src/gromacs/utility/allocator.h

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  35 /*! \file
  36  * \brief
  37  * Declares gmx::Allocator template whose allocation functionality is
  38  * configured both by type of object allocated and a policy class that
  39  * configures the necessary matching malloc and free operation.
  40  *
  41  * \author Erik Lindahl <erik.lindahl@gmail.com>
  42  * \author Mark Abraham <mark.j.abraham@gmail.com>
  43  * \inpublicapi
  44  * \ingroup module_utility
  45  */
  46 #ifndef GMX_UTILITY_ALLOCATOR_H
  47 #define GMX_UTILITY_ALLOCATOR_H
  48
  49 #include <cstddef>
  50
  51 #include <memory>
  52 #include <new>
  53
  54 #include "gromacs/utility/basedefinitions.h"
  55
  56 namespace gmx
  57 {
  58
  59 /*! \libinternal \brief Policy-based memory allocator.
  60  *
  61  *  \tparam T                 Type of objects to allocate
  62  *  \tparam AllocationPolicy  Policy of (matching) allocation and deallocation functions.
  63  *
  64  * This class can be used for the optional allocator template
  65  * parameter in standard library containers. It must be configured
  66  * with both the type of object to allocate, and an AllocationPolicy
  67  * which effectively wraps a matching pair of malloc and free
  68  * functions. This permits implementing a family of related allocators
  69  * e.g. with SIMD alignment, GPU host-side page locking, or perhaps
  70  * both, in a way that preserves a common programming interface and
  71  * duplicates minimal code.
  72  *
  73  * AllocationPolicy is used as a base class, so that if
  74  * AllocationPolicy is stateless, then the empty base optimization
  75  * will ensure that Allocation is also stateless, and objects made
  76  * with the Allocator will incur no size penalty. (Embedding an
  77  * AllocationPolicy object incurs a size penalty always, even if the
  78  * object is empty.) Normally a stateless allocator will be used.
  79  *
  80  * However, an AllocationPolicy with state might be desirable for
  81  * simplifying writing code that needs to allocate suitably for a
  82  * transfer to a GPU. That code needs to specify an Allocator that can
  83  * do the right job, which can be stateless. However, if we have code
  84  * that will not know until run time whether a GPU transfer will
  85  * occur, then the allocator needs to be aware of the state.  That
  86  * will increase the size of a container that uses the stateful
  87  * allocator.
  88  *
  89  * \throws std::bad_alloc Instead of a GROMACS exception object, we
  90  * throw the standard one on allocation failures to make it as
  91  * compatible as possible with the errors expected by code using the
  92  * standard library containers.
  93  *
  94  * \inlibraryapi
  95  * \ingroup module_utility
  96  */
  97 template<class T, typename AllocationPolicy>
  98 class Allocator : public AllocationPolicy
  99 {
 100 public:
 101     // The standard library specification for a custom allocator
 102     // requires this typedef, with this capitalization/underscoring.
 103     typedef T value_type; //!< Type of allocated elements
 104
 105     /*! \brief Constructor
 106      *
 107      * No constructor can be auto-generated in the presence of any
 108      * user-defined constructor, but we want the default constructor.
 109      */
 110     Allocator() = default;
 111
 112     /*! \brief Constructor to accept an AllocationPolicy.
 113      *
 114      * This is useful for AllocationPolicies with state.
 115      */
 116     Allocator(const AllocationPolicy& p) : AllocationPolicy(p) {}
 117
 118     /*! \brief Do the actual memory allocation
 119      *
 120      *  \param n    Number of elements of type T to allocate. n can be
 121      *              0 bytes, which will return a non-null properly aligned
 122      *              and padded pointer that should not be used.
 123      *  \param hint Optional value returned from previous call to allocate.
 124      *              For now this is not used.
 125      *  \return Pointer to allocated memory
 126      *
 127      *  \throws std::bad_alloc if the allocation fails.
 128      */
 129     value_type* allocate(std::size_t n, typename std::allocator<void>::const_pointer gmx_unused hint = nullptr)
 130     {
 131         void* p = AllocationPolicy::malloc(n * sizeof(T));
 132
 133         if (p == nullptr)
 134         {
 135             throw std::bad_alloc();
 136         }
 137         else
 138         {
 139             return static_cast<value_type*>(p);
 140         }
 141     }
 142
 143     /*! \brief Release memory
 144      *
 145      * \param p  Pointer to previously allocated memory returned from allocate()
 146      * \param n  number of objects previously passed to allocate()
 147      */
 148     void deallocate(value_type* p, std::size_t gmx_unused n) { AllocationPolicy::free(p); }
 149
 150     /*! \brief Return true if two allocators are identical
 151      *
 152      * This is a member function of the left-hand-side allocator.
 153      * Always true for stateless polcies. Has to be defined in the policy for stateful policies.
 154      * FUTURE: Can be removed with C++17 (is_always_equal)
 155      *
 156      * \todo Use std::is_empty_v when CUDA 11 is a requirement.
 157      */
 158     template<class T2, class A = AllocationPolicy, typename = std::enable_if_t<std::is_empty<A>::value>>
 159     bool operator==(const Allocator<T2, AllocationPolicy>& /*unused*/) const
 160     {
 161         return true;
 162     }
 163
 164     /*! \brief Return true if two allocators are different
 165      *
 166      * \param rhs Other allocator.
 167      *
 168      * This is a member function of the left-hand-side allocator.
 169      */
 170     template<class T2>
 171     bool operator!=(const Allocator<T2, AllocationPolicy>& rhs) const
 172     {
 173         return !(*this == rhs);
 174     }
 175 };
 176
 177 } // namespace gmx
 178
 179 #endif