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.
__kernel
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
image2d_t,
image3d_t,
image2d_array_t,
image1d_t,
image1d_buffer_t, and
image1d_array_t
refer to image memory objects allocated in the __global
address space.
Variables inside a __kernel
function not declared with an
address space qualifier, all variables inside non-kernel functions, and all function
arguments are in the __private
or private
address space. Variables declared as pointers are considered to point to the
__private
address space if an address space qualifier is not
specified.
The __global
, __constant
,
__local
, __private
,
global
, constant
, local
,
and private
names are reserved for use as address space qualifiers
and shall not be used otherwise.
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 space.
For example:
__private int f() { ... } // should generate an error __local int *f() { ... } // allowed __local int * __private f() { ... }; // should generate an error. |
// 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]; |