Use JavaScript runtime semantics when constant folding.

lib/compiler/implementation/js_backend/constant_system_javascript.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.
+
+const JAVA_SCRIPT_CONSTANT_SYSTEM = const JavaScriptConstantSystem();
+
+class JavaScriptBitNotOperation extends BitNotOperation {
+  const JavaScriptBitNotOperation();
+
+  Constant fold(Constant constant) {
+    if (JAVA_SCRIPT_CONSTANT_SYSTEM.isInt(constant)) {
+      // In JavaScript we don't check for -0 and treat it as if it was zero.
+      if (constant.isMinusZero()) constant = DART_CONSTANT_SYSTEM.createInt(0);
+      IntConstant intConstant = constant;
+      // We convert the result of bit-operations to 32 bit unsigned integers.
+      return JAVA_SCRIPT_CONSTANT_SYSTEM.createInt32(~intConstant.value);
+    }
+    return null;
+  }
+}
+
+/**
+ * In JavaScript we truncate the result to an unsigned 32 bit integer. Also, -0
+ * is treated as if it was the integer 0.
+ */
+class JavaScriptBinaryBitOperation implements BinaryOperation {
+  final BinaryBitOperation dartBitOperation;
+
+  const JavaScriptBinaryBitOperation(this.dartBitOperation);
+
+  bool isUserDefinable() => dartBitOperation.isUserDefinable();
+  SourceString get name() => dartBitOperation.name;
+
+  Constant fold(Constant left, Constant right) {
+    // In JavaScript we don't check for -0 and treat it as if it was zero.
+    if (left.isMinusZero()) left = DART_CONSTANT_SYSTEM.createInt(0);
+    if (right.isMinusZero()) right = DART_CONSTANT_SYSTEM.createInt(0);
+    IntConstant result = dartBitOperation.fold(left, right);
+    if (result != null) {
+      // We convert the result of bit-operations to 32 bit unsigned integers.
+      return JAVA_SCRIPT_CONSTANT_SYSTEM.createInt32(result.value);
+    }
+    return result;
+  }
+}
+
+class JavaScriptShiftRightOperation extends JavaScriptBinaryBitOperation {
+  const JavaScriptShiftRightOperation() : super(const ShiftRightOperation());
+
+  Constant fold(Constant left, Constant right) {
+    // Truncate the input value to 32 bits if necessary.
+    if (left.isInt()) {
+      IntConstant intConstant = left;
+      int value = intConstant.value;
+      int truncatedValue = value & JAVA_SCRIPT_CONSTANT_SYSTEM.BITS32;
+      // TODO(floitsch): we should treat the input to right shifts as unsigned.
+
+      // Sign-extend. A 32 bit complement-two value x can be computed by:
+      //    x_u - 2^32 (where x_u is its unsigned representation).
+      // Example: 0xFFFFFFFF - 0x100000000 => -1.
+      // We simply and with the sign-bit and multiply by two. If the sign-bit
+      // was set, then the result is 0. Otherwise it will become 2^32.
+      final int SIGN_BIT = 0x80000000;
+      truncatedValue -= 2 * (truncatedValue & SIGN_BIT);
+      if (value != truncatedValue) {
+        left = DART_CONSTANT_SYSTEM.createInt(truncatedValue);
+      }
+    }
+    return super.fold(left, right);
+  }
+}
+
+class JavaScriptNegateOperation implements UnaryOperation {
+  final NegateOperation dartNegateOperation = const NegateOperation();
+
+  const JavaScriptNegateOperation();
+
+  bool isUserDefinable() => dartNegateOperation.isUserDefinable();
+  SourceString get name() => dartNegateOperation.name;
+
+  Constant fold(Constant constant) {
+    if (constant.isInt()) {
+      IntConstant intConstant = constant;
+      if (intConstant.value == 0) {
+        return JAVA_SCRIPT_CONSTANT_SYSTEM.createDouble(-0.0);
+      }
+    }
+    return dartNegateOperation.fold(constant);
+  }
+}
+
+class JavaScriptBinaryArithmeticOperation implements BinaryOperation {
+  final BinaryOperation dartArithmeticOperation;
+
+  const JavaScriptBinaryArithmeticOperation(this.dartArithmeticOperation);
+
+  bool isUserDefinable() => dartArithmeticOperation.isUserDefinable();
+  SourceString get name() => dartArithmeticOperation.name;
+
+  Constant fold(Constant left, Constant right) {
+    Constant result = dartArithmeticOperation.fold(left, right);
+    if (result == null) return result;
+    return JAVA_SCRIPT_CONSTANT_SYSTEM.convertToJavaScriptConstant(result);
+  }
+}
+
+class JavaScriptIdentityOperation implements BinaryOperation {
+  final IdentityOperation dartIdentityOperation = const IdentityOperation();
+
+  const JavaScriptIdentityOperation();
+
+  bool isUserDefinable() => dartIdentityOperation.isUserDefinable();
+  SourceString get name() => dartIdentityOperation.name;
+
+  BoolConstant fold(Constant left, Constant right) {
+    BoolConstant result = dartIdentityOperation.fold(left, right);
+    if (result == null || result.value) return result;
+    // In JavaScript -0.0 === 0 and all doubles are equal to their integer
+    // values. Furthermore NaN !== NaN.
