memory.h

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 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
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 * Unless required by applicable law or agreed to in writing,
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// Acknowledgement: This file originates from incubator-tvm
#ifndef MXNET_RUNTIME_MEMORY_H_
#define MXNET_RUNTIME_MEMORY_H_

#include <cstdlib>
#include <utility>
#include <type_traits>
#include "object.h"

namespace mxnet {
namespace runtime {
template<typename T, typename... Args>
inline ObjectPtr<T> make_object(Args&&... args);

// Detail implementations after this
//
// The current design allows swapping the
// allocator pattern when necessary.
//
// Possible future allocator optimizations:
// - Arena allocator that gives ownership of memory to arena (deleter_= nullptr)
// - Thread-local object pools: one pool per size and alignment requirement.
// - Can specialize by type of object to give the specific allocator to each object.

template<typename Derived>
class ObjAllocatorBase {
 public:
  template<typename T, typename... Args>
  inline ObjectPtr<T> make_object(Args&&... args) {
    using Handler = typename Derived::template Handler<T>;
    static_assert(std::is_base_of<Object, T>::value,
                  "make can only be used to create Object");
    T* ptr = Handler::New(static_cast<Derived*>(this),
                         std::forward<Args>(args)...);
    ptr->type_index_ = T::RuntimeTypeIndex();
    ptr->deleter_ = Handler::Deleter();
    return ObjectPtr<T>(ptr);
  }

  template<typename ArrayType, typename ElemType, typename... Args>
  inline ObjectPtr<ArrayType> make_inplace_array(size_t num_elems, Args&&... args) {
    using Handler = typename Derived::template ArrayHandler<ArrayType, ElemType>;
    static_assert(std::is_base_of<Object, ArrayType>::value,
                  "make_inplace_array can only be used to create Object");
    ArrayType* ptr = Handler::New(static_cast<Derived*>(this),
                                  num_elems,
                                  std::forward<Args>(args)...);
    ptr->type_index_ = ArrayType::RuntimeTypeIndex();
    ptr->deleter_ = Handler::Deleter();
    return ObjectPtr<ArrayType>(ptr);
  }
};

// Simple allocator that uses new/delete.
class SimpleObjAllocator :
      public ObjAllocatorBase<SimpleObjAllocator> {
 public:
  template<typename T>
  class Handler {
   public:
    using StorageType = typename std::aligned_storage<sizeof(T), alignof(T)>::type;

    template<typename... Args>
    static T* New(SimpleObjAllocator*, Args&&... args) {
      // NOTE: the first argument is not needed for SimpleObjAllocator
      // It is reserved for special allocators that needs to recycle
      // the object to itself (e.g. in the case of object pool).
      //
      // In the case of an object pool, an allocator needs to create
      // a special chunk memory that hides reference to the allocator
      // and call allocator's release function in the deleter.

      // NOTE2: Use inplace new to allocate
      // This is used to get rid of warning when deleting a virtual
      // class with non-virtual destructor.
      // We are fine here as we captured the right deleter during construction.
      // This is also the right way to get storage type for an object pool.
      StorageType* data = new StorageType();
      new (data) T(std::forward<Args>(args)...);
      return reinterpret_cast<T*>(data);
    }

    static Object::FDeleter Deleter() {
      return Deleter_;
    }

   private:
    static void Deleter_(Object* objptr) {
      // NOTE: this is important to cast back to T*
      // because objptr and tptr may not be the same
      // depending on how sub-class allocates the space.
      T* tptr = static_cast<T*>(objptr);
      // It is important to do tptr->T::~T(),
      // so that we explicitly call the specific destructor
      // instead of tptr->~T(), which could mean the intention
      // call a virtual destructor(which may not be available and is not required).
      tptr->T::~T();
      delete reinterpret_cast<StorageType*>(tptr);
    }
  };

  // Array handler that uses new/delete.
  template<typename ArrayType, typename ElemType>
  class ArrayHandler {
   public:
    using StorageType = typename std::aligned_storage<sizeof(ArrayType), alignof(ArrayType)>::type;
    // for now only support elements that aligns with array header.
    static_assert(alignof(ArrayType) % alignof(ElemType) == 0 &&
                  sizeof(ArrayType) % alignof(ElemType) == 0,
                  "element alignment constraint");

    template<typename... Args>
    static ArrayType* New(SimpleObjAllocator*, size_t num_elems, Args&&... args) {
      // NOTE: the first argument is not needed for ArrayObjAllocator
      // It is reserved for special allocators that needs to recycle
      // the object to itself (e.g. in the case of object pool).
      //
      // In the case of an object pool, an allocator needs to create
      // a special chunk memory that hides reference to the allocator
      // and call allocator's release function in the deleter.
      // NOTE2: Use inplace new to allocate
      // This is used to get rid of warning when deleting a virtual
      // class with non-virtual destructor.
      // We are fine here as we captured the right deleter during construction.
      // This is also the right way to get storage type for an object pool.
      size_t unit = sizeof(StorageType);
      size_t requested_size = num_elems * sizeof(ElemType) + sizeof(ArrayType);
      size_t num_storage_slots = (requested_size + unit - 1) / unit;
      StorageType* data = new StorageType[num_storage_slots];
      new (data) ArrayType(std::forward<Args>(args)...);
      return reinterpret_cast<ArrayType*>(data);
    }

    static Object::FDeleter Deleter() {
      return Deleter_;
    }

   private:
    static void Deleter_(Object* objptr) {
      // NOTE: this is important to cast back to ArrayType*
      // because objptr and tptr may not be the same
      // depending on how sub-class allocates the space.
      ArrayType* tptr = static_cast<ArrayType*>(objptr);
      // It is important to do tptr->ArrayType::~ArrayType(),
      // so that we explicitly call the specific destructor
      // instead of tptr->~ArrayType(), which could mean the intention
      // call a virtual destructor(which may not be available and is not required).
      tptr->ArrayType::~ArrayType();
      StorageType* p = reinterpret_cast<StorageType*>(tptr);
      delete []p;
    }
  };
};

template<typename T, typename... Args>
inline ObjectPtr<T> make_object(Args&&... args) {
  return SimpleObjAllocator().make_object<T>(std::forward<Args>(args)...);
}

template<typename ArrayType, typename ElemType, typename... Args>
inline ObjectPtr<ArrayType> make_inplace_array_object(size_t num_elems, Args&&... args) {
  return SimpleObjAllocator().make_inplace_array<ArrayType, ElemType>(
    num_elems, std::forward<Args>(args)...);
}

}  // namespace runtime
}  // namespace mxnet
#endif  // MXNET_RUNTIME_MEMORY_H_