Method Sort

Source/Engine/Renderer/Utils/BitonicSort.cpp:79–145

}

void BitonicSort::Sort(GPUContext* context, GPUBuffer* indicesBuffer, GPUBuffer* keysBuffer, GPUBuffer* countBuffer, uint32 counterOffset, bool sortAscending, int32 maxElements)
{
    ASSERT(context && indicesBuffer && keysBuffer && countBuffer);
    if (checkIfSkipPass())
        return;
    PROFILE_GPU_CPU("Bitonic Sort");
    int32 maxNumElements = (int32)indicesBuffer->GetElementsCount();
    if (maxElements > 0 && maxElements < maxNumElements)
        maxNumElements = maxElements;
    const uint32 alignedMaxNumElements = Math::RoundUpToPowerOf2(maxNumElements);
    const uint32 maxIterations = (uint32)Math::Log2((float)Math::Max(2048u, alignedMaxNumElements)) - 10;

    // Setup constants buffer
    Data data;
    data.CounterOffset = counterOffset;
    data.NullItemKey = sortAscending ? MAX_float : -MAX_float;
    data.NullItemIndex = 0;
    data.KeySign = sortAscending ? -1.0f : 1.0f;
    data.MaxIterations = maxIterations;
    data.LoopK = 0;
    data.LoopJ = 0;
    context->UpdateCB(_cb, &data);
    context->BindCB(0, _cb);
    context->BindSR(0, countBuffer->View());

    // If item count is small we can do only presorting within a single dispatch thread group
    if (maxNumElements <= 2048)
    {
        // Use pre-sort with smaller thread group size (eg. for small particle emitters sorting)
        const int32 permutation = maxNumElements < 128 ? 1 : 0;
        context->BindUA(0, indicesBuffer->View());
        context->BindUA(1, keysBuffer->View());
        context->Dispatch(_preSortCS.Get(permutation), 1, 1, 1);
    }
    else
    {
        // Generate execute indirect arguments
        context->BindUA(0, _dispatchArgsBuffer->View());
        context->Dispatch(_indirectArgsCS, 1, 1, 1);

        // Pre-Sort the buffer up to k = 2048 (this also pads the list with invalid indices that will drift to the end of the sorted list)
        context->BindUA(0, indicesBuffer->View());
        context->BindUA(1, keysBuffer->View());
        context->DispatchIndirect(_preSortCS.Get(0), _dispatchArgsBuffer, 0);

        // We have already pre-sorted up through k = 2048 when first writing our list, so we continue sorting with k = 4096
        // For really large values of k, these indirect dispatches will be skipped over with thread counts of 0
        uint32 indirectArgsOffset = sizeof(GPUDispatchIndirectArgs);
        for (uint32 k = 4096; k <= alignedMaxNumElements; k *= 2)
        {
            for (uint32 j = k / 2; j >= 2048; j /= 2)
            {
                data.LoopK = k;
                data.LoopJ = j;
                context->UpdateCB(_cb, &data);

                context->DispatchIndirect(_outerSortCS, _dispatchArgsBuffer, indirectArgsOffset);
                indirectArgsOffset += sizeof(GPUDispatchIndirectArgs);