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       atom_xchg - 64-bit atom_xchg functions.

       long atom_xchg(volatile global(3clc) long *p, long val);

       long atom_xchg(volatile global(3clc) long *p, long val);

       ulong atom_xchg(volatile local(3clc) ulong *p, ulong val);

       ulong atom_xchg(volatile local(3clc) ulong *p, ulong val);


       Swaps the 32-bit value (referred to as old) stored at location pointed by p with new value
       given by val. The function returns old.

       An application that wants to use this extension will need to include the #pragma OPENCL
       EXTENSION cl_khr_int64_base_atomics : enable in the OpenCL program source.

       This function is performed in one atomic transaction. These transactions are atomic for
       the device executing these atomic functions. There is no guarantee of atomicity if the
       atomic operations to the same memory location are being performed by kernels executing on
       multiple devices.

       Atomic operations on 64-bit integers and 32-bit integers (and float) are also atomic with
       regard to each other.


       OpenCL Specification[1]


       atomicFunctions(3clc), atomic_xchg(3clc), cl_khr_int64_base_atomics(3clc)


       The Khronos Group


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        1. OpenCL Specification
           page 279, section 6.12.11 - Atomic Functions for 32-bit integers