Description
The __local or local address space name is
used to describe variables that need to be allocated in local memory and are shared by
all work-items of a work-group. Pointers to the __local address
space are allowed as arguments to functions (including kernel
functions). Variables allocated in the __local address space
inside a kernel function must occur at kernel function scope.
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
| // Examples of variables allocated in the __local address space // inside a kernel function kernel void my_func(...) { local float a; // A single float allocated // in local address space local float b[10]; // An array of 10 floats // allocated in local address space. if (...) { // example of variable in __local address space but not // declared at __kernel function scope. local float c; // not allowed. } } |
Variables allocated in the __local address space inside a
kernel function cannot be initialized.
| kernel void my_func(...) { local float a = 1; // not allowed local float b; b = 1; // allowed } |
NOTE: Variables allocated in the __local address
space inside a kernel function
are allocated for each work-group executing
the kernel and exist only for the lifetime of the work-group
executing the kernel.
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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