extensions/ext/cl_ext_atomic_counters_64.txt

Name Strings

    cl_ext_atomic_counters_64

    #pragma OPENCL EXTENSION cl_ext_atomic_counters_64 : enable

Contributors

    Benedict Gaster, AMD
    German Andryeyev, AMD
    Ofer Rosenberg, AMD
    Aaftab Munshi, Apple
    Robert Elliott, ARM

Contact

    Benedict Gaster, benedict 'dot' gaster 'at' amd 'dot' com

IP Status

    No known IP issues

Version

    Version 5, May 26, 2011

Number

    OpenCL Extension #13

Status

    Approved by contributing members

Extension Type

    OpenCL device extensions

Dependencies

    None

Overview

    This extension adds support in the OpenCL C language for 64-bit atomic
    counters.
    Atomic counter is a device-level counter that can be added / decremented
    by work-items in a work-group or across work-groups executing a kernel,
    where the atomicity of the operation is guaranteed.
    The access to the counter is done only via add/dec built-in functions,
    and as such no two workitems will see the same value returned in the case
    that a given kernel only increments or decrements the counter.

    This feature is highly useful for produce/consume to unordered queues.

Header File

    None

New Procedures and Functions

    None

New Tokens

    CL_DEVICE_MAX_ATOMIC_COUNTERS_EXT = 0x4032


Additions to Chapter 4.2 of the OpenCL 1.1 Specification

    An additional device query is added to table 4.3 (OpenCL Device Queries)
    and is defined as follows:

    CL_DEVICE_INFO                    | Return Type |  Description
    --------------------------------------------------------------------------
    CL_DEVICE_MAX_ATOMIC_COUNTERS_EXT | cl_uint     | Max number of atomic
                                                    | counters that can be
                                                    | used by a kernel.
                                                    | The minimum value is 8.



Additions to Chapter 6.1.3 of the OpenCL 1.1 Specification

    An additional data type is added to table 6.3 (Other Built-in Data Types)
    and is defined as follows:

    Type         | Description
    ---------------------------------------------------------------------------
    counter64_t  | A 64-bit atomic counter. Refer to section 6.11.XX (Atomic 
	             | Counter Functions) below for a detailed description of this
                 | type.

    Variables declared to be of type counter64_t can only be specified as
    arguments to a kernel or a non-kernel function. These types cannot be
    used in any expression evaluation, or have values assigned to it inside
    a kernel or a function. These types cannot be used to declare local
    variables or as the return type of a function.
    A counter64_t argument to a function cannot be modified.


Additions to Chapter 6.11 of the OpenCL 1.1 Specification

    Add a new section named “Atomic Counter Functions”, as follows:

    6.11.XX Atomic Counter Functions
    Atomic counter is a device-level counter that can be added / decremented
    by different workitems, where the atomicity of the operation is
    guaranteed.

    The access to the counter is done only via atomic inc/dec built-in
    functions, which return the old value of the counter. The inc/dec
    operation has a global scope across all workitems, and as such no two
    workitems will see the same value returned in the case that a given kernel
    only increments or decrements the counter, unless the counter overflows
    and then the behavior is undefined.

    Atomic counter cannot be assigned directly to a variable or a pointer, or
    used in any expression evaluation. In addition, the same counter cannot be
    incremented and decremented inside a single kernel.

    The Host initializes the atomic counter by passing a buffer object as an
    argument to the kernel. The counter initial value is taken from the first
    8 bytes of the buffer object. During the run of the kernel, the global
    buffer is not updated by the atomic operations done on the counter. At the
    end of the kernel’s run, the updated value of the counter is written to
    the buffer object.

    The following table describes the list of built-in atomic counter
    functions that can be used to increment and decrement a value of type
    counter64_t.


    Function                              | Description
    --------------------------------------------------------------------------
    ulong atomic_inc(counter64_t counter) | Read the value (referred to as old)
                                          | of counter. Compute (old + 1) and
                                          | update counter to this value. The
                                          | function returns old.
    --------------------------------------------------------------------------
    ulong atomic_dec(counter64_t counter) | Read the value (referred to as old)
                                          | of counter. Compute (old - 1) and
                                          | update counter to this value. The
                                          | function returns old




Interactions with other extensions

    None