _3 = x_30(D) + _2;
  _4 = *_3;
  _5 = _4 + 1;
  *_3 = _5;
  if (j_29 > 0)
    goto <bb 11>; [94.50%]

  <bb 11> [local count: 108546036]:

  <bb 3> [local count: 1014686026]:
  # j_40 = PHI <j_34(12), j_29(11)>
  # tmp_39 = PHI <_17(12), _4(11)>
  _6 = (long unsigned int) j_40;
  _7 = _6 * 4;
  _8 = x_30(D) + _7;
  _9 = *_8;
  _11 = j_29 - j_40;
  _12 = n_33(D) - _11;
  if (_9 < _12)
    goto <bb 14>; [5.50%]
  else
    goto <bb 4>; [94.50%]

and pcom replaces the _9 load like the following:

  <bb 3> [local count: 1014686026]:
  # j_40 = PHI <j_34(12), j_29(11)>
  # D__lsm0.5_25 = PHI <D__lsm0.5_18(12), _4(11)>
  tmp_39 = D__lsm0.5_25;
  _6 = (long unsigned int) j_40;
  _7 = _6 * 4;
  _8 = x_30(D) + _7;
  _9 = D__lsm0.5_25;
  _11 = j_29 - j_40;
  _12 = n_33(D) - _11;
  if (_9 < _12)
    goto <bb 14>; [5.50%]
  else
    goto <bb 4>; [94.50%]

but that fails to realize that while _4 loads from the same
address, the *_3 = _5 store clobbers the value.  So with pcom
applied we exit the loop immediately at the

  x[j] < n - (k - 1 -j))

test because we use the wrong loop entry value for x[j].  It somehow
gets derailed to use 'tmp' here (it doesn't actually even analyze
the load/store in BB2).

So it's probably wrong in determining

Store-loads chain 0x3410a90
  max distance 1, may reuse first
  inits _4
  references:
    *_15 (id 2, write)
      offset -1
      distance 0
    looparound ref
      in statement tmp_39 = PHI <_17(12), _4(11)>

      distance 1
    *_8 (id 0)
      offset 0
      distance 1

in particular identifying the looparound ref which is only partly
a looparund ref (the initial value isn't).  Indeed, prepare_initializers_chain
simply does

  /* If we have replaced some looparound phi nodes, use their initializers
     instead of creating our own.  */
  FOR_EACH_VEC_ELT (chain->refs, i, laref)
    {
      if (gimple_code (laref->stmt) != GIMPLE_PHI)
        continue;

      gcc_assert (laref->distance > 0);
      chain->inits[n - laref->distance]
        = PHI_ARG_DEF_FROM_EDGE (laref->stmt, entry);
    }

which would be OK, but find_looparound_phi simply looks at the def of
the loop entry value and matches it up with the chain, not considering
intermediate clobbering stmts.