Introduce (Heap)Vector trait covering zero'ed memory for unused slots.

Source/wtf/VectorTraits.h

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 18 matching lines @@
 using std::pair;
 
 namespace WTF {
 
     class AtomicString;
 
     template<typename T>
     struct VectorTraitsBase
     {
         static const bool needsDestruction = !IsTriviallyDestructible<T>::value;
+
         static const bool canInitializeWithMemset = IsTriviallyDefaultConstructible<T>::value;
+        // true iff memset(slot, 0, size) constructs an unused slot value that is valid for
+        // Oilpan to trace and if the value needs destruction, its destructor can be invoked
+        // over. The zero'ed value representing an unused slot in the vector's backing storage;
+        // it does not have to be equal to what its constructor(s) would create, only be
+        // valid for those two uses.
+        static const bool canClearUnusedSlotsWithMemset = IsTriviallyDefaultConstructible<T>::value;
+
         static const bool canMoveWithMemcpy = IsTriviallyMoveAssignable<T>::value;
         static const bool canCopyWithMemcpy = IsTriviallyCopyAssignable<T>::value;
         static const bool canFillWithMemset = IsTriviallyDefaultConstructible<T>::value && (sizeof(T) == sizeof(char));
         static const bool canCompareWithMemcmp = IsScalar<T>::value; // Types without padding.
         template<typename U = void>
         struct NeedsTracingLazily {
             static const bool value = NeedsTracing<T>::value;
         };
         static const WeakHandlingFlag weakHandlingFlag = NoWeakHandlingInCollections; // We don't support weak handling in vectors.
     };
 
     template<typename T>
     struct VectorTraits : VectorTraitsBase<T> { };
 
     // Classes marked with SimpleVectorTraits will use memmov, memcpy, memcmp
     // instead of constructors, copy operators, etc for initialization, move
     // and comparison.
     template<typename T>
     struct SimpleClassVectorTraits : VectorTraitsBase<T>
     {
         static const bool canInitializeWithMemset = true;
+        static const bool canClearUnusedSlotsWithMemset = true;
         static const bool canMoveWithMemcpy = true;
         static const bool canCompareWithMemcmp = true;
     };
 
     // We know OwnPtr and RefPtr are simple enough that initializing to 0 and
     // moving with memcpy (and then not destructing the original) will totally
     // work.
     template<typename P>
     struct VectorTraits<RefPtr<P>> : SimpleClassVectorTraits<RefPtr<P>> { };
 
@@ skipping 16 matching lines @@
     {
         typedef VectorTraits<First> FirstTraits;
         typedef VectorTraits<Second> SecondTraits;
 
         static const bool needsDestruction = FirstTraits::needsDestruction || SecondTraits::needsDestruction;
         static const bool canInitializeWithMemset = FirstTraits::canInitializeWithMemset && SecondTraits::canInitializeWithMemset;
         static const bool canMoveWithMemcpy = FirstTraits::canMoveWithMemcpy && SecondTraits::canMoveWithMemcpy;
         static const bool canCopyWithMemcpy = FirstTraits::canCopyWithMemcpy && SecondTraits::canCopyWithMemcpy;
         static const bool canFillWithMemset = false;
         static const bool canCompareWithMemcmp = FirstTraits::canCompareWithMemcmp && SecondTraits::canCompareWithMemcmp;
+        static const bool canClearUnusedSlotsWithMemset = FirstTraits::canClearUnusedSlotsWithMemset && SecondTraits::canClearUnusedSlotsWithMemset;
         template <typename U = void>
         struct NeedsTracingLazily {
             static const bool value = ShouldBeTraced<FirstTraits>::value || ShouldBeTraced<SecondTraits>::value;
         };
         static const WeakHandlingFlag weakHandlingFlag = NoWeakHandlingInCollections; // We don't support weak handling in vectors.
     };
 
 } // namespace WTF
 
 #define WTF_ALLOW_MOVE_INIT_AND_COMPARE_WITH_MEM_FUNCTIONS(ClassName) \
 namespace WTF { \
 static_assert(!IsTriviallyDefaultConstructible<ClassName>::value || !IsTriviallyMoveAssignable<ClassName>::value || !IsScalar<ClassName>::value, "macro not needed"); \
     template<> \
     struct VectorTraits<ClassName> : SimpleClassVectorTraits<ClassName> { }; \
 }
 
 #define WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS(ClassName) \
 namespace WTF { \
 static_assert(!IsTriviallyDefaultConstructible<ClassName>::value || !IsTriviallyMoveAssignable<ClassName>::value, "macro not needed"); \
     template<> \
     struct VectorTraits<ClassName> : VectorTraitsBase<ClassName> \
     { \
         static const bool canInitializeWithMemset = true; \
+        static const bool canClearUnusedSlotsWithMemset = true; \
         static const bool canMoveWithMemcpy = true; \
     }; \
 }
 
 #define WTF_ALLOW_INIT_WITH_MEM_FUNCTIONS(ClassName) \
 namespace WTF { \
 static_assert(!IsTriviallyDefaultConstructible<ClassName>::value, "macro not needed"); \
     template<> \
     struct VectorTraits<ClassName> : VectorTraitsBase<ClassName> \
     { \
         static const bool canInitializeWithMemset = true; \
+        static const bool canClearUnusedSlotsWithMemset = true; \
     }; \
 }
 
+#define WTF_ALLOW_CLEAR_UNUSED_SLOTS_WITH_MEM_FUNCTIONS(ClassName) \
+namespace WTF { \
+static_assert(!IsTriviallyDefaultConstructible<ClassName>::value, "macro not needed"); \
+    template<> \
+    struct VectorTraits<ClassName> : VectorTraitsBase<ClassName> \
+    { \
+        static const bool canClearUnusedSlotsWithMemset = true; \
+    }; \
+}
+
 using WTF::VectorTraits;
 using WTF::SimpleClassVectorTraits;
 
 #endif // WTF_VectorTraits_h