c10/util/AlignOf.h

//===--- AlignOf.h - Portable calculation of type alignment -----*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the AlignedCharArray and AlignedCharArrayUnion classes.
//
//===----------------------------------------------------------------------===//

// ATen: modified from llvm::AlignOf
// replaced LLVM_ALIGNAS with alignas

#pragma once

#include <cstddef>

namespace c10 {

/// \struct AlignedCharArray
/// \brief Helper for building an aligned character array type.
///
/// This template is used to explicitly build up a collection of aligned
/// character array types. We have to build these up using a macro and explicit
/// specialization to cope with MSVC (at least till 2015) where only an
/// integer literal can be used to specify an alignment constraint. Once built
/// up here, we can then begin to indirect between these using normal C++
/// template parameters.

// MSVC requires special handling here.
#ifndef _MSC_VER

template <size_t Alignment, size_t Size>
struct AlignedCharArray {
  alignas(Alignment) char buffer[Size];
};

#else // _MSC_VER

/// \brief Create a type with an aligned char buffer.
template <size_t Alignment, size_t Size>
struct AlignedCharArray;

// We provide special variations of this template for the most common
// alignments because __declspec(align(...)) doesn't actually work when it is
// a member of a by-value function argument in MSVC, even if the alignment
// request is something reasonably like 8-byte or 16-byte. Note that we can't
// even include the declspec with the union that forces the alignment because
// MSVC warns on the existence of the declspec despite the union member forcing
// proper alignment.

template <size_t Size>
struct AlignedCharArray<1, Size> {
  union {
    char aligned;
    char buffer[Size];
  };
};

template <size_t Size>
struct AlignedCharArray<2, Size> {
  union {
    short aligned;
    char buffer[Size];
  };
};

template <size_t Size>
struct AlignedCharArray<4, Size> {
  union {
    int aligned;
    char buffer[Size];
  };
};

template <size_t Size>
struct AlignedCharArray<8, Size> {
  union {
    double aligned;
    char buffer[Size];
  };
};

// The rest of these are provided with a __declspec(align(...)) and we simply
// can't pass them by-value as function arguments on MSVC.

#define AT_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
  template <size_t Size>                          \
  struct AlignedCharArray<x, Size> {              \
    __declspec(align(x)) char buffer[Size];       \
  };

AT_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16)
AT_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32)
AT_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64)
AT_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128)

#undef AT_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT

#endif // _MSC_VER

namespace detail {
template <
    typename T1,
    typename T2 = char,
    typename T3 = char,
    typename T4 = char,
    typename T5 = char,
    typename T6 = char,
    typename T7 = char,
    typename T8 = char,
    typename T9 = char,
    typename T10 = char>
class AlignerImpl {
  T1 t1;
  T2 t2;
  T3 t3;
  T4 t4;
  T5 t5;
  T6 t6;
  T7 t7;
  T8 t8;
  T9 t9;
  T10 t10;

  AlignerImpl() = delete;
};

template <
    typename T1,
    typename T2 = char,
    typename T3 = char,
    typename T4 = char,
    typename T5 = char,
    typename T6 = char,
    typename T7 = char,
    typename T8 = char,
    typename T9 = char,
    typename T10 = char>
union SizerImpl {
  char arr1[sizeof(T1)], arr2[sizeof(T2)], arr3[sizeof(T3)], arr4[sizeof(T4)],
      arr5[sizeof(T5)], arr6[sizeof(T6)], arr7[sizeof(T7)], arr8[sizeof(T8)],
      arr9[sizeof(T9)], arr10[sizeof(T10)];
};
} // end namespace detail

/// \brief This union template exposes a suitably aligned and sized character
/// array member which can hold elements of any of up to ten types.
///
/// These types may be arrays, structs, or any other types. The goal is to
/// expose a char array buffer member which can be used as suitable storage for
/// a placement new of any of these types. Support for more than ten types can
/// be added at the cost of more boilerplate.
template <
    typename T1,
    typename T2 = char,
    typename T3 = char,
    typename T4 = char,
    typename T5 = char,
    typename T6 = char,
    typename T7 = char,
    typename T8 = char,
    typename T9 = char,
    typename T10 = char>
struct AlignedCharArrayUnion
    : AlignedCharArray<
          alignof(detail::AlignerImpl<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10>),
          sizeof(::c10::detail::
                     SizerImpl<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10>)> {};
} // end namespace c10