Making std::vector allocate aligned memory

Question

Is it possible to make std::vector of custom structs allocate aligned memory for further processing with SIMD instructions? If it is possible to do with Allocator, does anyone happen to have such an allocator he could share?

Answer 1

Edit: I removed the inheritance of std::allocator as suggested by GManNickG and made the alignment parameter a compile time thing.

I recently wrote this piece of code. It's not tested as much as I would like it so go on and report errors. :-)

enum class Alignment : size_t {     Normal = sizeof(void*),     SSE    = 16,     AVX    = 32, };   namespace detail {     void* allocate_aligned_memory(size_t align, size_t size);     void deallocate_aligned_memory(void* ptr) noexcept; }   template <typename T, Alignment Align = Alignment::AVX> class AlignedAllocator;   template <Alignment Align> class AlignedAllocator<void, Align> { public:     typedef void*             pointer;     typedef const void*       const_pointer;     typedef void              value_type;      template <class U> struct rebind { typedef AlignedAllocator<U, Align> other; }; };   template <typename T, Alignment Align> class AlignedAllocator { public:     typedef T         value_type;     typedef T*        pointer;     typedef const T*  const_pointer;     typedef T&        reference;     typedef const T&  const_reference;     typedef size_t    size_type;     typedef ptrdiff_t difference_type;      typedef std::true_type propagate_on_container_move_assignment;      template <class U>     struct rebind { typedef AlignedAllocator<U, Align> other; };  public:     AlignedAllocator() noexcept     {}      template <class U>     AlignedAllocator(const AlignedAllocator<U, Align>&) noexcept     {}      size_type     max_size() const noexcept     { return (size_type(~0) - size_type(Align)) / sizeof(T); }      pointer     address(reference x) const noexcept     { return std::addressof(x); }      const_pointer     address(const_reference x) const noexcept     { return std::addressof(x); }      pointer     allocate(size_type n, typename AlignedAllocator<void, Align>::const_pointer = 0)     {         const size_type alignment = static_cast<size_type>( Align );         void* ptr = detail::allocate_aligned_memory(alignment , n * sizeof(T));         if (ptr == nullptr) {             throw std::bad_alloc();         }          return reinterpret_cast<pointer>(ptr);     }      void     deallocate(pointer p, size_type) noexcept     { return detail::deallocate_aligned_memory(p); }      template <class U, class ...Args>     void     construct(U* p, Args&&... args)     { ::new(reinterpret_cast<void*>(p)) U(std::forward<Args>(args)...); }      void     destroy(pointer p)     { p->~T(); } };   template <typename T, Alignment Align> class AlignedAllocator<const T, Align> { public:     typedef T         value_type;     typedef const T*  pointer;     typedef const T*  const_pointer;     typedef const T&  reference;     typedef const T&  const_reference;     typedef size_t    size_type;     typedef ptrdiff_t difference_type;      typedef std::true_type propagate_on_container_move_assignment;      template <class U>     struct rebind { typedef AlignedAllocator<U, Align> other; };  public:     AlignedAllocator() noexcept     {}      template <class U>     AlignedAllocator(const AlignedAllocator<U, Align>&) noexcept     {}      size_type     max_size() const noexcept     { return (size_type(~0) - size_type(Align)) / sizeof(T); }      const_pointer     address(const_reference x) const noexcept     { return std::addressof(x); }      pointer     allocate(size_type n, typename AlignedAllocator<void, Align>::const_pointer = 0)     {         const size_type alignment = static_cast<size_type>( Align );         void* ptr = detail::allocate_aligned_memory(alignment , n * sizeof(T));         if (ptr == nullptr) {             throw std::bad_alloc();         }          return reinterpret_cast<pointer>(ptr);     }      void     deallocate(pointer p, size_type) noexcept     { return detail::deallocate_aligned_memory(p); }      template <class U, class ...Args>     void     construct(U* p, Args&&... args)     { ::new(reinterpret_cast<void*>(p)) U(std::forward<Args>(args)...); }      void     destroy(pointer p)     { p->~T(); } };  template <typename T, Alignment TAlign, typename U, Alignment UAlign> inline bool operator== (const AlignedAllocator<T,TAlign>&, const AlignedAllocator<U, UAlign>&) noexcept { return TAlign == UAlign; }  template <typename T, Alignment TAlign, typename U, Alignment UAlign> inline bool operator!= (const AlignedAllocator<T,TAlign>&, const AlignedAllocator<U, UAlign>&) noexcept { return TAlign != UAlign; } 

The implementation for the actual allocate calls is posix only but you can extent that easily.

void* detail::allocate_aligned_memory(size_t align, size_t size) {     assert(align >= sizeof(void*));     assert(nail::is_power_of_two(align));      if (size == 0) {         return nullptr;     }      void* ptr = nullptr;     int rc = posix_memalign(&ptr, align, size);      if (rc != 0) {         return nullptr;     }      return ptr; }   void detail::deallocate_aligned_memory(void *ptr) noexcept {     return free(ptr); } 

Needs C++11, btw.

Answer 2

In the upcoming version 1.56, the Boost library will include Boost.Align. Among other memory alignment helpers it provides boost::alignment::aligned_allocator, which can be used a drop-in replacement for std::allocator and allows you to specify an alignment. See the documentation on https://boostorg.github.io/align/