Address Space Qualifier.

__local
local

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

General information about address space qualifiers

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, and private names are reserved for use as address space qualifiers and shall not be used otherwise. The generic and generic names are reserved for future use.

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.

Also see

Specification