[interpreter] Add a fast path for dynamic local load

src/interpreter/interpreter-assembler.cc

Index: src/interpreter/interpreter-assembler.cc
diff --git a/src/interpreter/interpreter-assembler.cc b/src/interpreter/interpreter-assembler.cc
index 2ea7e85acd409946cf43e273c42c93d3b6c01b7e..7a75a9c7d06fde3812ffeb655374516982a54984 100644
--- a/src/interpreter/interpreter-assembler.cc
+++ b/src/interpreter/interpreter-assembler.cc
@@ -114,6 +114,41 @@ Node* InterpreterAssembler::GetContextAtDepth(Node* context, Node* depth) {
   return cur_context.value();
 }
 
+void InterpreterAssembler::GotoIfHasContextExtensionUpToDepth(Node* context,
+                                                              Node* depth,
+                                                              Label* target) {
+  Variable cur_context(this, MachineRepresentation::kTaggedPointer);
+  cur_context.Bind(context);
+
+  Variable cur_depth(this, MachineRepresentation::kWord32);
+  cur_depth.Bind(depth);
+
+  Variable* context_search_loop_variables[2] = {&cur_depth, &cur_context};
+  Label context_search(this, 2, context_search_loop_variables);
+
+  // Loop until the depth is 0.
+  Goto(&context_search);
+  Bind(&context_search);
+  {
+    // TODO(leszeks): We only need to do this check if the context had a sloppy
+    // eval, we could pass in a context chain bitmask to figure out which
+    // contexts actually need to be checked.

[rmcilroy, 2016/09/16 08:56:55]

It would probably take as long to check the bitmap as to do this check here (and
would increase bytecode size), so assuming this check is only for performance
and not correctness, I wouldn't bother.

[Leszek Swirski, 2016/09/16 10:46:54]

I see your point, but it would e.g. make a big difference to the TurboFan graph
generated by our graph builder. Even if not, we should definitely still only
look as far up in the chain as the deepest sloppy eval.

+
+    Node* extension_slot =
+        LoadContextSlot(cur_context.value(), Context::EXTENSION_INDEX);
+
+    // Jump to the target if the extension slot is not a hole
+    GotoIf(Word32NotEqual(extension_slot, TheHoleConstant()), target);
+
+    cur_depth.Bind(Int32Sub(cur_depth.value(), Int32Constant(1)));
+    cur_context.Bind(
+        LoadContextSlot(cur_context.value(), Context::PREVIOUS_INDEX));
+
+    GotoIf(Word32NotEqual(cur_depth.value(), Int32Constant(0)),
+           &context_search);
+  }
+}
+
 Node* InterpreterAssembler::BytecodeOffset() {
   return bytecode_offset_.value();
 }