dtensor.hpp

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// Copyright (c) 2010-2020, Lawrence Livermore National Security, LLC. Produced
// at the Lawrence Livermore National Laboratory. All Rights reserved. See files
// LICENSE and NOTICE for details. LLNL-CODE-806117.
//
// This file is part of the MFEM library. For more information and source code
// availability visit https://mfem.org.
//
// MFEM is free software; you can redistribute it and/or modify it under the
// terms of the BSD-3 license. We welcome feedback and contributions, see file
// CONTRIBUTING.md for details.

#ifndef MFEM_DTENSOR
#define MFEM_DTENSOR

#include "../general/backends.hpp"

namespace mfem
{

/// A Class to compute the real index from the multi-indices of a tensor
template <int N, int Dim, typename T, typename... Args>
class TensorInd
{
public:
   MFEM_HOST_DEVICE
   static inline int result(const int* sizes, T first, Args... args)
   {
#if !(defined(MFEM_USE_CUDA) || defined(MFEM_USE_HIP))
      MFEM_ASSERT(first<sizes[N-1],"Trying to access out of boundary.");
#endif
      return first + sizes[N - 1] * TensorInd < N + 1, Dim, Args... >
             ::result(sizes, args...);
   }
};

// Terminal case
template <int Dim, typename T, typename... Args>
class TensorInd<Dim, Dim, T, Args...>
{
public:
   MFEM_HOST_DEVICE
   static inline int result(const int* sizes, T first, Args... args)
   {
#if !(defined(MFEM_USE_CUDA) || defined(MFEM_USE_HIP))
      MFEM_ASSERT(first<sizes[Dim-1],"Trying to access out of boundary.");
#endif
      return first;
   }
};


/// A class to initialize the size of a Tensor
template <int N, int Dim, typename T, typename... Args>
class Init
{
public:
   static inline int result(int* sizes, T first, Args... args)
   {
      sizes[N - 1] = first;
      return first * Init < N + 1, Dim, Args... >::result(sizes, args...);
   }
};

// Terminal case
template <int Dim, typename T, typename... Args>
class Init<Dim, Dim, T, Args...>
{
public:
   static inline int result(int* sizes, T first, Args... args)
   {
      sizes[Dim - 1] = first;
      return first;
   }
};


/// A basic generic Tensor class, appropriate for use on the GPU
template<int Dim, typename Scalar = double>
class DeviceTensor
{
protected:
   int capacity;
   Scalar *data;
   int sizes[Dim];

public:
   /// Default constructor
   DeviceTensor() = delete;

   /// Constructor to initialize a tensor from the Scalar array _data
   template <typename... Args>
   DeviceTensor(Scalar* _data, Args... args)
   {
      static_assert(sizeof...(args) == Dim, "Wrong number of arguments");
      // Initialize sizes, and compute the number of values
      const long int nb = Init<1, Dim, Args...>::result(sizes, args...);
      capacity = nb;
      data = (capacity > 0) ? _data : NULL;
   }

   /// Copy constructor
   MFEM_HOST_DEVICE DeviceTensor(const DeviceTensor& t)
   {
      capacity = t.capacity;
      for (int i = 0; i < Dim; ++i)
      {
         sizes[i] = t.sizes[i];
      }
      data = t.data;
   }

   /// Conversion to `Scalar *`.
   inline operator Scalar *() const { return data; }

   /// Const accessor for the data
   template <typename... Args> MFEM_HOST_DEVICE inline
   Scalar& operator()(Args... args) const
   {
      static_assert(sizeof...(args) == Dim, "Wrong number of arguments");
      return data[ TensorInd<1, Dim, Args...>::result(sizes, args...) ];
   }

   /// Subscript operator where the tensor is viewed as a 1D array.
   MFEM_HOST_DEVICE inline Scalar& operator[](int i) const
   {
      return data[i];
   }
};


/** @brief Wrap a pointer as a DeviceTensor with automatically deduced template
    parameters */
template <typename T, typename... Dims>
inline DeviceTensor<sizeof...(Dims),T> Reshape(T *ptr, Dims... dims)
{
   return DeviceTensor<sizeof...(Dims),T>(ptr, dims...);
}


typedef DeviceTensor<1,int> DeviceArray;
typedef DeviceTensor<1,double> DeviceVector;
typedef DeviceTensor<2,double> DeviceMatrix;

} // mfem namespace

#endif // MFEM_DTENSOR