Make closures optimizable by Crankshaft compiler.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 153bd9aae9cb02a5121a45ce991363a50f109f94..7705d0059936ba7e46c1535fc8eda556e0604a47 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -2930,6 +2930,27 @@ void HGraphBuilder::VisitVariableProxy(VariableProxy* expr) {
       BAILOUT("unsupported context for arguments object");
     }
     ast_context()->ReturnValue(environment()->Lookup(variable));
+  } else if (variable->AsSlot() != NULL &&
+             variable->AsSlot()->type() == Slot::CONTEXT) {
+    if (variable->mode() == Variable::CONST) {
+      BAILOUT("reference to const context slot");
+    }
+    Slot* slot = variable->AsSlot();
+    CompilationInfo* info = graph()->info();
+    int context_chain_length = info->function()->scope()->
+        ContextChainLength(slot->var()->scope());
+    ASSERT(context_chain_length >= 0);
+    Handle<JSFunction> closure = info->closure();
+    Context* context = closure->context();
+    for (int i = 0; i < context_chain_length; i++) {
+      context = context->closure()->context();
+    }
+    // TODO(antonm): if slot's value is not modified by closures, instead
+    // of reading it out of context, we could just embed the value as
+    // a constant.
+    HLoadContextSlot* instr =
+        new HLoadContextSlot(Handle<Context>(context), slot->index());
+    ast_context()->ReturnInstruction(instr, expr->id());
   } else if (variable->is_global()) {
     LookupResult lookup;
     LookupGlobalPropertyCell(variable, &lookup, false);
@@ -3441,6 +3462,9 @@ void HGraphBuilder::VisitAssignment(Assignment* expr) {
     if (proxy->IsArguments()) BAILOUT("assignment to arguments");
 
     // Handle the assignment.
+    if (var->AsSlot() != NULL && var->AsSlot()->type() == Slot::CONTEXT) {
+      BAILOUT("context slot assignment");
+    }
     if (var->is_global()) {
       VISIT_FOR_VALUE(expr->value());
       HandleGlobalVariableAssignment(var, Top(), expr->position(), expr->id());