bit_field.hpp

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/*! \file
    \brief Portable bit field that supports byte and word straddling that can
           be used in unions to bit-wise define parameters.
*/

#pragma once

#include <cute/config.hpp>

#include <cute/numeric/int.hpp>   // uint_bit_t

namespace cute
{

class dummy_type {};

template <uint32_t BitStart, uint32_t NumBits, class OtherValueType = dummy_type>
struct bit_field
{
  static_assert(0 < NumBits && NumBits <= 64, "bit_fields with more than 64 bits are not supported.");

  // value_type: Use the smallest value type that fits NumBits
  static constexpr uint32_t value_type_bits = (NumBits <=  8) ?  8 :
                                              (NumBits <= 16) ? 16 :
                                              (NumBits <= 32) ? 32 : 64;
  using value_type   = cute::uint_bit_t<value_type_bits>;
  // storage_type: Use the smallest storage_type that avoids boundary crossing
  static constexpr uint32_t storage_type_bits = (BitStart /  8 == (BitStart + NumBits - 1) /  8) ?  8 :
                                                (BitStart / 16 == (BitStart + NumBits - 1) / 16) ? 16 :
                                                (BitStart / 32 == (BitStart + NumBits - 1) / 32) ? 32 : 64;
  using storage_type = cute::uint_bit_t<storage_type_bits>;

  static_assert(sizeof(OtherValueType) == sizeof(value_type) || is_same<OtherValueType,dummy_type>::value,
                "sizeof(OtherValueType) must be same as sizeof(value_type).");

  // Number of storage values needed: ceil_div(BitStart + NumBits, storage_type_bits)
  static constexpr uint32_t N      = (BitStart + NumBits + storage_type_bits - 1) / storage_type_bits;
  // Index of storage value for BitStart
  static constexpr uint32_t idx    = BitStart / storage_type_bits;
  // Bit of data_[idx] for BitStart
  static constexpr uint32_t bit_lo = BitStart % storage_type_bits;
  // Number of bits in data_[idx] used for NumBits if straddling, else 0
  static constexpr uint32_t bit_hi = (idx + 1 < N) ? (storage_type_bits - bit_lo) : 0;

private:
  // MSVC issues warning C4293 ("shift count negative or too big, undefined behavior")
  // if we use NumBits directly in the shift expression, even if the shift occurs
  // in the branch of a ternary expression where NumBits is known to be less than
  // the number of bits of the value being shifted.
  static constexpr uint32_t MollifiedNumBits = NumBits > 63u ? 63u : NumBits;
public:

  // NumBits mask
  static constexpr value_type   mask    = (NumBits < 64u) ? ((uint64_t(1) << MollifiedNumBits) - 1) : uint64_t(-1);
  // NumBits mask for BitStart
  static constexpr storage_type mask_lo = storage_type(mask) << bit_lo;
  // NumBits mask for leftover bits in data_[idx+1] if straddling, else 0
  static constexpr storage_type mask_hi = (idx + 1 < N) ? (storage_type(mask) >> bit_hi) : 0;

  storage_type data_[N];

  // Get value
  CUTE_HOST_DEVICE constexpr
  value_type get() const {
    storage_type result = (data_[idx] & mask_lo) >> bit_lo;
    if constexpr (bit_hi) {
      result |= (data_[idx+1] & mask_hi) << bit_hi;
    }
    return static_cast<value_type>(result);
  }

  // Set value
  CUTE_HOST_DEVICE constexpr
  void set(value_type x) {
    storage_type item = static_cast<storage_type>(x & mask);
    data_[idx] = static_cast<storage_type>((data_[idx] & ~mask_lo) | (item << bit_lo));
    if constexpr (bit_hi) {
      data_[idx+1] = static_cast<storage_type>((data_[idx+1] & ~mask_hi) | (item >> bit_hi));
    }
  }

  // Assign value
  CUTE_HOST_DEVICE constexpr
  bit_field& operator=(value_type x) {
    set(x);
    return *this;
  }

  // Cast to value
  CUTE_HOST_DEVICE constexpr
  operator value_type () const {
    return get();
  }

  // Assign OtherValueType
  CUTE_HOST_DEVICE constexpr
  bit_field& operator=(OtherValueType x) {
    return *this = *reinterpret_cast<value_type*>(&x);
  }

  // Cast to OtherValueType
  CUTE_HOST_DEVICE constexpr
  operator OtherValueType () const {
    value_type x = get();
    return *reinterpret_cast<OtherValueType*>(&x);
  }
};

} // end namespace cute