Prune variables that are not live at block entry from the environment.

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 732 matching lines @@
   } else if (block_entry->IsCatchBlockEntry()) {
     // Add real definitions for all locals and parameters.
     for (intptr_t i = 0; i < env->length(); ++i) {
       ParameterInstr* param = new ParameterInstr(i, block_entry);
       param->set_ssa_temp_index(alloc_ssa_temp_index());  // New SSA temp.
       (*env)[i] = param;
       block_entry->AsCatchBlockEntry()->initial_definitions()->Add(param);
     }
   }
 
+  // Prune non-live variables at block entry by replacing their environment
+  // slots with null.
+  BitVector* live_in = variable_liveness->GetLiveInSet(block_entry);
+  for (intptr_t i = 0; i < variable_count(); i++) {
+    if (!live_in->Contains(i)) {
+      (*env)[i] = constant_null();
+    }
+  }
+
   // Attach environment to the block entry.
   AttachEnvironment(block_entry, env);
 
   // 2. Process normal instructions.
   for (ForwardInstructionIterator it(block_entry); !it.Done(); it.Advance()) {
     Instruction* current = it.Current();
 
     // Attach current environment to the instructions that need it.
     if (current->NeedsEnvironment()) {
       AttachEnvironment(current, env);
@@ skipping 371 matching lines @@
 }
 
 
 bool BlockEffects::IsSideEffectFreePath(BlockEntryInstr* from,
                                         BlockEntryInstr* to) const {
   return available_at_[to->postorder_number()]->Contains(
       from->postorder_number());
 }
 
 }  // namespace dart