VM: Fix imprecision in array bounds check elimination.

runtime/vm/compiler.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/compiler.h"
 
 #include "vm/assembler.h"
 
 #include "vm/ast_printer.h"
 #include "vm/block_scheduler.h"
@@ skipping 586 matching lines @@
         DEBUG_ASSERT(flow_graph->VerifyUseLists());
 
         if (FLAG_common_subexpression_elimination ||
             FLAG_loop_invariant_code_motion) {
           flow_graph->ComputeBlockEffects();
         }
 
         if (FLAG_common_subexpression_elimination) {
           if (DominatorBasedCSE::Optimize(flow_graph)) {
             DEBUG_ASSERT(flow_graph->VerifyUseLists());
+            optimizer.Canonicalize();
             // Do another round of CSE to take secondary effects into account:
             // e.g. when eliminating dependent loads (a.x[0] + a.x[0])
             // TODO(fschneider): Change to a one-pass optimization pass.
-            DominatorBasedCSE::Optimize(flow_graph);
+            if (DominatorBasedCSE::Optimize(flow_graph)) {
+              optimizer.Canonicalize();
+            }
             DEBUG_ASSERT(flow_graph->VerifyUseLists());
           }
         }
 
         // Run loop-invariant code motion right after load elimination since it
         // depends on the numbering of loads from the previous load-elimination.
         if (FLAG_loop_invariant_code_motion) {
           LICM licm(flow_graph);
           licm.Optimize();
           DEBUG_ASSERT(flow_graph->VerifyUseLists());
@@ skipping 1207 matching lines @@
       isolate->set_background_compiler(task);
       start_task = true;
     }
   }
   if (start_task) {
     Dart::thread_pool()->Run(isolate->background_compiler());
   }
 }
 
 }  // namespace dart