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..63206914a1a046f15a2d4fc7dacff7b96e81cab6 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);
@@ -1036,9 +1054,34 @@ class StatementRewriter extends Transformer implements Pass {
       Expression s,
       Expression t,
       Expression condition) {
-    if (s is Assign && t is Assign && s.variable == t.variable) {
-      Expression values = new Conditional(condition, s.value, t.value);
-      return new CombinedAssigns(s, t, new CombinedExpressions(values));
+    if (s is Assign && t is Assign &&
+        s.variable == t.variable) {
+      return new CombinedAssigns(s, t,
+          combineAsConditional(s.value, t.value, condition));
+    }
+    if (s is SetStatic &&
+        t is SetStatic &&
+        s.element == t.element) {
+      return new CombinedSetStatics(s, t,
+            combineAsConditional(s.value, t.value, condition));
+    }
+    if (s is SetField &&
+        t is SetField &&
+        s.compound == null &&
+        t.compound == null &&
+        s.field == t.field &&
+        samePrimary(s.object, t.object)) {
+      return new CombinedSetFields(s, t,
+          combineAsConditional(s.value, t.value, condition));
+    }
+    if (s is SetIndex &&
+        t is SetIndex &&
+        s.compound == null &&
+        t.compound == null &&
+        samePrimary(s.object, t.object) &&
+        samePrimary(s.index, t.index)) {
+      return new CombinedSetIndexes(s, t,
+          combineAsConditional(s.value, t.value, condition));
     }
     return new CombinedExpressions(new Conditional(condition, s, t));
   }
@@ -1281,12 +1324,12 @@ abstract class CombinedExpressions {
   factory CombinedExpressions(Expression e) = GenericCombinedExpressions;
 }
 
-/// Combines assignments of form `[variable] := E1` and `[variable] := E2` into
-/// a single assignment of form `[variable] := combine(E1, E2)`.
+/// Combines assignments of form `v := E1` and `v := E2` into
+/// a single assignment of form `v := combine(E1, E2)`.

[sra1, 2016/01/22 03:22:04]

We probably want to avoid this where one arm is a constant that the JS VM can
tell does not need a write-barrier. 

[asgerf, 2016/01/22 18:54:47]

Good point.  I looked at the IR generated by V8 in the two cases and it does
indeed hit a write-barrier it would not hit otherwise.

 class CombinedAssigns implements CombinedExpressions {
   Assign assign1, assign2;
   CombinedExpressions value;
-  Expression combined;
+  Assign combined;
 
   CombinedAssigns(this.assign1, this.assign2, this.value) {
     assert(assign1.variable == assign2.variable);
@@ -1300,10 +1343,74 @@ class CombinedAssigns implements CombinedExpressions {
   }
 }
 
+/// Combines `static = E1` and `static = E2` into `static = combine(E1, E2)`.
+class CombinedSetStatics implements CombinedExpressions {
+  SetStatic assign1, assign2;
+  CombinedExpressions value;
+  SetStatic combined;
+
+  CombinedSetStatics(this.assign1, this.assign2, this.value) {
+    assert(assign1.element == assign2.element);
+    // TODO(asgerf): Can we combine source information from the two assignments?
+    combined = new SetStatic(assign1.element, value.combined,
+        assign1.sourceInformation);
+  }
+
+  void uncombine() {
+    value.uncombine();
+  }
+}
+
+/// Combines `v.field = E1` and `v.field = E2` into `v.field = combine(E1, E2)`.
+class CombinedSetFields implements CombinedExpressions {
+  SetField assign1, assign2;
+  CombinedExpressions value;
+  SetField combined;
+
+  CombinedSetFields(this.assign1, this.assign2, this.value) {
+    assert(samePrimary(assign1.object, assign2.object));
+    assert(assign1.field == assign2.field);
+    assert(assign1.compound == null);
+    assert(assign2.compound == null);
+    destroyPrimaryExpression(assign2.object);
+    combined = new SetField(assign1.object, assign1.field, value.combined);
+  }
+
+  void uncombine() {
+    value.uncombine();
+    restorePrimaryExpression(assign2.object);
+  }
+}
+
+/// Combines `v[index] = E1` and `v[index] = E2` into
+/// `v[index] = combine(E1, E2)`.
+class CombinedSetIndexes implements CombinedExpressions {
+  SetIndex assign1, assign2;
+  CombinedExpressions value;
+  SetIndex combined;
+
+  CombinedSetIndexes(this.assign1, this.assign2, this.value) {
+    assert(samePrimary(assign1.object, assign2.object));
+    assert(samePrimary(assign1.index, assign2.index));
+    assert(assign1.compound == null);
+    assert(assign2.compound == null);
+    destroyPrimaryExpression(assign2.object);
+    destroyPrimaryExpression(assign2.index);
+    combined = new SetIndex(assign1.object, assign1.index, value.combined,
+        compound: assign1.compound);
+  }
+
+  void uncombine() {
+    value.uncombine();
+    restorePrimaryExpression(assign2.object);
+    restorePrimaryExpression(assign2.index);
+  }
+}
+
 /// Combines two variable uses into one.
 class CombinedUses implements CombinedExpressions {
   VariableUse use1, use2;
-  Expression combined;
+  VariableUse combined;
 
   CombinedUses(this.use1, this.use2) {
     assert(use1.variable == use2.variable);
@@ -1356,3 +1463,31 @@ 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);
+  }
+}
+
+/// Increment the reference count for [e] if it is a variable use.
+void restorePrimaryExpression(Expression e) {
+  if (e is VariableUse) {
+    ++e.variable.readCount;
+  } else {
+    assert(e is This);
+  }
+}