Enable redundancy elimination for array loads.

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/code_generator.h"
@@ skipping 197 matching lines @@
         if (FLAG_constant_propagation ||
             FLAG_common_subexpression_elimination) {
           flow_graph->ComputeUseLists();
         }
         if (FLAG_constant_propagation) {
           ConstantPropagator::Optimize(flow_graph);
           // A canonicalization pass to remove e.g. smi checks on smi constants.
           optimizer.OptimizeComputations();
         }
         if (FLAG_common_subexpression_elimination) {
-          DominatorBasedCSE::Optimize(flow_graph);
+          if (DominatorBasedCSE::Optimize(flow_graph)) {
+            // 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 (FLAG_loop_invariant_code_motion &&
             (parsed_function.function().deoptimization_counter() <
              (FLAG_deoptimization_counter_threshold - 1))) {
           LICM::Optimize(flow_graph);
         }
 
         if (FLAG_range_analysis) {
           // We have to perform range analysis after LICM because it
           // optimistically moves CheckSmi through phis into loop preheaders
@@ skipping 360 matching lines @@
     result = isolate->object_store()->sticky_error();
     isolate->object_store()->clear_sticky_error();
     isolate->set_long_jump_base(base);
     return result.raw();
   }
   UNREACHABLE();
   return Object::null();
 }
 
 }  // namespace dart