cuda::bitfield_extract

template <typename T>
[[nodiscard]] constexpr T
bitfield_extract(T value, int start, int width) noexcept;

The function extracts a bitfield from a value and returns it in the lower bits. bitfield_extract() computes (value >> start) & bitfield, where bitfield is a sequence of bits of width width.

Parameters

• value: The value to apply the bitfield.

• start: Initial position of the bitfield.

• width: Width of the bitfield.

Return value

• (value >> start) & bitfield.

Constraints

• T is an unsigned integer type.

Preconditions

• start >= 0 && start < num_bits(T)

• width >= 0 && width <= num_bits(T)

• start + width <= num_bits(T)

Performance considerations

The function performs the following operations in CUDA for uint8_t, uint16_t, uint32_t:

• SM < 70: BFE

• SM >= 70: BMSK, bitwise operation x2

Note

When the input values are run-time values that the compiler can resolve at compile-time, e.g. an index of a loop with a fixed number of iterations, using the function could not be optimal.

Note

GCC <= 8 uses a slow path with more instructions even in CUDA

Example

#include <cuda/bit>
#include <cuda/std/cassert>

__global__ void bitfield_insert_kernel() {
    assert(cuda::bitfield_extract(~0u, 0, 4) == 0b1111);
    assert(cuda::bitfield_extract(0b1011000, 3, 4) == 0b1011);
}

int main() {
    bitfield_insert_kernel<<<1, 1>>>();
    cudaDeviceSynchronize();
    return 0;
}

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