NAME¶
__constant - Address Space Qualifier.
DESCRIPTION¶
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.
NOTES¶
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.
Example¶
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. |
SPECIFICATION¶
OpenCL Specification[1]
SEE ALSO¶
global(3clc),
local(3clc),
private(3clc)
AUTHORS¶
The Khronos Group
COPYRIGHT¶
Copyright © 2007-2011 The Khronos Group Inc.
Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and/or associated documentation files (the
"Materials"), to deal in the Materials without restriction,
including without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Materials, and to permit
persons to whom the Materials are furnished to do so, subject to the condition
that this copyright notice and permission notice shall be included in all
copies or substantial portions of the Materials.
NOTES¶
- 1.
- OpenCL Specification
page 222, section 6.5 - Address Space Qualifiers