dart2js cps: More rewritings for compound assignments.

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

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 1c96301711577753d98346431fcc4e8fb7d3cc06..80ffa43c9cf5d24f8718e494abefa67be34142b9 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -752,16 +752,26 @@ class StatementRewriter extends Transformer implements Pass {
     return node;
   }
 
-  bool sameVariable(Expression e1, Expression e2) {
-    return e1 is VariableUse && e2 is VariableUse && e1.variable == e2.variable;
-  }
-
   bool isCompoundableBuiltin(Expression e) {
     return e is ApplyBuiltinOperator &&
-           e.arguments.length == 2 &&
+           e.arguments.length >= 2 &&
            isCompoundableOperator(e.operator);
   }
 
+  /// Converts a compoundable operator application into the right-hand side for
+  /// use in a compound assignment, discarding the left-hand value.
+  ///
+  /// For example, for `x + y + z` it returns `y + z`.
+  Expression contractCompoundableBuiltin(ApplyBuiltinOperator e) {
+    assert(isCompoundableBuiltin(e));
+    if (e.arguments.length > 2) {
+      assert(e.operator == BuiltinOperator.StringConcatenate);
+      return new ApplyBuiltinOperator(e.operator, e.arguments.skip(1).toList());
+    } else {
+      return e.arguments[1];
+    }
+  }
+
   void destroyVariableUse(VariableUse node) {
     --node.variable.readCount;
   }
@@ -772,13 +782,12 @@ class StatementRewriter extends Transformer implements Pass {
     if (isCompoundableBuiltin(node.value)) {
       ApplyBuiltinOperator rhs = node.value;
       Expression left = rhs.arguments[0];
-      Expression right = rhs.arguments[1];
       if (left is GetField &&
           left.field == node.field &&
-          sameVariable(left.object, node.object)) {
-        destroyVariableUse(left.object);
+          samePrimary(left.object, node.object)) {
+        destroyPrimaryExpression(left.object);
         node.compound = rhs.operator;
-        node.value = right;
+        node.value = contractCompoundableBuiltin(rhs);
       }
     }
     node.object = visitExpression(node.object);
@@ -798,6 +807,16 @@ class StatementRewriter extends Transformer implements Pass {
   Expression visitSetStatic(SetStatic node) {
     allowRhsPropagation.add(true);
     node.value = visitExpression(node.value);
+    if (isCompoundableBuiltin(node.value)) {
+      ApplyBuiltinOperator rhs = node.value;
+      Expression left = rhs.arguments[0];
+      if (left is GetStatic &&
+          left.element == node.element &&
+          !left.useLazyGetter) {
+        node.compound = rhs.operator;
+        node.value = contractCompoundableBuiltin(rhs);
+      }
+    }
     allowRhsPropagation.removeLast();
     return node;
   }
@@ -857,14 +876,13 @@ class StatementRewriter extends Transformer implements Pass {
     if (isCompoundableBuiltin(node.value)) {
       ApplyBuiltinOperator rhs = node.value;
       Expression left = rhs.arguments[0];
-      Expression right = rhs.arguments[1];
       if (left is GetIndex &&
-          sameVariable(left.object, node.object) &&
-          sameVariable(left.index, node.index)) {
-        destroyVariableUse(left.object);
-        destroyVariableUse(left.index);
+          samePrimary(left.object, node.object) &&
+          samePrimary(left.index, node.index)) {
+        destroyPrimaryExpression(left.object);
+        destroyPrimaryExpression(left.index);
         node.compound = rhs.operator;
-        node.value = right;
+        node.value = contractCompoundableBuiltin(rhs);
       }
     }
     node.index = visitExpression(node.index);
@@ -1356,3 +1374,22 @@ class VariableUseVisitor extends RecursiveVisitor {
     new VariableUseVisitor(callback).visitExpression(node);
   }
 }
+
+bool sameVariable(Expression e1, Expression e2) {
+  return e1 is VariableUse && e2 is VariableUse && e1.variable == e2.variable;
+}
+
+/// True if [e1] and [e2] are primary expressions (expressions without
+/// subexpressions) with the same value.
+bool samePrimary(Expression e1, Expression e2) {
+  return sameVariable(e1, e2) || (e1 is This && e2 is This);
+}
+
+/// Decrement the reference count for [e] if it is a variable use.
+void destroyPrimaryExpression(Expression e) {
+  if (e is VariableUse) {
+    --e.variable.readCount;
+  } else {
+    assert(e is This);
+  }
+}