GCC's linear-time DF calculation, pasted by Dan Berlin.

-

 static void
 compute_dominance_frontiers_1 (bitmap_head *frontiers)
 {
   edge p;
   edge_iterator ei;
   basic_block b;
   FOR_EACH_BB_FN (b, cfun)
     {
       if (EDGE_COUNT (b->preds) >= 2)
     {
       FOR_EACH_EDGE (p, ei, b->preds)
         {
           basic_block runner = p->src;
           basic_block domsb;
           if (runner == ENTRY_BLOCK_PTR_FOR_FN (cfun))
         continue;

           domsb = get_immediate_dominator (CDI_DOMINATORS, b);
           while (runner != domsb)
         {
           if (!bitmap_set_bit (&frontiers[runner->index],
                        b->index))
             break;
           runner = get_immediate_dominator (CDI_DOMINATORS,
                             runner);
         }
         }
     }
     }
 }