cps-ir: Merge variables based on the source variable's name.

pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart b/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
index 9ca641e87c458a81bbfcc0818aeb7d8b02002798..32a8cffb065d6e313599dec255ff5bc4a3d16b9f 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
@@ -34,7 +34,7 @@ class VariableMerger extends RecursiveVisitor implements Pass {
       builder.build(node.parameters, body);
       _computeLiveness(builder.blocks);
       Map<Variable, Variable> subst =
-          _computeRegisterAllocation(builder.blocks);
+          _computeRegisterAllocation(builder.blocks, node.parameters);
       new SubstituteVariables(subst).apply(node);
     });
   }
@@ -355,20 +355,20 @@ const bool NO_PRESERVE_VARS = const bool.fromEnvironment('NO_PRESERVE_VARS');
 ///
 /// We never merge variables that originated from distinct source variables,
 /// so we build a separate register interference graph for each source variable.
-Map<Variable, Variable> _computeRegisterAllocation(List<Block> blocks) {
+Map<Variable, Variable> _computeRegisterAllocation(List<Block> blocks,
+                                                   List<Variable> parameters) {
   Map<Variable, Set<Variable>> interference = <Variable, Set<Variable>>{};
 
   /// Group for the given variable. We attempt to merge variables in the same
   /// group.
-  /// By default, variables are grouped based on their source variable, but
-  /// this can be disabled for testing purposes.
-  Local group(Variable variable) {
-    if (NO_PRESERVE_VARS) {
-      // Parameters may not occur more than once in a parameter list,
-      // so except for parameters, we try to merge all variables.
-      return variable.element is ParameterElement ? variable.element : null;
-    }
-    return variable.element;
+  /// By default, variables are grouped based on their source variable name,
+  /// but this can be disabled for testing purposes.
+  String group(Variable variable) {
+    if (NO_PRESERVE_VARS) return '';
+    // Group variables based on the source variable's name, not its element,
+    // so if multiple locals are declared with the same name, they will
+    // map to the same (hoisted) variable in the output.
+    return variable.element == null ? '' : variable.element.name;
   }
 
   Set<Variable> empty = new Set<Variable>();
@@ -377,7 +377,7 @@ Map<Variable, Variable> _computeRegisterAllocation(List<Block> blocks) {
   // live after the assignment (if it came from the same source variable).
   for (Block block in blocks) {
     // Group the liveOut set by source variable.
-    Map<Local, Set<Variable>> liveOut = <Local, Set<Variable>>{};
+    Map<String, Set<Variable>> liveOut = <String, Set<Variable>>{};
     for (Variable variable in block.liveOut) {
       liveOut.putIfAbsent(
           group(variable),
@@ -414,10 +414,21 @@ Map<Variable, Variable> _computeRegisterAllocation(List<Block> blocks) {
   List<Variable> variables = interference.keys.toList();
   variables.sort((x, y) => interference[y].length - interference[x].length);
 
-  Map<Local, List<Variable>> registers = <Local, List<Variable>>{};
+  Map<String, List<Variable>> registers = <String, List<Variable>>{};
   Map<Variable, Variable> subst = <Variable, Variable>{};
 
+  // Parameters are special in that they must have a ParameterElement and
+  // cannot be merged with each other. Ensure that they are not substituted.
+  // Other variables can still be substituted by a parameter.
+  for (Variable parameter in parameters) {
+    subst[parameter] = parameter;
+    registers[group(parameter)] = <Variable>[parameter];
+  }
+
   for (Variable v1 in variables) {
+    // Parameters have already been assigned a substitute; skip those.
+    if (subst.containsKey(v1)) continue;
+
     List<Variable> register = registers[group(v1)];
 
     // Optimization: For the first variable in a group, allocate a new color