cfganal.c (compute_dominance_frontiers_1): Replace with new algorithm

2005-01-19  Daniel Berlin  <dberlin@dberlin.org>

	* cfganal.c (compute_dominance_frontiers_1): Replace with new algorithm
	(compute_dominance_frontiers): Ditto.

From-SVN: r93922

gcc/ChangeLog

+2005-01-19  Daniel Berlin  <dberlin@dberlin.org>
+
+	* cfganal.c (compute_dominance_frontiers_1): Replace with new algorithm
+	(compute_dominance_frontiers): Ditto.
+
 2005-01-19  Ralf Corsepius  <ralf.corsepius@rtems.org>
 
 	PR target/19529

gcc/cfganal.c

@@ -933,80 +933,66 @@ dfs_enumerate_from (basic_block bb, int reverse,
 }
 
 
-/* Computing the Dominance Frontier:
+/* Compute dominance frontiers, ala Harvey, Ferrante, et al.
+   
+   This algorithm can be found in Timothy Harvey's PhD thesis, at
+   http://www.cs.rice.edu/~harv/thesis.pdf in the section on iterative
+   dominance algorithms.
 
-   As described in Morgan, section 3.5, this may be done simply by
-   walking the dominator tree bottom-up, computing the frontier for
-   the children before the parent.  When considering a block B,
-   there are two cases:
+   First, we identify each join point, j (any node with more than one
+   incoming edge is a join point). 
 
-   (1) A flow graph edge leaving B that does not lead to a child
-   of B in the dominator tree must be a block that is either equal
-   to B or not dominated by B.  Such blocks belong in the frontier
-   of B.
+   We then examine each predecessor, p, of j and walk up the dominator tree
+   starting at p. 
+   
+   We stop the walk when we reach j's immediate dominator - j is in the
+   dominance frontier of each of  the nodes in the walk, except for j's
+   immediate dominator. Intuitively, all of the rest of j's dominators are
+   shared by j's predecessors as well.
+   Since they dominate j, they will not have j in their dominance frontiers.
+
+   The number of nodes touched by this algorithm is equal to the size 
+   of the dominance frontiers, no more, no less.
+*/
 
-   (2) Consider a block X in the frontier of one of the children C
-   of B.  If X is not equal to B and is not dominated by B, it
-   is in the frontier of B.  */
 
 static void
-compute_dominance_frontiers_1 (bitmap *frontiers, basic_block bb, sbitmap done)
+compute_dominance_frontiers_1 (bitmap *frontiers)
 {
-  edge e;
+  edge p;
   edge_iterator ei;
-  basic_block c;
-
-  SET_BIT (done, bb->index);
-
-  /* Do the frontier of the children first.  Not all children in the
-     dominator tree (blocks dominated by this one) are children in the
-     CFG, so check all blocks.  */
-  for (c = first_dom_son (CDI_DOMINATORS, bb);
-       c;
-       c = next_dom_son (CDI_DOMINATORS, c))
-    {
-      if (! TEST_BIT (done, c->index))
-    	compute_dominance_frontiers_1 (frontiers, c, done);
-    }
-      
-  /* Find blocks conforming to rule (1) above.  */
-  FOR_EACH_EDGE (e, ei, bb->succs)
-    {
-      if (e->dest == EXIT_BLOCK_PTR)
-	continue;
-      if (get_immediate_dominator (CDI_DOMINATORS, e->dest) != bb)
-	bitmap_set_bit (frontiers[bb->index], e->dest->index);
-    }
-
-  /* Find blocks conforming to rule (2).  */
-  for (c = first_dom_son (CDI_DOMINATORS, bb);
-       c;
-       c = next_dom_son (CDI_DOMINATORS, c))
+  basic_block b;
+  FOR_EACH_BB (b)
     {
-      unsigned x;
-      bitmap_iterator bi;
-
-      EXECUTE_IF_SET_IN_BITMAP (frontiers[c->index], 0, x, bi)
+      if (EDGE_COUNT (b->preds) >= 2)
 	{
-	  if (get_immediate_dominator (CDI_DOMINATORS, BASIC_BLOCK (x)) != bb)
-	    bitmap_set_bit (frontiers[bb->index], x);
+	  FOR_EACH_EDGE (p, ei, b->preds)
+	    {
+	      basic_block runner = p->src;
+	      basic_block domsb;
+	      if (runner == ENTRY_BLOCK_PTR)
+		continue;
+	      
+	      domsb = get_immediate_dominator (CDI_DOMINATORS, b);
+	      while (runner != domsb)
+		{
+		  bitmap_set_bit (frontiers[runner->index], 
+				  b->index);
+		  runner = get_immediate_dominator (CDI_DOMINATORS,
+						    runner);
+		}
+	    }
 	}
     }
-}
-
+}	      
+  
 
 void
 compute_dominance_frontiers (bitmap *frontiers)
 {
-  sbitmap done = sbitmap_alloc (last_basic_block);
-
   timevar_push (TV_DOM_FRONTIERS);
 
-  sbitmap_zero (done);
-
-  compute_dominance_frontiers_1 (frontiers, EDGE_SUCC (ENTRY_BLOCK_PTR, 0)->dest, done);
-
-  sbitmap_free (done);
+  compute_dominance_frontiers_1 (frontiers);
 
   timevar_pop (TV_DOM_FRONTIERS);
 }