bitmask.rst

bitmask

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

The function reverses the order of bits in a value.

Parameters

• start: starting position of the bitmask
• width: width of the bitmask

Return value

• bitmask of size width starting at start

Preconditions

• Compile-time: T is an unsigned integral type (including 128-bit integers).

Performance considerations

The function performs the following operations in device code:

• uint8_t, uint16_t, uint32_t: BMSK
• uint64_t: SHL x4, UADD x2
• uint128_t: SHL x8, UADD x4

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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