Compute default throws behavior from JavaScript.

pkg/compiler/lib/src/native/js.dart

Index: pkg/compiler/lib/src/native/js.dart
diff --git a/pkg/compiler/lib/src/native/js.dart b/pkg/compiler/lib/src/native/js.dart
index 1cbe8da5c8fb978d81a05f35d7e040febc8b2e19..2203b55f19394d1e10bb928db11461a0dee10283 100644
--- a/pkg/compiler/lib/src/native/js.dart
+++ b/pkg/compiler/lib/src/native/js.dart
@@ -81,3 +81,187 @@ class SideEffectsVisitor extends js.BaseVisitor {
     node.visitChildren(this);
   }
 }
+
+
+/// ThrowBehaviorVisitor generates a NativeThrowBehavior describing the
+/// exception behavior of a JavaScript expression.
+///
+/// The result is semi-conservative, giving reasonable results for many simple
+/// JS fragments. The non-conservative part is the assumption that binary
+/// operators are used on 'good' operands that do not force arbirary code to be
+/// executed via conversions (valueOf() and toString() methods).
+///
+/// In many cases a JS fragment has more precise behavior. In these cases the
+/// behavior should be described as a property of the JS fragment. For example,
+/// Object.keys(#) has a TypeError on null / undefined, which can only be known
+/// in the calling context.
+///
+class ThrowBehaviorVisitor extends js.BaseVisitor<NativeThrowBehavior> {
+
+  ThrowBehaviorVisitor();
+
+  NativeThrowBehavior analyze(js.Node node) {
+    return visit(node);
+  }
+
+  // TODO(sra): Add [sequence] functionality to NativeThrowBehavior.
+  /// Returns the combined behavior of sequential execution of code having
+  /// behavior [first] followed by code having behavior [second].
+  static NativeThrowBehavior sequence(NativeThrowBehavior first,
+      NativeThrowBehavior second) {
+    if (first == NativeThrowBehavior.MUST) return first;
+    if (second == NativeThrowBehavior.MUST) return second;
+    if (second == NativeThrowBehavior.NEVER) return first;
+    if (first == NativeThrowBehavior.NEVER) return second;
+    // Both are one of MAY or MAY_THROW_ONLY_ON_FIRST_ARGUMENT_ACCESS.
+    return NativeThrowBehavior.MAY;
+  }
+
+  // TODO(sra): Add [choice] functionality to NativeThrowBehavior.
+  /// Returns the combined behavior of a choice between two paths with behaviors
+  /// [first] and [second].
+  static NativeThrowBehavior choice(NativeThrowBehavior first,
+      NativeThrowBehavior second) {
+    if (first == second) return first;  // Both paths have same behaviour.
+    return NativeThrowBehavior.MAY;
+  }
+
+  NativeThrowBehavior visit(js.Node node) {
+    return node.accept(this);
+  }
+
+  NativeThrowBehavior visitNode(js.Node node) {
+    return NativeThrowBehavior.MAY;
+  }
+
+  NativeThrowBehavior visitLiteral(js.Literal node) {
+    return NativeThrowBehavior.NEVER;
+  }
+
+  NativeThrowBehavior visitInterpolatedExpression(js.InterpolatedNode node) {
+    return NativeThrowBehavior.NEVER;
+  }
+
+  NativeThrowBehavior visitInterpolatedSelector(js.InterpolatedNode node) {
+    return NativeThrowBehavior.NEVER;
+  }
+
+  NativeThrowBehavior visitObjectInitializer(js.ObjectInitializer node) {
+    NativeThrowBehavior result = NativeThrowBehavior.NEVER;
+    for (js.Property property in node.properties) {
+      result = sequence(result, visit(property));
+    }
+    return result;
+  }
+
+  NativeThrowBehavior visitProperty(js.Property node) {
+    return sequence(visit(node.name), visit(node.value));
+  }
+
+  NativeThrowBehavior visitAssignment(js.Assignment node) {
+    // TODO(sra): Can we make "#.p = #" be null(1)?
+    return NativeThrowBehavior.MAY;
+  }
+
+  NativeThrowBehavior visitCall(js.Call node) {
+    return NativeThrowBehavior.MAY;
+  }
+
+  NativeThrowBehavior visitNew(js.New node) {
+    // TODO(sra): `new Array(x)` where `x` is a small number.
+    return NativeThrowBehavior.MAY;
+  }
+
+  NativeThrowBehavior visitBinary(js.Binary node) {
+    NativeThrowBehavior left = visit(node.left);
+    NativeThrowBehavior right = visit(node.right);
+    switch (node.op) {
+      // We make the non-conservative assumption that these operations are not
+      // used in ways that force calling arbitrary code via valueOf or
+      // toString().
+      case "*":
+      case "/":
+      case "%":
+      case "+":
+      case "-":
+      case "<<":
+      case ">>":
+      case ">>>":
+      case "<":
+      case ">":
+      case "<=":
+      case ">=":
+      case "==":
+      case "===":
+      case "!=":
+      case "!==":
+      case "&":
+      case "^":
+      case "|":
+        return sequence(left, right);
+
+      case ',':
+        return sequence(left, right);
+
+      case "&&":
+      case "||":
+        return choice(left, sequence(left, right));
+
+      case "instanceof":
+      case "in":
+      default:
+        return NativeThrowBehavior.MAY;
+    }
+  }
+
+  NativeThrowBehavior visitThrow(js.Throw node) {
+    return NativeThrowBehavior.MUST;
+  }
+
+  NativeThrowBehavior visitPrefix(js.Prefix node) {
+    if (node.op == 'typeof' && node.argument is js.VariableUse)
+      return NativeThrowBehavior.NEVER;
+    NativeThrowBehavior result = visit(node.argument);
+    switch (node.op) {
+      case '!':
+      case '~':
+      case 'void':
+      case 'typeof':
+        return result;
+      default:
+        return NativeThrowBehavior.MAY;
+    }
+  }
+
+  NativeThrowBehavior visitVariableUse(js.VariableUse node) {
+    // We could get a ReferenceError unless the variable is in scope. The AST
+    // could distinguish in-scope and out-of scope references. For JS fragments,
+    // the only use of VariableUse should be for global references. Certain
+    // global names are almost certainly not reference errors, e.g 'Array'.
+    switch (node.name) {
+      case 'Array':
+      case 'Object':
+        return NativeThrowBehavior.NEVER;
+      default:
+        return NativeThrowBehavior.MAY;
+    }
+  }
+
+  NativeThrowBehavior visitAccess(js.PropertyAccess node) {
+    // TODO(sra): We need a representation where the nsm guard behaviour is
+    // maintained when combined with other throwing behaviour.
+    js.Node receiver = node.receiver;
+    NativeThrowBehavior first = visit(receiver);
+    NativeThrowBehavior second = visit(node.selector);
+
+    if (receiver is js.InterpolatedExpression &&
+        receiver.isPositional &&
+        receiver.nameOrPosition == 0) {
+      first = NativeThrowBehavior.MAY_THROW_ONLY_ON_FIRST_ARGUMENT_ACCESS;
+    } else {
+      first = NativeThrowBehavior.MAY;
+    }
+
+    return sequence(first, second);
+  }
+}