Improve load forwarding:

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 273 matching lines @@
       }
 
       // The final canonicalization pass before the code generation.
       if (FLAG_propagate_types) {
         // Recompute types after code movement was done to ensure correct
         // reaching types for hoisted values.
         FlowGraphTypePropagator propagator(flow_graph);
         propagator.Propagate();
         DEBUG_ASSERT(flow_graph->VerifyUseLists());
       }
-      optimizer.Canonicalize();
+      if (optimizer.Canonicalize()) {
+        // To fully remove redundant boxing (e.g. BoxDouble used only in
+        // environments and UnboxDouble instructions) instruction we
+        // first need to replace all their uses and then fold them away.
+        // For now we just repeat Canonicalize twice to do that.
+        // TODO(vegorov): implement a separate representation folding pass.
+        optimizer.Canonicalize();
+      }
       DEBUG_ASSERT(flow_graph->VerifyUseLists());
 
       // Perform register allocation on the SSA graph.
       FlowGraphAllocator allocator(*flow_graph);
       allocator.AllocateRegisters();
 
       if (FLAG_print_flow_graph || FLAG_print_flow_graph_optimized) {
         FlowGraphPrinter::PrintGraph("After Optimizations", flow_graph);
       }
     }
@@ skipping 396 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