Online 'SSA conversion' of mutable parameters.

pkg/compiler/lib/src/ssa/optimize.dart

 // 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.
 
 import '../common/codegen.dart' show CodegenWorkItem;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../core_types.dart' show CoreClasses;
@@ skipping 229 matching lines @@
     ConstantValue value = getConstantFromType(node);
     if (value != null) {
       HConstant constant = graph.addConstant(value, compiler);
       for (HInstruction user in node.usedBy.toList()) {
         user.changeUse(node, constant);
       }
     }
   }
 
   HInstruction visitParameterValue(HParameterValue node) {
+    // [HParameterValue]s are either the value of the parameter (in fully SSA
+    // converted code), or the mutable variable containing the value (in
+    // incompletely SSA converted code, e.g. methods containing exceptions).
+    //
     // If the parameter is used as a mutable variable we cannot replace the
     // variable with a value.
-    if (node.usedAsVariable()) return node;
+    //
+    // If the parameter is used as a mutable variable but never written, first
+    // convert to a value parameter.
+
+    if (node.usedAsVariable()) {
+      if (!node.usedBy.every((user) => user is HLocalGet)) return node;
+      // Trivial SSA-conversion. Replace all HLocalGet instructions with the
+      // parameter.
+      for (HLocalGet user in node.usedBy.toList()) {
+        user.block.rewrite(user, node);
+        user.block.remove(user);
+      }
+    }
+
     propagateConstantValueToUses(node);
     return node;
   }
 
   HInstruction visitBoolify(HBoolify node) {
     List<HInstruction> inputs = node.inputs;
     assert(inputs.length == 1);
     HInstruction input = inputs[0];
     if (input.isBoolean(compiler)) return input;
 
@@ skipping 2144 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }