Avoid initialization checks when loading from an immutable context variable.

src/compiler/ast-graph-builder.cc

Index: src/compiler/ast-graph-builder.cc
diff --git a/src/compiler/ast-graph-builder.cc b/src/compiler/ast-graph-builder.cc
index b6c6e0fb3f5bc1a0f396d178d09931f978332bda..6661b4025cd495e1d3022a5908b1af04c4a2d45f 100644
--- a/src/compiler/ast-graph-builder.cc
+++ b/src/compiler/ast-graph-builder.cc
@@ -2595,9 +2595,29 @@ Node* AstGraphBuilder::BuildVariableLoad(Variable* variable,
       const Operator* op =
           javascript()->LoadContext(depth, variable->index(), immutable);
       Node* value = NewNode(op, current_context());
-      // TODO(titzer): initialization checks are redundant for already
-      // initialized immutable context loads, but only specialization knows.
-      // Maybe specializer should be a parameter to the graph builder?
+
+      if (immutable) {
+        // TODO(titzer): initialization checks are redundant for already
+        // initialized immutable context loads, but only specialization knows.
+        // Below copied from context specialization reduction - refactor?
+        // Maybe specializer should be a parameter to the graph builder?
+        HeapObjectMatcher<Context> m(NodeProperties::GetValueInput(value, 0));
+        // If the context is not constant, no reduction can occur.
+        if (m.HasValue()) {
+          // Find the right parent context.
+          Context* context = *m.Value().handle();
+          for (size_t i = depth; i > 0; --i) {
+            context = context->previous();
+          }
+          Handle<Object> value2 = Handle<Object>(
+              context->get(static_cast<int>(variable->index())),
+                           jsgraph_->isolate());
+          if (!value2->IsUndefined() && !value2->IsTheHole()) {
+            return value;
+          }
+        }
+      }
+
       if (mode == CONST_LEGACY) {
         // Perform check for uninitialized legacy const variables.
         Node* undefined = jsgraph()->UndefinedConstant();