Avoid initialization checks when loading from an immutable context variable.

src/compiler/ast-graph-builder.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/ast-graph-builder.h"
 
 #include "src/compiler.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/control-builders.h"
 #include "src/compiler/linkage.h"
@@ skipping 2577 matching lines @@
       }
       return value;
     }
     case Variable::CONTEXT: {
       // Context variable (potentially up the context chain).
       int depth = current_scope()->ContextChainLength(variable->scope());
       bool immutable = variable->maybe_assigned() == kNotAssigned;
       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();
         value = BuildHoleCheckSilent(value, undefined, value);
       } else if (mode == LET || mode == CONST) {
         // Perform check for uninitialized let/const variables.
         value = BuildHoleCheckThrow(value, variable, value, bailout_id);
       }
       return value;
     }
@@ skipping 622 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8