cuda::bitmask

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cuda::bitmask#

2026-10-01

3 min read time

Applies to Linux and Windows

This page documents cuda::bitmask, which generates an integer bitmask of a specified width starting at a given bit position.

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

The function generates a bitmask of size width starting at position start.

Parameters

  • start: Starting position of the bitmask.

  • width: Width of the bitmask.

Return value

Bitmask of size width starting at start.

Constraints

T is an unsigned integral 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 device code:

  • uint8_t, uint16_t, uint32_t: BMSK

  • uint64_t: SHL x4, ADD 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>
#include <cuda/std/cstdint>

__global__ void bitmask_kernel() {
    assert(cuda::bitmask<uint32_t>(2, 4) == 0b111100u);
    assert(cuda::bitmask<uint64_t>(1, 3) == uint64_t{0b1110});
}

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

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