Eliminate phis that do not reach any non-environment uses.

runtime/vm/intermediate_language.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/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/flow_graph_compiler.h"
@@ skipping 392 matching lines @@
     for (intptr_t i = 0; i < var_count; i++) {
       phis_->Add(NULL);
     }
   }
   ASSERT((*phis_)[var_index] == NULL);
   (*phis_)[var_index] = new PhiInstr(PredecessorCount());
   phi_count_++;
 }
 
 
+void JoinEntryInstr::RemoveDeadPhis() {
+  if (phis_ == NULL) return;
+
+  for (intptr_t i = 0; i < phis_->length(); i++) {
+    PhiInstr* phi = (*phis_)[i];
+    if ((phi != NULL) && !phi->is_alive()) {
+      (*phis_)[i] = NULL;
+      phi_count_--;
+    }
+  }
+
+  // Check if we removed all phis.
+  if (phi_count_ == 0) phis_ = NULL;
+}
+
+
 intptr_t Instruction::SuccessorCount() const {
   return 0;
 }
 
 
 BlockEntryInstr* Instruction::SuccessorAt(intptr_t index) const {
   // Called only if index is in range.  Only control-transfer instructions
   // can have non-zero successor counts and they override this function.
   UNREACHABLE();
   return NULL;
@@ skipping 739 matching lines @@
   if (compiler->is_ssa()) {
     ASSERT(locs()->in(0).IsRegister());
     __ PushRegister(locs()->in(0).reg());
   }
 }
 
 
 #undef __
 
 }  // namespace dart