[ES6 modules] UninstantiatedInclusiveDescendants: skip recursion for instantiated sub-trees.

third_party/WebKit/Source/core/loader/modulescript/ModuleTreeLinker.cpp

 // Copyright 2017 The Chromium 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 "core/loader/modulescript/ModuleTreeLinker.h"
 
 #include "bindings/core/v8/ScriptModule.h"
 #include "core/dom/AncestorList.h"
 #include "core/dom/ModuleScript.h"
 #include "core/loader/modulescript/ModuleScriptFetchRequest.h"
@@ skipping 433 matching lines @@
 
   // Step 4. While stack is not empty:
   while (!stack.IsEmpty()) {
     // Step 4.1. Let current the result of popping from stack.
     ModuleScript* current = stack.TakeFirst();
 
     // Step 4.2. Assert: current is a module script (i.e., it is not "fetching"
     // or null).
     DCHECK(current);
 
+    // [nospec, optimization]
+    // We can safely skip recursing into the descendants if the module graph
+    // node status is "instantiated". The sub-graph is guaranteed to be all
+    // "instantiated", thus never would end up in the "uninstantiated set".
+    if (current->InstantiationState() ==
+        ModuleInstantiationState::kInstantiated)
+      continue;
+
     // Step 4.3. If inclusive descendants and stack both do not contain current,
     // then:
     if (inclusive_descendants.Contains(current))
       continue;
     if (std::find(stack.begin(), stack.end(), current) != stack.end())
       continue;
 
     // Step 4.3.1. Append current to inclusive descendants.
     inclusive_descendants.insert(current);
 
@@ skipping 48 matching lines @@
   HeapHashSet<Member<ModuleScript>> uninstantiated_set;
   for (const auto& script : inclusive_descendants) {
     if (script->InstantiationState() ==
         ModuleInstantiationState::kUninstantiated)
       uninstantiated_set.insert(script);
   }
   return uninstantiated_set;
 }
 
 }  // namespace blink