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       __constant - Address Space Qualifier.

       __constant constant


       OpenCL implements the following disjoint address spaces: __global, __local, __constant,
       and __private The address space qualifier may be used in variable declarations to specify
       the region of memory that is used to allocate the object. The C syntax for type qualifiers
       is extended in OpenCL to include an address space name as a valid type qualifier. If the
       type of an object is qualified by an address space name, the object is allocated in the
       specified address name; otherwise, the object is allocated in the generic address space.

       The address space names without the __prefix i.e.  global, local, constant and private may
       be substituted for the corresponding address space names with the __prefix.

       The generic address space name for arguments to a function in a program, or local
       variables of a function is __private. All function arguments shall be in the __private
       address space.

       functionQualifiers(3clc) function arguments declared to be a pointer of a type can point
       to one of the following address spaces only: __global, __local or __constant. A pointer to
       address space A can only be assigned to a pointer to the same address space A. Casting a
       pointer to address space A to a pointer to address space B is illegal.

       Function arguments of type otherDataTypes(3clc), otherDataTypes(3clc),
       otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), and otherDataTypes(3clc)
       refer to image memory objects allocated in the __global address space.


       The __constant or constant address space name is used to describe variables allocated in
       global memory and which are accessed inside a kernel(s) as read-only variables. These
       readonly variables can be accessed by all (global) work-items of the kernel during its
       execution. Pointers to the __constant address space are allowed as arguments to functions
       (including kernel functions) and for variables declared inside functions.

       All string literal storage shall be in the __constant address space.

       NOTE: Each argument to a kernel that is a pointer to the __constant address space is
       counted separately towards the maximum number of such arguments, defined as
       CL_DEVICE_MAX_CONSTANT_ARGS are described in the table for clGetDeviceInfo(3clc).

       Variables in the program scope or the outermost scope of kernel functions can be declared
       in the __constant address space. These variables are required to be initialized and the
       values used to initialize these variables must be a compile time constant. Writing to such
       a variable results in a compile-time error.

       Implementations are not required to aggregate these declarations into the fewest number of
       constant arguments. This behavior is implementation defined.

       Thus portable code must conservatively assume that each variable declared inside a
       function or in program scope allocated in the __constant address space counts as a
       separate constant argument.

       There is no generic address space name for program scope variables. All program scope
       variables must be declared in the __constant address space. For example:

       // declares a pointer p in the __private address
       space that // points to an int object in address
       space __global __global int *p;

       // declares an array of 4 floats in the
       __private address space.  float x[4];

       There is no address space for function return values. Using an address space qualifier in
       a function return type declaration will generate a compilation error, unless the return
       type is declared as a pointer type and the qualifier is used on the points-to address

       For example:

       __private int f() { ... } // should generate an
       error __local int *f() { ... } // allowed
       __local int * __private f() { ... }; // should
       generate an error.


       OpenCL Specification[1]


       global(3clc), local(3clc), private(3clc)


       The Khronos Group


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        1. OpenCL Specification
           page 222, section 6.5 - Address Space Qualifiers