std::bit_expand

template< class T >
constexpr T bit_expand( T x, T m ) noexcept;

Selects the least significant bits of x and places them where m has a 1-bit. The remaining bits are 0.

This overload participates in overload resolution only if T is an unsigned integer type (that is, unsignedchar, unsignedshort, unsignedint, unsignedlong, unsignedlonglong, or an extended unsigned integer type).

Parameters

Return value

x with the bit deposit through the mask m applied.

Notes

The function is has the same result as the PDEP x86_64 and PDEP ARM instructions.

Possible implementation

template<typename T, typename ... U>
concept neither = (!std::same_as<T, U> && ...);

template<std::unsigned_integral T>
    requires neither<T, bool, char, char8_t, char16_t, char32_t, wchar_t>
constexpr T bit_compress(T x) noexcept
{
    T result = 0;
    for (int i = 0, j = 0; i < std::numeric_limits<T>::digits; ++i)
    {
        bool mask_bit = (m >> i) & 1;
        result |= (mask_bit & (x >> j)) << i;
        j += mask_bit;
    }
    return result;
}

Example

#include <bit>
#include <cstdint>

static_assert(
    std::bit_expand(
        std::uint16_t{0xABCD}, // source
        std::uint16_t{0x0F0F}) // mask
    ==  std::uint16_t{0x0C0D}  // result
);
static_assert(
    std::bit_expand(
        std::uint8_t{0b1010'1001}, // source
        std::uint8_t{0b0011'0011}) // mask
    ==  std::uint8_t{0b0010'0001}  // result
);

int main() {}

See also