Avoid creating unnecessary environments during SSA renaming.

runtime/vm/flow_graph.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/flow_graph.h"
 
 #include "vm/bit_vector.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/intermediate_language.h"
 #include "vm/longjump.h"
@@ skipping 505 matching lines @@
           (*env)[i] = phi;
           phi->set_ssa_temp_index(alloc_ssa_temp_index());  // New SSA temp.
         }
       }
     }
   }
 
   // 2. Process normal instructions.
   for (ForwardInstructionIterator it(block_entry); !it.Done(); it.Advance()) {
     Instruction* current = it.Current();
-    // Attach current environment to the instruction. First, each instruction
-    // gets a full copy of the environment. Later we optimize this by
-    // eliminating unnecessary environments.
-    // TODO(zerny): Avoid creating unnecessary environments. Note that some
-    // optimizations need deoptimization info for non-deoptable instructions,
-    // eg, LICM on GOTOs.
-    current->set_env(Environment::From(*env,
-                                       num_non_copied_params_,
-                                       parsed_function_.function()));
+    // Attach current environment to the instructions that can deoptimize and
+    // at goto instructions. Optimizations like LICM expect an environment at
+    // gotos.
+    if (current->CanDeoptimize() || current->IsGoto()) {
+      current->set_env(Environment::From(*env,
+                                         num_non_copied_params_,
+                                         parsed_function_.function()));
+    }
     if (current->CanDeoptimize()) {
       current->env()->set_deopt_id(current->deopt_id());
     }
 
     // 2a. Handle uses:
     // Update expression stack environment for each use.
     // For each use of a LoadLocal or StoreLocal: Replace it with the value
     // from the environment.
     for (intptr_t i = current->InputCount() - 1; i >= 0; --i) {
       Value* v = current->InputAt(i);
@@ skipping 363 matching lines @@
          !it.Done();
          it.Advance()) {
       ++size;
     }
   }
   return size;
 }
 
 
 }  // namespace dart