Introduce --no-prune-dead-locals flag to disable dead locals pruning.

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/growable_array.h"
 #include "vm/report.h"
 
 namespace dart {
 
+DEFINE_FLAG(bool, prune_dead_locals, true, "optimize dead locals away");
 DECLARE_FLAG(bool, reorder_basic_blocks);
 DECLARE_FLAG(bool, trace_optimization);
 DECLARE_FLAG(bool, verify_compiler);
 
 
 FlowGraph::FlowGraph(const FlowGraphBuilder& builder,
                      GraphEntryInstr* graph_entry,
                      intptr_t max_block_id)
   : isolate_(Isolate::Current()),
     parent_(),
@@ skipping 840 matching lines @@
       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)) {
+    if (FLAG_prune_dead_locals && !live_in->Contains(i)) {
       (*env)[i] = constant_dead();
     }
   }
 
   // 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();
@@ skipping 47 matching lines @@
           (store != NULL) ||
           (push != NULL) ||
           (drop != NULL) ||
           (constant != NULL)) {
         Definition* result = NULL;
         if (store != NULL) {
           // Update renaming environment.
           intptr_t index = store->local().BitIndexIn(num_non_copied_params_);
           result = store->value()->definition();
 
-          if (variable_liveness->IsStoreAlive(block_entry, store)) {
+          if (!FLAG_prune_dead_locals ||
+              variable_liveness->IsStoreAlive(block_entry, store)) {
             (*env)[index] = result;
           } else {
             (*env)[index] = constant_dead();
           }
         } else if (load != NULL) {
           // The graph construction ensures we do not have an unused LoadLocal
           // computation.
           ASSERT(definition->HasTemp());
           intptr_t index = load->local().BitIndexIn(num_non_copied_params_);
           result = (*env)[index];
 
           PhiInstr* phi = result->AsPhi();
           if ((phi != NULL) && !phi->is_alive()) {
             phi->mark_alive();
             live_phis->Add(phi);
           }
 
-          if (variable_liveness->IsLastLoad(block_entry, load)) {
+          if (FLAG_prune_dead_locals &&
+              variable_liveness->IsLastLoad(block_entry, load)) {
             (*env)[index] = constant_dead();
           }
 
           // Record captured parameters so that they can be skipped when
           // emitting sync code inside optimized try-blocks.
           if (load->local().is_captured_parameter()) {
             intptr_t index = load->local().BitIndexIn(num_non_copied_params_);
             captured_parameters_->Add(index);
           }
 
@@ skipping 336 matching lines @@
 }
 
 
 bool BlockEffects::IsSideEffectFreePath(BlockEntryInstr* from,
                                         BlockEntryInstr* to) const {
   return available_at_[to->postorder_number()]->Contains(
       from->postorder_number());
 }
 
 }  // namespace dart