VVMPlaceholder.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#if WITH_VERSE_VM || defined(__INTELLISENSE__)
 
#include "VVMSuspension.h"
 
namespace Verse
{
struct VPlaceholder : public VCell
{
    DECLARE_DERIVED_VCPPCLASSINFO(COREUOBJECT_API, VCell);
    COREUOBJECT_API static TGlobalTrivialEmergentTypePtr<&StaticCppClassInfo> GlobalTrivialEmergentType;
 
    enum class EMode : uint8
    {
        HasSuspension,
        HasValue,
        HasParent,
    };
 
    TWriteBarrier<VValue> Value;
 
    static VPlaceholder& New(FAllocationContext Context, uint16 SplitDepth)
    {
        return *new (Context.AllocateFastCell(sizeof(VPlaceholder))) VPlaceholder(Context, SplitDepth);
    }
 
    uint16 SplitDepth() const { return Misc2And3; }
    bool HasSuspension() const { return Mode() == EMode::HasSuspension; }
    bool HasValue() const { return Mode() == EMode::HasValue; }
    bool HasParent() const { return Mode() == EMode::HasParent; }
 
    VSuspension* GetSuspension() const
    {
        checkSlow(HasSuspension());
        if (Value.Get().IsUninitialized())
        {
            return nullptr;
        }
        else
        {
            return &Value.Get().AsCell().StaticCast<VSuspension>();
        }
    }
 
    VValue GetValue() const
    {
        checkSlow(HasValue());
        return Value.Get();
    }
 
    VPlaceholder* GetParent() const
    {
        checkSlow(HasParent());
        return &Value.Get().AsPlaceholder();
    }
 
    void Unify(FAllocationContext Context, VPlaceholder& Other)
    {
        checkSlow(HasSuspension() && Other.HasSuspension());
        if (this == &Other)
        {
            return;
        }
        if (!GetSuspension())
        {
            SetParentTrailed(Context, Other);
            return;
        }
        if (!Other.GetSuspension())
        {
            Other.SetParentTrailed(Context, *this);
            return;
        }
        // No need to trail the copying of suspensions.  Any unification that
        // needs to be undone will also result in all related suspensions
        // having `bFailed` failure contexts.
        GetSuspension()->Last().Next.Set(Context, Other.GetSuspension());
        Other.SetParentTrailed(Context, *this);
    }
 
    VSuspension* SetValue(FAccessContext Context, VValue InValue)
    {
        checkSlow(!InValue.IsPlaceholder());
        VSuspension* Result = GetSuspension();
        Mode() = EMode::HasValue;
        Value.Set(Context, InValue);
        return Result;
    }
 
    VSuspension* SetValueTrailed(FAllocationContext Context, VValue InValue)
    {
        checkSlow(!InValue.IsPlaceholder());
        VSuspension* Result = GetSuspension();
        SetModeTrailed(Context, EMode::HasValue);
        Value.SetTrailed(Context, InValue);
        return Result;
    }
 
    COREUOBJECT_API VValue Follow();
 
    // TODO: For perf, think through if FIFO or LIFO will be faster.
    void EnqueueSuspension(FAccessContext Context, VSuspension& Suspension)
    {
        VPlaceholder& Root = Follow().GetRootPlaceholder();
        if (VSuspension* PreviousSuspension = Root.GetSuspension())
        {
            Suspension.Next.Set(Context, PreviousSuspension);
        }
        Root.Value.Set(Context, Suspension);
    }
 
private:
    void SetParent(FAccessContext Context, VPlaceholder& Other)
    {
        Mode() = EMode::HasParent;
        Value.Set(Context, VValue::Placeholder(Other));
    }
 
    void SetParentTrailed(FAllocationContext Context, VPlaceholder& Other)
    {
        SetModeTrailed(Context, EMode::HasParent);
        Value.SetTrailed(Context, VValue::Placeholder(Other));
    }
 
    EMode& Mode() const { return *BitCast<EMode*>(&Misc3); }
 
    void SetModeTrailed(FAllocationContext Context, EMode InMode);
 
    VPlaceholder(FAllocationContext Context, uint16 InputSplitDepth)
        : VCell(Context, &GlobalTrivialEmergentType.Get(Context))
    {
        checkSlow(SplitDepth() == InputSplitDepth);
        Misc2And3 = InputSplitDepth;
        Mode() = EMode::HasSuspension;
        checkSlow(!GetSuspension());
    }
};
} // namespace Verse
#endif // WITH_VERSE_VM