[Bug tree-optimization/107407] [12/13 Regression] Wrong code at -Os on x86_64-linux-gnu since r12-383-g32955416d8040b1f

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107407

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
  Deleted dead store: MEM <int[4]> [(int *)&c][_6] = 3;

Further reduced testcase, we mistreat SSA name indexes with must aliases.
We do have

          /* If we visit this PHI by following a backedge then we have to
             make sure ref->ref only refers to SSA names that are invariant
             with respect to the loop represented by this PHI node.  */
          if (dominated_by_p (CDI_DOMINATORS, gimple_bb (stmt),
                              gimple_bb (temp))
              && !for_each_index (ref->ref ? &ref->ref : &ref->base,
                                  check_name, gimple_bb (temp)))
            return DSE_STORE_LIVE;

that is supposed to catch this but somehow it doesn't trigger here.

int *a;
int c[4];
int d;

static int
f(char k, int j)
{
  for (; k <= 3; k++)
    {
      a = &c[k];
      for (; d <= 1; d++)
        *a = 3;
    }
  *a = 0;
}

int main()
{
  int i;
  f(0, 0);
  if (c[0] != 3)
    __builtin_abort ();
}


So what happens is that we elide processing of this check with

          /* In addition to kills we can remove defs whose only use
             is another def in defs.  That can only ever be PHIs of which
             we track two for simplicity reasons, the first and last in
             {first,last}_phi_def (we fail for multiple PHIs anyways).
             We can also ignore defs that feed only into
             already visited PHIs.  */
          else if (single_imm_use (vdef, &use_p, &use_stmt)
                   && (use_stmt == first_phi_def
                       || use_stmt == last_phi_def
                       || (gimple_code (use_stmt) == GIMPLE_PHI
                           && bitmap_bit_p (visited,
                                            SSA_NAME_VERSION
                                              (PHI_RESULT (use_stmt))))))

where we have the last PHI being the outer loop virtual PHI and the first
PHI being the loop exit PHI of the outer loop and we've already processed
the single immediate use of the outer loop PHI, the inner loop PHI.  But
we still have to perform the above check!

It's easiest to perform the check when we visit the PHI node instead of
delaying it to the actual processing loop.