Reland r17365.

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 193 matching lines @@
 
       // Use propagated class-ids to optimize further.
       optimizer.ApplyClassIds();
 
       // Recompute use lists after applying class ids.
       flow_graph->ComputeUseLists();
 
       // Do optimizations that depend on the propagated type information.
       optimizer.Canonicalize();
 
-      // Unbox doubles.
       flow_graph->ComputeUseLists();
-      optimizer.SelectRepresentations();
 
-      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.Canonicalize();
       }
+
+      // Unbox doubles. Performed after constant propagation to minimize
+      // interference from phis merging double values and tagged
+      // values comming from dead paths.
+      flow_graph->ComputeUseLists();
+      optimizer.SelectRepresentations();
+      flow_graph->ComputeUseLists();
+
       if (FLAG_common_subexpression_elimination) {
         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() <
@@ skipping 403 matching lines @@
       Object::Handle(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