[Bug tree-optimization/84960] [8 Regression] ICE in GIMPLE pass: isolate-paths

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84960

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Slightly better one without the weird __builtin_prefetch.

/* PR tree-optimization/84960 */
/* { dg-do compile } */
/* { dg-options "-O2 -fwrapv" } */

void
foo (int a, float b, void *c)
{
lab:
  if ((b - (a %= 0) < 1) * -1)
    ;
  else
    {
      long f = a;
      __builtin_unreachable ();
      c = &&lab;
    }
  goto *c;
}

Seems ssa_redirect_edges attempts to redirect ENTRY -> 2 edge to some other
basic block and the bb 2, which contains a forced label is then removed as
unreachable and removing unreachable blocks implies:
          if (label_stmt
              && (FORCED_LABEL (gimple_label_label (label_stmt))
                  || DECL_NONLOCAL (gimple_label_label (label_stmt))))
            {
              basic_block new_bb;
              gimple_stmt_iterator new_gsi;

              /* A non-reachable non-local label may still be referenced.
                 But it no longer needs to carry the extra semantics of
                 non-locality.  */
              if (DECL_NONLOCAL (gimple_label_label (label_stmt)))
                {
                  DECL_NONLOCAL (gimple_label_label (label_stmt)) = 0;
                  FORCED_LABEL (gimple_label_label (label_stmt)) = 1;
                }

              new_bb = bb->prev_bb;
              new_gsi = gsi_start_bb (new_bb);
              gsi_remove (&i, false);
              gsi_insert_before (&new_gsi, stmt, GSI_NEW_STMT);
            }
but here bb->prev_bb is ENTRY_BLOCK, into which we obviously can't insert
anything.