..
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.. meta::
  :description: API reference for cuda::bitmask, which generates a bitmask of a specified width starting at a given bit position in libhipcxx for HIP.
  :keywords: libhipcxx, ROCm, HIP, C++, bitmask, bit manipulation, unsigned integer, BMSK

.. _libcudacxx-extended-api-bit-bitmask:

``cuda::bitmask``
=================

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

.. code:: cpp

   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
-------

.. code:: cpp

    #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>>>();
        hipDeviceSynchronize();
        return 0;
    }

..
   `See it on Godbolt 🔗 <https://godbolt.org/z/PPqP8rTPd>`_