Description
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.
Variables in the program scope must be declared in the
__constant address space. Variables
in 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.
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 *).
Example
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. |
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