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DirectML helper functions

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DMLCalcBufferTensorSize

This helper function calculates the minimum number of bytes required to store a buffer tensor with the specified type, sizes, and strides. The formula can be expressed as the following.

IndexOfLastElement = dot(Sizes - 1, Strides);
MinimumImpliedSizeInBytes = roundup((IndexOfLastElement + 1) * ElementSizeInBytes, 4)

In other words, the minimum size of a tensor is the index of the one-past-the-end element, multiplied by the element size (for example, 2 bytes for a FLOAT16 tensor). Additionally, DirectML requires that all bound buffers must have a total size that is DWORD-aligned, and hence the minimum implied size in bytes must be rounded up to the nearest 4-byte boundary.

inline UINT64 DMLCalcBufferTensorSize(
    DML_TENSOR_DATA_TYPE dataType,
    UINT dimensionCount,
    _In_reads_(dimensionCount) const UINT* sizes,
    _In_reads_opt_(dimensionCount) const UINT* strides)
{
    UINT elementSizeInBytes = 0;
    switch (dataType)
    {
    case DML_TENSOR_DATA_TYPE_FLOAT32:
    case DML_TENSOR_DATA_TYPE_UINT32:
    case DML_TENSOR_DATA_TYPE_INT32:
        elementSizeInBytes = 4;
        break;

    case DML_TENSOR_DATA_TYPE_FLOAT16:
    case DML_TENSOR_DATA_TYPE_UINT16:
    case DML_TENSOR_DATA_TYPE_INT16:
        elementSizeInBytes = 2;
        break;

    case DML_TENSOR_DATA_TYPE_UINT8:
    case DML_TENSOR_DATA_TYPE_INT8:
        elementSizeInBytes = 1;
        break;

    case DML_TENSOR_DATA_TYPE_FLOAT64:
    case DML_TENSOR_DATA_TYPE_UINT64:
    case DML_TENSOR_DATA_TYPE_INT64:
        elementSizeInBytes = 8;
        break;

    default:
        return 0; // Invalid data type
    }

    UINT64 minimumImpliedSizeInBytes = 0;
    if (!strides)
    {
        minimumImpliedSizeInBytes = sizes[0];
        for (UINT i = 1; i < dimensionCount; ++i)
        {
            minimumImpliedSizeInBytes *= sizes[i];
        }
        minimumImpliedSizeInBytes *= elementSizeInBytes;
    }
    else
    {
        UINT indexOfLastElement = 0;
        for (UINT i = 0; i < dimensionCount; ++i)
        {
            indexOfLastElement += (sizes[i] - 1) * strides[i];
        }

        minimumImpliedSizeInBytes = (static_cast<UINT64>(indexOfLastElement) + 1) * elementSizeInBytes;
    }

    // Round up to the nearest 4 bytes.
    minimumImpliedSizeInBytes = (minimumImpliedSizeInBytes + 3) & ~3ull;

    return minimumImpliedSizeInBytes;
}

CalculateStrides

This helper function calculates strides for 4D tensors with either NCHW or NHWC layout, and optional broadcasting.

enum class Layout
{
    NCHW,
    NHWC
};

// Given dimension sizes (in NCHW order), calculates the strides to achieve a desired layout.
std::array<uint32_t, 4> CalculateStrides(
        Layout layout, 
        std::array<uint32_t, 4> sizes, 
        std::array<bool, 4> broadcast)
{
    enum DML_ORDER { N, C, H, W };

    uint32_t n = broadcast[N] ? 1 : sizes[N];
    uint32_t c = broadcast[C] ? 1 : sizes[C];
    uint32_t h = broadcast[H] ? 1 : sizes[H];
    uint32_t w = broadcast[W] ? 1 : sizes[W];

    uint32_t nStride = 0, cStride = 0, hStride = 0, wStride = 0;

    switch (layout)
    {
    case Layout::NCHW:
        nStride = broadcast[N] ? 0 : c * h * w;
        cStride = broadcast[C] ? 0 : h * w;
        hStride = broadcast[H] ? 0 : w;
        wStride = broadcast[W] ? 0 : 1;
        break;

    case Layout::NHWC:
        nStride = broadcast[N] ? 0 : h * w * c;
        hStride = broadcast[H] ? 0 : w * c;
        wStride = broadcast[W] ? 0 : c;
        cStride = broadcast[C] ? 0 : 1;
        break;
    }

    return { nStride, cStride, hStride, wStride };
}

See also