Remove dead phis as soon as they are discovered.

runtime/vm/flow_graph_builder.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_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/code_descriptors.h"
 #include "vm/dart_entry.h"
@@ skipping 179 matching lines @@
     caller_graph->set_max_block_id(join_id);
     JoinEntryInstr* join =
         new JoinEntryInstr(join_id, CatchClauseNode::kInvalidTryIndex);
     for (intptr_t i = 0; i < num_exits; ++i) {
       LastInstructionAt(i)->Goto(join);
       // Directly add the predecessors of the join in ascending block id order.
       join->predecessors_.Add(ExitBlockAt(i));
     }
     // If the call has uses, create a phi of the returns.
     if (call->HasUses()) {
-      // Environment count: length before call - argument count (+ return)
-      intptr_t env_count = call->env()->Length() - call->ArgumentCount();
       // Add a phi of the return values.
-      join->InsertPhi(env_count, env_count + 1);
-      PhiInstr* phi = join->phis()->Last();
+      PhiInstr* phi = new PhiInstr(join, num_exits);
       phi->set_ssa_temp_index(caller_graph->alloc_ssa_temp_index());
       phi->mark_alive();
       for (intptr_t i = 0; i < num_exits; ++i) {
         phi->SetInputAt(i, ValueAt(i));
       }
+      join->InsertPhi(phi);
       // Replace uses of the call with the phi.
       call->ReplaceUsesWith(phi);
     } else {
       // In the case that the result is unused, remove the return value uses
       // from their definition's use list.
       for (intptr_t i = 0; i < num_exits; ++i) {
         ValueAt(i)->RemoveFromUseList();
       }
     }
     // Remove the call from the graph.
@@ skipping 3044 matching lines @@
   intptr_t len = OS::SNPrint(NULL, 0, kFormat, function_name, reason) + 1;
   char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
   OS::SNPrint(chars, len, kFormat, function_name, reason);
   const Error& error = Error::Handle(
       LanguageError::New(String::Handle(String::New(chars))));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }
 
 
 }  // namespace dart