+    if (left.isNum() && right.isNum()) {
+      NumConstant leftNum = left;
+      NumConstant rightNum = right;
+      double leftDouble = leftNum.value.toDouble();
+      double rightDouble = rightNum.value.toDouble();
+      return new BoolConstant(leftDouble == rightDouble);
+    }
+    return result;
+  }
+}
+
+/**
+ * Constant system following the semantics for Dart code that has been
+ * compiled to JavaScript.
+ */
+class JavaScriptConstantSystem implements ConstantSystem {
+  const int BITS31 = 0x8FFFFFFF;
+  const int BITS32 = 0xFFFFFFFF;
+  // The maximum integer value a double can represent without losing
+  // precision.
+  const int BITS53 = 0x1FFFFFFFFFFFFF;
+
+  final add = const JavaScriptBinaryArithmeticOperation(const AddOperation());
+  final bitAnd = const JavaScriptBinaryBitOperation(const BitAndOperation());
+  final bitNot = const JavaScriptBitNotOperation();
+  final bitOr = const JavaScriptBinaryBitOperation(const BitOrOperation());
+  final bitXor = const JavaScriptBinaryBitOperation(const BitXorOperation());
+  final booleanAnd = const BooleanAndOperation();
+  final booleanOr = const BooleanOrOperation();
+  final divide =
+      const JavaScriptBinaryArithmeticOperation(const DivideOperation());
+  final equal = const EqualsOperation();
+  final greaterEqual = const GreaterEqualOperation();
+  final greater = const GreaterOperation();
+  final identity = const JavaScriptIdentityOperation();
+  final lessEqual = const LessEqualOperation();
+  final less = const LessOperation();
+  final modulo =
+      const JavaScriptBinaryArithmeticOperation(const ModuloOperation());
+  final multiply =
+      const JavaScriptBinaryArithmeticOperation(const MultiplyOperation());
+  final negate = const JavaScriptNegateOperation();
+  final not = const NotOperation();
+  final shiftLeft =
+      const JavaScriptBinaryBitOperation(const ShiftLeftOperation());
+  final shiftRight = const JavaScriptShiftRightOperation();
+  final subtract =
+      const JavaScriptBinaryArithmeticOperation(const SubtractOperation());
+  final truncatingDivide = const JavaScriptBinaryArithmeticOperation(
+      const TruncatingDivideOperation());
+
+  const JavaScriptConstantSystem();
+
+  /**
+   * Returns true if the given [value] fits into a double without losing
+   * precision.
+   */
+  bool integerFitsIntoDouble(int value) {
+    int absValue = value.abs();
+    return (absValue & BITS53) == absValue;
+  }
+
+  NumConstant convertToJavaScriptConstant(NumConstant constant) {
+    if (constant.isInt()) {
+      IntConstant intConstant = constant;
+      int intValue = intConstant.value;
+      if (!integerFitsIntoDouble(intValue)) {
+        return new DoubleConstant(intValue.toDouble());
+      }
+    } else if (constant.isDouble()) {
+      DoubleConstant doubleResult = constant;
+      double doubleValue = doubleResult.value;
+      if (!doubleValue.isInfinite() && !doubleValue.isNaN() &&
+          !constant.isMinusZero()) {
+        int intValue = doubleValue.toInt();
+        if (intValue == doubleValue && integerFitsIntoDouble(intValue)) {
+          return new IntConstant(intValue);
+        }
+      }
+    }
+    return constant;
+  }
+
+  NumConstant createInt(int i)
+      => convertToJavaScriptConstant(new IntConstant(i));
+  NumConstant createInt32(int i) => new IntConstant(i & BITS32);
+  NumConstant createDouble(double d)
+      => convertToJavaScriptConstant(new DoubleConstant(d));
+  StringConstant createString(DartString string, Node diagnosticNode)
+      => new StringConstant(string, diagnosticNode);
+  BoolConstant createBool(bool value) => new BoolConstant(value);
+  NullConstant createNull() => new NullConstant();
+
+  // Integer checks don't verify that the number is not -0.0.
+  bool isInt(Constant constant) => constant.isInt() || constant.isMinusZero();
+  bool isDouble(Constant constant)
+      => constant.isDouble() && !constant.isMinusZero();
+  bool isString(Constant constant) => constant.isString();
+  bool isBool(Constant constant) => constant.isBool();
+  bool isNull(Constant constant) => constant.isNull();
+}