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

runtime/vm/intermediate_language.cc

 // Copyright (c) 2013, 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/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cpu.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_allocator.h"
@@ skipping 19 matching lines @@
     "Propagate IC data from unoptimized to optimized IC calls.");
 DEFINE_FLAG(bool, two_args_smi_icd, true,
     "Generate special IC stubs for two args Smi operations");
 DEFINE_FLAG(bool, unbox_numeric_fields, true,
     "Support unboxed double and float32x4 fields.");
 DECLARE_FLAG(bool, enable_type_checks);
 DECLARE_FLAG(bool, eliminate_type_checks);
 DECLARE_FLAG(bool, trace_optimization);
 DECLARE_FLAG(bool, trace_constant_propagation);
 DECLARE_FLAG(bool, throw_on_javascript_int_overflow);
+DECLARE_FLAG(bool, prune_dead_locals);
 
 Definition::Definition(intptr_t deopt_id)
     : Instruction(deopt_id),
       range_(NULL),
       type_(NULL),
       temp_index_(-1),
       ssa_temp_index_(-1),
       input_use_list_(NULL),
       env_use_list_(NULL),
       constant_value_(Object::ZoneHandle(ConstantPropagator::Unknown())) {
@@ skipping 1010 matching lines @@
   // Currently, phis are stored in a sparse array that holds the phi
   // for variable with index i at position i.
   // TODO(fschneider): Store phis in a more compact way.
   if (phis_ == NULL) {
     phis_ = new ZoneGrowableArray<PhiInstr*>(var_count);
     for (intptr_t i = 0; i < var_count; i++) {
       phis_->Add(NULL);
     }
   }
   ASSERT((*phis_)[var_index] == NULL);
-  (*phis_)[var_index] = new PhiInstr(this, PredecessorCount());
+  PhiInstr* phi = new PhiInstr(this, PredecessorCount());
+  (*phis_)[var_index] = phi;
+  if (!FLAG_prune_dead_locals) {
+    phi->mark_alive();
+  }
 }
 
 
 void JoinEntryInstr::InsertPhi(PhiInstr* phi) {
   // Lazily initialize the array of phis.
   if (phis_ == NULL) {
     phis_ = new ZoneGrowableArray<PhiInstr*>(1);
   }
   phis_->Add(phi);
 }
@@ skipping 2277 matching lines @@
     case Token::kTRUNCDIV: return 0;
     case Token::kMOD: return 1;
     default: UNIMPLEMENTED(); return -1;
   }
 }
 
 
 #undef __
 
 }  // namespace dart