bb-reorder: Improve the simple algorithm for -Os (PR67864)

As the PR points out, the "simple" reorder algorithm makes bigger code
than the STC algorithm did, for -Os, for x86.  I now tested it for many
different targets and it turns out to be worse everywhere.

This simple patch tunes "simple" a bit; this makes it better than STC
almost everywhere.  The only exceptions (for the targets where I have
results) are x86 and mn10300.  For those targets it may be best to switch
the default algorithm for -Os to STC.


2015-10-11  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/67864
	* gcc/bb-reorder (reorder_basic_blocks_simple): Prefer existing
	fallthrough edges for conditional jumps.  Don't sort candidate
	edges if not optimizing for speed.

From-SVN: r228692

gcc/ChangeLog

+2015-10-11  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	PR rtl-optimization/67864
+	* gcc/bb-reorder (reorder_basic_blocks_simple): Prefer existing
+	fallthrough edges for conditional jumps.  Don't sort candidate
+	edges if not optimizing for speed.
+
 2015-10-10  Trevor Saunders  <tbsaunde+gcc@tbsaunde.org>
 
 	* defaults.h (REVERSE_CONDITION): New default definition.

gcc/bb-reorder.c

@@ -2318,16 +2318,24 @@ reorder_basic_blocks_simple (void)
 
       if (any_condjump_p (end))
 	{
-	  edges[n++] = EDGE_SUCC (bb, 0);
-	  edges[n++] = EDGE_SUCC (bb, 1);
+	  edge e0 = EDGE_SUCC (bb, 0);
+	  edge e1 = EDGE_SUCC (bb, 1);
+	  /* When optimizing for size it is best to keep the original
+	     fallthrough edges.  */
+	  if (e1->flags & EDGE_FALLTHRU)
+	    std::swap (e0, e1);
+	  edges[n++] = e0;
+	  edges[n++] = e1;
 	}
       else if (single_succ_p (bb))
 	edges[n++] = EDGE_SUCC (bb, 0);
     }
 
-  /* Sort the edges, the most desirable first.  */
+  /* Sort the edges, the most desirable first.  When optimizing for size
+     all edges are equally desirable.  */
 
-  std::stable_sort (edges, edges + n, edge_order);
+  if (optimize_function_for_speed_p (cfun))
+    std::stable_sort (edges, edges + n, edge_order);
 
   /* Now decide which of those edges to make fallthrough edges.  We set
      BB_VISITED if a block already has a fallthrough successor assigned