CUDA Kernel API
===============

Kernel declaration
------------------

The ``@cuda.jit`` decorator is used to create a CUDA kernel:

.. autofunction:: numba.cuda.jit

.. autoclass:: numba.cuda.compiler.AutoJitCUDAKernel
   :members: inspect_asm, inspect_llvm, inspect_types, specialize, extensions

Individual specialized kernels are instances of
:class:`numba.cuda.compiler.CUDAKernel`:

.. autoclass:: numba.cuda.compiler.CUDAKernel
   :members: bind, ptx, device, inspect_llvm, inspect_asm, inspect_types

Intrinsic Attributes and Functions
----------------------------------

The remainder of the attributes and functions in this section may only be called
from within a CUDA Kernel.

Thread Indexing
~~~~~~~~~~~~~~~

.. attribute:: numba.cuda.threadIdx

    The thread indices in the current thread block, accessed through the
    attributes ``x``, ``y``, and ``z``. Each index is an integer spanning the
    range from 0 inclusive to the corresponding value of the attribute in
    :attr:`numba.cuda.blockDim` exclusive.

.. attribute:: numba.cuda.blockIdx

    The block indices in the grid of thread blocks, accessed through the
    attributes ``x``, ``y``, and ``z``. Each index is an integer spanning the
    range from 0 inclusive to the corresponding value of the attribute in
    :attr:`numba.cuda.gridDim` exclusive.

.. attribute:: numba.cuda.blockDim

    The shape of a block of threads, as declared when instantiating the
    kernel.  This value is the same for all threads in a given kernel, even
    if they belong to different blocks (i.e. each block is "full").

.. attribute:: numba.cuda.gridDim

    The shape of the grid of blocks, accessed through the attributes ``x``,
    ``y``, and ``z``.

.. attribute:: numba.cuda.laneid

    The thread index in the current warp, as an integer spanning the range
    from 0 inclusive to the :attr:`numba.cuda.warpsize` exclusive.

.. attribute:: numba.cuda.warpsize

    The size in threads of a warp on the GPU. Currently this is always 32.

.. function:: numba.cuda.grid(ndim)

   Return the absolute position of the current thread in the entire
   grid of blocks.  *ndim* should correspond to the number of dimensions
   declared when instantiating the kernel.  If *ndim* is 1, a single integer
   is returned.  If *ndim* is 2 or 3, a tuple of the given number of
   integers is returned.

   Computation of the first integer is as follows::

      cuda.threadIdx.x + cuda.blockIdx.x * cuda.blockDim.x

   and is similar for the other two indices, but using the ``y`` and ``z``
   attributes.

.. function:: numba.cuda.gridsize(ndim)

   Return the absolute size (or shape) in threads of the entire grid of
   blocks. *ndim* should correspond to the number of dimensions declared when
   instantiating the kernel.

   Computation of the first integer is as follows::

       cuda.blockDim.x * cuda.gridDim.x

   and is similar for the other two indices, but using the ``y`` and ``z``
   attributes.

Memory Management
~~~~~~~~~~~~~~~~~

.. function:: numba.cuda.shared.array(shape, dtype)

   Creates an array in the local memory space of the CUDA kernel with
   the given ``shape`` and ``dtype``.

   Returns an array with its content uninitialized.

   .. note:: All threads in the same thread block sees the same array.

.. function:: numba.cuda.local.array(shape, dtype)

   Creates an array in the local memory space of the CUDA kernel with the
   given ``shape`` and ``dtype``.

   Returns an array with its content uninitialized.

   .. note:: Each thread sees a unique array.

.. function:: numba.cuda.const.array_like(ary)

   Copies the ``ary`` into constant memory space on the CUDA kernel at compile
   time.

   Returns an array like the ``ary`` argument.

   .. note:: All threads and blocks see the same array.

Synchronization and Atomic Operations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. function:: numba.cuda.atomic.add(array, idx, value)

    Perform ``array[idx] += value``. Support int32, int64, float32 and
    float64 only. The ``idx`` argument can be an integer or a tuple of integer
    indices for indexing into multiple dimensional arrays. The number of element
    in ``idx`` must match the number of dimension of ``array``.

    Returns the value of ``array[idx]`` before the storing the new value.
    Behaves like an atomic load.

.. function:: numba.cuda.atomic.max(array, idx, value)

    Perform ``array[idx] = max(array[idx], value)``. Support int32, int64,
    float32 and float64 only. The ``idx`` argument can be an integer or a
    tuple of integer indices for indexing into multiple dimensional arrays.
    The number of element in ``idx`` must match the number of dimension of
    ``array``.

    Returns the value of ``array[idx]`` before the storing the new value.
    Behaves like an atomic load.


.. function:: numba.cuda.syncthreads

    Synchronize all threads in the same thread block.  This function implements
    the same pattern as barriers in traditional multi-threaded programming: this
    function waits until all threads in the block call it, at which point it
    returns control to all its callers.

.. function:: numba.cuda.syncthreads_count(predicate)

    An extension to :attr:`numba.cuda.syncthreads` where the return value is a count
    of the threads where ``predicate`` is true.

.. function:: numba.cuda.syncthreads_and(predicate)

    An extension to :attr:`numba.cuda.syncthreads` where 1 is returned if ``predicate`` is
    true for all threads or 0 otherwise.

.. function:: numba.cuda.syncthreads_or(predicate)

    An extension to :attr:`numba.cuda.syncthreads` where 1 is returned if ``predicate`` is
    true for any thread or 0 otherwise.

