Make use lists into doubly-linked lists.

runtime/vm/flow_graph_optimizer.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/flow_graph_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/flow_graph_compiler.h"
@@ skipping 2211 matching lines @@
   }
 
   return dom_block->Dominates(use_block);
 }
 
 
 void RangeAnalysis::RenameDominatedUses(Definition* def,
                                         Instruction* dom,
                                         Definition* other) {
   Value* next_use = NULL;
-  Value* prev_use = NULL;
   for (Value* use = def->input_use_list();
        use != NULL;
        use = next_use) {
     next_use = use->next_use();
 
     // Skip dead phis.
-    if (use->instruction()->IsPhi() &&
-        !use->instruction()->AsPhi()->is_alive()) {
-      prev_use = use;
-      continue;
-    }
+    PhiInstr* phi = use->instruction()->AsPhi();
+    if ((phi != NULL) && !phi->is_alive()) continue;
 
     if (IsDominatedUse(dom, use)) {
-      if (prev_use != NULL) {
-        prev_use->set_next_use(next_use);
-      } else {
-        def->set_input_use_list(next_use);
-      }
+      use->RemoveFromInputUseList();

[Kevin Millikin (Google), 2013/01/31 12:44:54]

In another change, I will create a single operation for the pattern of
RemoveFromList/set_definition/AddToList.

       use->set_definition(other);
       use->AddToInputUseList();
-    } else {
-      prev_use = use;
     }
   }
 }
 
 
 // For a comparison operation return an operation for the equivalent flipped
 // comparison: a (op) b === b (op') a.
 static Token::Kind FlipComparison(Token::Kind op) {
   switch (op) {
     case Token::kEQ: return Token::kEQ;
@@ skipping 2350 matching lines @@
 
   if (FLAG_trace_constant_propagation) {
     OS::Print("\n==== After constant propagation ====\n");
     FlowGraphPrinter printer(*graph_);
     printer.PrintBlocks();
   }
 }
 
 
 }  // namespace dart