tree-ssa-threadupdate.c (thread_through_all_blocks): Thread blocks is post order.

	* tree-ssa-threadupdate.c (thread_through_all_blocks): Thread
	blocks is post order.

From-SVN: r254752

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 9cba109ec59946690262880211423a071c766a64..c404eb8e5a7de82abdc5386fa7affc5f2c2dcd3e 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2017-11-14  Jeff Law  <law@redhat.com>
+
+       * tree-ssa-threadupdate.c (thread_through_all_blocks): Thread
+       blocks is post order.
+
 2017-11-15  Alexandre Oliva <aoliva@redhat.com>
 
        * dumpfile.h (TDF_COMPARE_DEBUG): New.

diff --git a/gcc/tree-ssa-threadupdate.c b/gcc/tree-ssa-threadupdate.c
index 3d3aeab2a6668b1cf0f1337555536eaa676230b2..045905eceb7290c3ebf25d6b621f501222eaeabf 100644 (file)
--- a/gcc/tree-ssa-threadupdate.c
+++ b/gcc/tree-ssa-threadupdate.c
@@ -2174,7 +2174,6 @@ thread_through_all_blocks (bool may_peel_loop_headers)
 {
   bool retval = false;
   unsigned int i;
-  bitmap_iterator bi;
   struct loop *loop;
   auto_bitmap threaded_blocks;
 
@@ -2278,14 +2277,33 @@ thread_through_all_blocks (bool may_peel_loop_headers)
 
   initialize_original_copy_tables ();
 
-  /* First perform the threading requests that do not affect
-     loop structure.  */
-  EXECUTE_IF_SET_IN_BITMAP (threaded_blocks, 0, i, bi)
-    {
-      basic_block bb = BASIC_BLOCK_FOR_FN (cfun, i);
+  /* The order in which we process jump threads can be important.
+
+     Consider if we have two jump threading paths A and B.  If the
+     target edge of A is the starting edge of B and we thread path A
+     first, then we create an additional incoming edge into B->dest that
+     we can not discover as a jump threading path on this iteration.
+
+     If we instead thread B first, then the edge into B->dest will have
+     already been redirected before we process path A and path A will
+     natually, with no further work, target the redirected path for B.
 
-      if (EDGE_COUNT (bb->preds) > 0)
-       retval |= thread_block (bb, true);
+     An post-order is sufficient here.  Compute the ordering first, then
+     process the blocks.  */
+  if (!bitmap_empty_p (threaded_blocks))
+    {
+      int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
+      unsigned int postorder_num = post_order_compute (postorder, false, false);
+      for (unsigned int i = 0; i < postorder_num; i++)
+       {
+         unsigned int indx = postorder[i];
+         if (bitmap_bit_p (threaded_blocks, indx))
+           {
+             basic_block bb = BASIC_BLOCK_FOR_FN (cfun, indx);
+             retval |= thread_block (bb, true);
+           }
+       }
+      free (postorder);
     }
 
   /* Then perform the threading through loop headers.  We start with the