    .. warning:: All syncthreads functions must be called by every thread in the
                 thread-block. Falling to do so may result in undefined behavior.

Memory Fences
~~~~~~~~~~~~~

The memory fences are used to guarantee the effect of memory operations
are visible by other threads within the same thread-block, the same GPU device,
and the same system (across GPUs on global memory). Memory loads and stores
are guaranteed to not move across the memory fences by optimization passes.

.. warning:: The memory fences are considered to be advanced API and most
             usercases should use the thread barrier (e.g. ``syncthreads()``).



.. function:: numba.cuda.threadfence

   A memory fence at device level (within the GPU).

.. function:: numba.cuda.threadfence_block

   A memory fence at thread block level.

.. function:: numba.cuda.threadfence_system


   A memory fence at system level (across GPUs).

Warp Intrinsics
~~~~~~~~~~~~~~~~~~

All warp level operations require at least CUDA 9. The argument ``membermask`` is
a 32 bit integer mask with each bit corresponding to a thread in the warp, with 1
meaning the thread is in the subset of threads within the function call. The
``membermask`` must be all 1 if the GPU compute capability is below 7.x.

.. function:: numba.cuda.syncwarp(membermask)

   Synchronize a masked subset of the threads in a warp.

.. function:: numba.cuda.all_sync(membermask, predicate)

    If the ``predicate`` is true for all threads in the masked warp, then
    a non-zero value is returned, otherwise 0 is returned.

.. function:: numba.cuda.any_sync(membermask, predicate)

    If the ``predicate`` is true for any thread in the masked warp, then
    a non-zero value is returned, otherwise 0 is returned.

.. function:: numba.cuda.eq_sync(membermask, predicate)

    If the boolean ``predicate`` is the same for all threads in the masked warp,
    then a non-zero value is returned, otherwise 0 is returned.

.. function:: numba.cuda.ballot_sync(membermask, predicate)

    Returns a mask of all threads in the warp whose ``predicate`` is true,
    and are within the given mask.

.. function:: numba.cuda.shfl_sync(membermask, value, src_lane)

    Shuffles ``value`` across the masked warp and returns the ``value``
    from ``src_lane``. If this is outside the warp, then the
    given ``value`` is returned.

.. function:: numba.cuda.shfl_up_sync(membermask, value, delta)

    Shuffles ``value`` across the masked warp and returns the ``value``
    from ``laneid - delta``. If this is outside the warp, then the
    given ``value`` is returned.

.. function:: numba.cuda.shfl_down_sync(membermask, value, delta)

    Shuffles ``value`` across the masked warp and returns the ``value``
    from ``laneid + delta``. If this is outside the warp, then the
    given ``value`` is returned.

.. function:: numba.cuda.shfl_xor_sync(membermask, value, lane_mask)

    Shuffles ``value`` across the masked warp and returns the ``value``
    from ``laneid ^ lane_mask``.

.. function:: numba.cuda.match_any_sync(membermask, value, lane_mask)

    Returns a mask of threads that have same ``value`` as the given ``value``
    from within the masked warp.

.. function:: numba.cuda.match_all_sync(membermask, value, lane_mask)

    Returns a tuple of (mask, pred), where mask is a mask of threads that have
    same ``value`` as the given ``value`` from within the masked warp, if they
    all have the same value, otherwise it is 0. And pred is a boolean of whether
    or not all threads in the mask warp have the same warp.


Integer Intrinsics
~~~~~~~~~~~~~~~~~~

A subset of the CUDA Math API's integer intrinsics are available. For further
documentation, including semantics, please refer to the `CUDA Toolkit
documentation
<https://docs.nvidia.com/cuda/cuda-math-api/group__CUDA__MATH__INTRINSIC__INT.html>`_.


.. function:: numba.cuda.popc

   Returns the number of set bits in the given value.

.. function:: numba.cuda.brev

   Reverses the bit pattern of an integer value, for example 0b10110110
   becomes 0b01101101.

.. function:: numba.cuda.clz

   Counts the number of leading zeros in a value.

.. function:: numba.cuda.ffs

   Find the position of the least significant bit set to 1 in an integer.


Floating Point Intrinsics
~~~~~~~~~~~~~~~~~~~~~~~~~

A subset of the CUDA Math API's floating point intrinsics are available. For further
documentation, including semantics, please refer to the `single
<https://docs.nvidia.com/cuda/cuda-math-api/group__CUDA__MATH__SINGLE.html>`_ and
`double <https://docs.nvidia.com/cuda/cuda-math-api/group__CUDA__MATH__DOUBLE.html>`_
precision parts of the CUDA Toolkit documentation.


.. function:: numba.cuda.fma

   Perform the fused multiply-add operation. Named after the ``fma`` and ``fmaf`` in
   the C api, but maps to the ``fma.rn.f32`` and ``fma.rn.f64`` (round-to-nearest-even)
   PTX instructions.


Control Flow Instructions
~~~~~~~~~~~~~~~~~~~~~~~~~

A subset of the CUDA's control flow instructions are directly available as
intrinsics. Avoiding branches is a key way to improve CUDA performance, and
using these intrinsics mean you don't have to rely on the ``nvcc`` optimizer
identifying and removing branches. For further documentation, including
semantics, please refer to the `relevant CUDA Toolkit documentation
<https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#comparison-and-selection-instructions>`_.


.. function:: numba.cuda.selp

    Select between two expressions, depending on the value of the first
    argument. Similar to LLVM's ``select`` instruction.
