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.. meta::
  :description: API reference for cuda::bitfield_extract, which extracts a bitfield from an unsigned integer value and returns it in the lower bits in libhipcxx.
  :keywords: libhipcxx, ROCm, HIP, C++, bitfield_extract, bit manipulation, unsigned integer, BFE

.. _libcudacxx-extended-api-bit-bitfield_extract:

``cuda::bitfield_extract``
==========================

This page documents ``cuda::bitfield_extract``, which extracts a bitfield of a specified width from a value and returns it in the lower bits.

.. code:: cpp

   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
-------

.. code:: cpp

    #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;
    }

`See it on Godbolt 🔗 <https://godbolt.org/z/WvqfG9nbP>`_