PR tree-optimization/58380

	* tree-ssa-threadupdate.c (thread_block): Recognize another case
	of threading through a buried loop header.

	* tree-ssa-threadedge.c (thread_around_empty_blocks): Correct
	return value for single successor case.

	* g++.dg/torture/pr58380.C: New test.

2013-09-10  Jeff Law  <law@redhat.com>

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@202489 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 21 insertions, 8 deletions

diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
index 14bc4e37329..839f1859c1d 100644
--- a/gcc/tree-ssa-threadedge.c
+++ b/gcc/tree-ssa-threadedge.c
@@ -770,7 +770,19 @@ thread_around_empty_blocks (edge taken_edge,
   gsi = gsi_start_nondebug_bb (bb);
 
   /* If the block has no statements, but does have a single successor, then
-     it's just a forwarding block and we can thread through it trivially.  */
+     it's just a forwarding block and we can thread through it trivially. 
+
+     However, note that just threading through empty blocks with single
+     successors is not inherently profitable.  For the jump thread to
+     be profitable, we must avoid a runtime conditional.
+
+     By taking the return value from the recursive call, we get the
+     desired effect of returning TRUE when we found a profitable jump
+     threading opportunity and FALSE otherwise. 
+
+     This is particularly important when this routine is called after
+     processing a joiner block.  Returning TRUE too aggressively in
+     that case results in pointless duplication of the joiner block.  */
   if (gsi_end_p (gsi))
     {
       if (single_succ_p (bb))
@@ -781,15 +793,16 @@ thread_around_empty_blocks (edge taken_edge,
 	    {
 	      bitmap_set_bit (visited, taken_edge->dest->index);
 	      path->safe_push (taken_edge);
-	      thread_around_empty_blocks (taken_edge,
-					  dummy_cond,
-					  handle_dominating_asserts,
-					  simplify,
-					  visited,
-					  path);
-	      return true;
+	      return thread_around_empty_blocks (taken_edge,
+						 dummy_cond,
+						 handle_dominating_asserts,
+						 simplify,
+						 visited,
+						 path);
 	    }
 	}
+
+      /* We have a block with no statements, but multiple successors?  */
       return false;
     }