[turbofan] Specialize loads of the native context.

src/compiler/js-native-context-specialization.cc

Index: src/compiler/js-native-context-specialization.cc
diff --git a/src/compiler/js-native-context-specialization.cc b/src/compiler/js-native-context-specialization.cc
index a09e2f67591c9efdf7e5fcf35e71b11787f62639..cfe4d9ac26980888fa3fa7f936d33321598535ee 100644
--- a/src/compiler/js-native-context-specialization.cc
+++ b/src/compiler/js-native-context-specialization.cc
@@ -38,6 +38,8 @@ JSNativeContextSpecialization::JSNativeContextSpecialization(
 
 Reduction JSNativeContextSpecialization::Reduce(Node* node) {
   switch (node->opcode()) {
+    case IrOpcode::kJSLoadContext:
+      return ReduceJSLoadContext(node);
     case IrOpcode::kJSLoadNamed:
       return ReduceJSLoadNamed(node);
     case IrOpcode::kJSStoreNamed:
@@ -52,6 +54,21 @@ Reduction JSNativeContextSpecialization::Reduce(Node* node) {
   return NoChange();
 }
 
+Reduction JSNativeContextSpecialization::ReduceJSLoadContext(Node* node) {
+  DCHECK_EQ(IrOpcode::kJSLoadContext, node->opcode());
+  ContextAccess const& access = ContextAccessOf(node->op());
+  Handle<Context> native_context;
+  // Specialize JSLoadContext(NATIVE_CONTEXT_INDEX) to the known native
+  // context (if any), so we can constant-fold those fields, which is
+  // safe, since the NATIVE_CONTEXT_INDEX slot is always immutable.
+  if (access.index() == Context::NATIVE_CONTEXT_INDEX &&
+      GetNativeContext(node).ToHandle(&native_context)) {
+    Node* value = jsgraph()->HeapConstant(native_context);
+    ReplaceWithValue(node, value);
+    return Replace(value);
+  }
+  return NoChange();
+}
 
 Reduction JSNativeContextSpecialization::ReduceNamedAccess(
     Node* node, Node* value, MapHandleList const& receiver_maps,