std::bit_compress

 

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Defined in header <bit>
template< class T > constexpr T bit_compress( T x, T mask ) noexcept;		(since C++29)

See More

Selects the bits of x where mask has a 1-bit, and packs them contiguously to the right. The remaining bits are 0.

Parameters

x	-	the source value for packing
mask	-	the bit-mask for packing
Type requirements
T	-	must be an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type) in order to participate in overload resolution.

Return value

x with the bit-packing through the mask mask applied.

Notes

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

Feature-test macro	Value	Std	Feature
__cpp_lib_bitops	202607L	(C++29)	Bit permutations

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 source, T mask) noexcept { T result{}; for (T source_mask{1}, result_mask{1}; source_mask; source_mask <<= 1) if (source_mask & mask) result |= source_mask & mask & source ? result_mask : 0, result_mask <<= 1; return result; }

Example

Run this code

#include <bit>
#include <cstdint>

static_assert(
    std::bit_compress(
        std::uint16_t{0xABCD}, // source
        std::uint16_t{0x0F0F}) // mask
    ==  std::uint16_t{0x00BD}  // result
    and
    std::bit_compress(
        std::uint8_t{0b0100'1001}, // source
        std::uint8_t{0b1100'1100}) // mask
    ==  std::uint8_t{0b0000'0110}  // result
);

int main() {}

See also

bit_expand (C++29)

expands bits from an operand using a mask (PDEP) (function template)

External links

1.	What is a fast fallback algorithm which emulates PDEP and PEXT in software? — SO
2.	Reference implementation of C++26/29 bit permutation functions — github.com
3.	ZP7: Zach's Peppy Parallel-Prefix-Popcountin' PEXT/PDEP — github.com
4.	Henry S. Warren, Jr. Hacker's Delight, 2nd Edition, 2013, pp.150–161.