NaniteOrderedScatterUpdater.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "CoreMinimal.h"
#include "Containers/HashTable.h"
 
class FRDGBuilder;
class FRDGBufferUAV;
 
namespace Nanite
{
 
enum class EScatterOp
{
    Or = 0,
    And = 1,
    Write = 2,
};
 
// Helper to emulate serial buffer updates on the GPU
class FOrderedScatterUpdater
{
public:
    FOrderedScatterUpdater(uint32 NumExpectedElements);
 
    void Add(EScatterOp Op, uint32 Offset, uint32 Value)
    {
        check(IsAligned(Offset, 4));
 
        Updates.Add(FUpdate(Op, Offset, Value));
    }
 
    // Call this if there can be multiple updates to the same address.
    // When executed on the GPU, these updates can otherwise be unordered.
    // For a given address ResolveOverwrites removes all updates except the last one.
    void ResolveOverwrites(bool bVerify);
 
    void Flush(FRDGBuilder& GraphBuilder, FRDGBufferUAV* UAV);
    
    uint32 Num() const
    {
        return Updates.Num();
    }
 
private:
    class FUpdate
    {
    public:
        FUpdate() :
            Op_Offset(0xFFFFFFFCu), Value(0u)
        {
        }
 
        FUpdate(EScatterOp InOp, uint32 InOffset, uint32 InValue) :
            Op_Offset(InOffset | (uint32)InOp),
            Value(InValue)
        {
        }
 
        EScatterOp  GetOp()     const { return EScatterOp(Op_Offset & 3u); }
        uint32      GetOffset() const { return Op_Offset & ~3u; }
        uint32      GetValue()  const { return Value; }
 
        uint32 WriteMask() const
        {
            switch (GetOp())
            {
            case EScatterOp::Or: return Value;
            case EScatterOp::And: return ~Value;
            case EScatterOp::Write: return 0xFFFFFFFFu;
            default:
                check(false);
                return 0xFFFFFFFFu;
            }
        }
    private:
        uint32 Op_Offset;
        uint32 Value;
    };
 
    FHashTable HashTable;
    TArray<FUpdate> Updates;
    uint32 HashShift;
};
 
} // namespace Nanite