Description
The __global or global address
space name is used to refer to memory objects (buffer or image objects)
allocated from the global memory pool.
A buffer memory object can be declared as a pointer to a scalar, vector or user-defined struct. This allows the kernel to read and/or write any location in the buffer.
The actual size of the array memory object is determined when the memory object is allocated via appropriate API calls in the host code.
As image objects are always allocated from the global
address space, the __global or
global qualifier should not be
specified for image types. The elements of an image object
cannot be directly accessed. Built-in functions to
read from and write to an image object are
provided.
Variables defined at program scope and static
variables inside a function can also be declared
in the global address space.
They can be defined with any valid OpenCL C data type except
for those in table 6.3. In particular, such
program scope variables may be of any user-defined
type, or a pointer to a user-defined type. In
the presence of shared virtual memory, these pointers
or pointer members should work as expected as
long as they are shared virtual memory pointers
and the referenced storage has been mapped appropriately.
These variables in the global
address space have the same lifetime as the program,
and their values persist between calls to
any of the kernels in the program. These variables
are not shared across devices. They have
distinct storage.
Program scope and static variables in the
global address space may be initialized, but only
with constant expressions.
The const qualifier can also be used
with the __global qualifier to
specify a read-only buffer memory object.
Notes
OpenCL implements the following disjoint named 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 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.
The address space for a variable at program scope
or a static variable inside a function can either
be __global or
__constant, but defaults to
__global if not specified.
OpenCL 2.0 adds support for an unnamed generic
address space. Pointers that are declared
without pointing to a named address space point to the
generic address space. Before
referring to the region pointed to, the pointer
must be associated with a named address space.
Functions may be written with arguments and return values that point to the
generic address space.
kernel function arguments declared to be a pointer or
an array of a type must point to one of
the named address spaces __global,
__local or __constant.
The named address spaces are a subset of the generic
address space except for the constant
address space.
A pointer to address space A can only be assigned to
a pointer to the same address space A or a
pointer to the generic address
space. Casting a pointer to address space A to a pointer to
address space B is illegal if A and B are named
address spaces and A is not the same as B.
The __global, __constant,
__local,
__private, __generic,
global,
constant, local,
private and
generic names are reserved for use as address space
qualifiers and shall not be used otherwise.
The size of pointers to different address spaces
may differ. It is not correct to assume
that, for example, sizeof(__global int *)
always equals sizeof(__local int *).
Examples
General qualifier examples follow:
| // declares a pointer p in the private address space that // points to an object in address space global global int *p; void foo (...) { // declares an array of 4 floats in the private address space float x[4]; ... } |
Example:
| private int f() { ... } // should generate an error local int *f() { ... } // allowed local int * private f() { ... }; // should generate an error. |
| global float4 *color; // An array of float4 elements typedef struct { float a[3]; int b[2]; } foo_t; global foo_t *my_info; // An array of foo_t elements. |
| global int foo; // OK. int foo; // OK. Declared in the global address space global uchar buf[512]; // OK. global int baz = 12; // OK. Initialization is allowed static global int bat; // OK. Internal linkage global uchar bigbuf[CL_DEVICE_MAX_GLOBAL_VARIABLE_SIZE]; // OK. static int foo; // OK. Declared in the global address space static global int foo; // OK. int *foo; // OK. foo is allocated in global address space. // pointer to foo in generic address space void func(...) { int *foo; // OK. foo is allocated in private address space. // foo points to a location in generic address space. ... } global int * global ptr; // OK. int * global ptr; // OK. constant int *global ptr=&baz; // error since baz is in global address // space. global int * constant ptr = &baz; // OK // Pointers work. Also, initialization to a constant known at // program load time global int *global baz_ptr = &baz; global image2d_t im; // Error. Invalid type for program scope // variables global event_t ev; // Error. Invalid type for program scope variables global int *bad_ptr; // Error. No implicit address space |
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