[oilpan] Incremental marking: DCHECK for deleted values

Since df7973c4 there is no way to
trigger individual write barriers on deleted values, because:
a. write barriers are emitted for complete backing stores and their
   processing already takes care of deleted values
b. for moving and copying the reinsertion takes care of omitting
   deleted values

Bug: chromium:757440
Change-Id: I43fb8f43936d471e7ad6d1fb7f0da6c8bd626e52
Reviewed-on: https://chromium-review.googlesource.com/992932Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548399}

third_party/WebKit/Source/platform/heap/HeapAllocator.h

@@ -230,6 +230,7 @@ class PLATFORM_EXPORT HeapAllocator {
       DCHECK(!thread_state->Heap().GetStackFrameDepth().IsEnabled());
       // No weak handling for write barriers. Modifying weakly reachable objects
       // strongifies them for the current cycle.
+      DCHECK(!Traits::kCanHaveDeletedValue || !Traits::IsDeletedValue(*object));
       TraceCollectionIfEnabled<
           WTF::kNoWeakHandling, T, Traits>::Trace(thread_state
                                                       ->CurrentVisitor(),
@@ -254,6 +255,8 @@ class PLATFORM_EXPORT HeapAllocator {
       // No weak handling for write barriers. Modifying weakly reachable objects
       // strongifies them for the current cycle.
       while (len-- > 0) {
+        DCHECK(!Traits::kCanHaveDeletedValue ||
+               !Traits::IsDeletedValue(*array));
         TraceCollectionIfEnabled<
             WTF::kNoWeakHandling, T, Traits>::Trace(thread_state
                                                         ->CurrentVisitor(),

third_party/WebKit/Source/platform/heap/IncrementalMarkingTest.cpp

@@ -236,8 +236,6 @@ class Object : public GarbageCollected<Object> {
 // =============================================================================
 
 TEST(IncrementalMarkingTest, EnableDisableBarrier) {
-  Object* object = Object::Create();
-  BasePage* page = PageFromObject(object);
   EXPECT_FALSE(ThreadState::Current()->IsIncrementalMarking());
   ThreadState::Current()->EnableIncrementalMarkingBarrier();
   EXPECT_TRUE(ThreadState::Current()->IsIncrementalMarking());

third_party/WebKit/Source/platform/heap/Visitor.h

@@ -81,6 +81,7 @@ class PLATFORM_EXPORT Visitor {
   // Member version of the one-argument templated trace method.
   template <typename T>
   void Trace(const Member<T>& t) {
+    DCHECK(!t.IsHashTableDeletedValue());
     Trace(t.Get());
   }
 
@@ -155,6 +156,8 @@ class PLATFORM_EXPORT Visitor {
 
     if (!t.Get())
       return;
+
+    DCHECK(!t.IsHashTableDeletedValue());
     VisitWeak(const_cast<void*>(reinterpret_cast<const void*>(t.Get())),
               reinterpret_cast<void**>(
                   const_cast<typename std::remove_const<T>::type**>(t.Cell())),

third_party/WebKit/Source/platform/wtf/HashTraits.h

@@ -85,6 +85,8 @@ struct GenericHashTraitsBase<false, T> {
 
   static const WeakHandlingFlag kWeakHandlingFlag =
       IsWeak<T>::value ? kWeakHandling : kNoWeakHandling;
+
+  static constexpr bool kCanHaveDeletedValue = true;
 };
 
 // Default integer traits disallow both 0 and -1 as keys (max value instead of

third_party/WebKit/Source/platform/wtf/VectorTraits.h

@@ -64,6 +64,10 @@ struct VectorTraitsBase {
   };
   // We don't support weak handling in vectors.
   static const WeakHandlingFlag kWeakHandlingFlag = kNoWeakHandling;
+
+  // Vectors do not support deleting values.
+  static constexpr bool kCanHaveDeletedValue = false;
+  static bool IsDeletedValue(T value) { return false; }
 };
 
 template <typename T>
@@ -142,6 +146,10 @@ struct VectorTraits<std::pair<First, Second>> {
   };
   // We don't support weak handling in vectors.
   static const WeakHandlingFlag kWeakHandlingFlag = kNoWeakHandling;
+
+  // Vectors do not support deleting values.
+  static constexpr bool kCanHaveDeletedValue = false;
+  static bool IsDeletedValue(std::pair<First, Second> value) { return false; }
 };
 
 }  // namespace WTF