VVMWeakKeyMap.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#if WITH_VERSE_VM || defined(__INTELLISENSE__)
 
#include "Containers/Map.h"
 
namespace Verse
{
 
struct FMarkStack;
struct VCell;
 
// This is a collection of "transposed" weak maps.
//
// A transposed weak map maps maps to values. I.e. while a weak map is a Key -> Value map, a transposed weak map
// is a Map -> Value map.
//
// A weak key map is a collection of transposed weak maps keyed by key. I.e. a weak key map is a Key -> Map -> Value
// map.
//
// For every weak map entry in the world of the form Key -> Value where Map is the owning map (the one that would do
// the logic in VisitReferences that says "if Key is marked then mark Value"), there must be a weak key map entry
// of the form Key -> Map -> Value. This ensures that if Key is not yet marked before the Map runs its
// VisitReferences, then the weak key map that owns the Key will do the marking by running "if Map is marked then
// mark Value" logic.
//
// You could imagine there being one FWeakKeyMap globally for the whole GC. That would work. But we hang FWeakKeyMap
// off libpas's verse_heap_page_header (aka Verse::FHeapPageHeader). There is one of those per "page" for objects
// in segregated storage, and one globally for all large objects. Libpas manages the synchronization that protects
// these maps.
//
// Most of this is "owned" by VCell. VCell accesses this via FWeakKeyMapGuard.
struct FWeakKeyMap final
{
    FWeakKeyMap();
    ~FWeakKeyMap();
 
    bool IsEmpty() const
    {
        return InternalMap.IsEmpty();
    }
 
    // This is just to support testing.
    bool HasEntriesForKey(VCell* Key) const;
 
    void Add(VCell* Key, VCell* Map, VCell* Value);
    void Remove(VCell* Key, VCell* Map);
 
    template <typename TVisitor>
    void Visit(VCell* Key, TVisitor& Visitor);
 
    void ConductCensus();
 
    size_t GetAllocatedSize() const
    {
        return InternalMap.GetAllocatedSize();
    }
 
private:
    TMap<VCell*, TMap<VCell*, VCell*>> InternalMap;
};
 
} // namespace Verse
#endif // WITH_VERSE_VM