Name

    ARB_shader_ballot

Name Strings

    GL_ARB_shader_ballot

Contact

    Timothy Lottes (timothy.lottes 'at' amd.com)

Contributors

    Timothy Lottes, AMD
    Graham Sellers, AMD
    Daniel Rakos, AMD
    Jeannot Breton, NVIDIA
    Pat Brown, NVIDIA
    Eric Werness, NVIDIA
    Mark Kilgard, NVIDIA
    Jeff Bolz, NVIDIA

Notice

    Copyright (c) 2015 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete. Approved by the ARB on June 26, 2015.
    Ratified by the Khronos Board of Promoters on August 7, 2015.

Version

    Last Modified Date: 03/18/2017
    Revision: 7

Number

    ARB Extension #183

Dependencies

    This extension is written against Revision 5 of the version 4.50 of the
    OpenGL Shading Language Specification, dated January 30, 2015.

    This extension requires GL_ARB_gpu_shader_int64.

Overview

    This extension provides the ability for a group of invocations which
    execute in lockstep to do limited forms of cross-invocation communication
    via a group broadcast of a invocation value, or broadcast of a bitarray
    representing a predicate value from each invocation in the group.

New Procedures and Functions

    None.

New Tokens

    None.

IP Status

    None.

Modifications to the OpenGL Shading Language Specification, Version 4.50

    Including the following line in a shader can be used to control the
    language features described in this extension:

      #extension GL_ARB_shader_ballot : <behavior>

    where <behavior> is as specified in section 3.3.

    New preprocessor #defines are added to the OpenGL Shading Language:

      #define GL_ARB_shader_ballot               1

Additions to Chapter 7 of the OpenGL Shading Language Specification
(Built-in Variables)

    Modify Section 7.4, Built-In Uniform State, p. 133

    (Add to the list of built-in uniform variable declaration)

        uniform uint  gl_SubGroupSizeARB;

    (Add this paragraph at the end of this section)

    A sub-group is a collection of invocations which execute in lockstep.
    The variable <gl_SubGroupSizeARB> is the maximum number of invocations
    in a sub-group. The maximum <gl_SubGroupSizeARB> supported in this
    extension is 64.

    Modify Section 7.1, Built-in Languages Variable, p. 110

    (Add to the list of built-in variables for the compute, vertex, geometry,
     tessellation control, tessellation evaluation and fragment languages)

        in uint     gl_SubGroupInvocationARB;
        in uint64_t gl_SubGroupEqMaskARB;
        in uint64_t gl_SubGroupGeMaskARB;
        in uint64_t gl_SubGroupGtMaskARB;
        in uint64_t gl_SubGroupLeMaskARB;
        in uint64_t gl_SubGroupLtMaskARB;

    (Add those paragraphs at the end of this section)

    The variable <gl_SubGroupInvocationARB> holds the index of the invocation within
    sub-group. This variable is in the range 0 to <gl_SubGroupSizeARB>-1, where
    <gl_SubGroupSizeARB> is the total number of invocations in a sub-group.

    The <gl_SubGroup??MaskARB> variables provide a bitmask for all invocations,
    with one bit per invocation starting with the least significant bit,
    according to the following table,

        variable               equation for bit values
        --------------------   ------------------------------------
        gl_SubGroupEqMaskARB   bit index == gl_SubGroupInvocationARB
        gl_SubGroupGeMaskARB   bit index >= gl_SubGroupInvocationARB
        gl_SubGroupGtMaskARB   bit index >  gl_SubGroupInvocationARB
        gl_SubGroupLeMaskARB   bit index <= gl_SubGroupInvocationARB
        gl_SubGroupLtMaskARB   bit index <  gl_SubGroupInvocationARB

Additions to Chapter 8 of the OpenGL Shading Language Specification
(Built-in Functions)

    Add Section 8.18, Shader Invocation Group Functions

    Syntax:

        uint64_t ballotARB(bool value);

    The function ballotARB() returns a bitfield containing the result of
    evaluating the expression <value> in all active invocations in the
    sub-group. An active invocation is one that is executing the ballotARB()
    call. The sub-group may have inactive invocations for example due to
    exit of the shader, or divergent branching. Sub-groups of up to 64
    invocations may be represented by the return value of ballotARB(). Bits
    for each invocation are packed in least significant bit ordering. If
    <value> evaluates to true for an active invocation then the corresponding
    bit is set to one in the result, otherwise it is zero. Bits corresponding
    to invocations that are not active or that do not exist in the sub group
    (because, for example, they are at bit positions beyond the sub-group
    size) are set to zero. The following trivial assumptions can be made:

        * ballotARB(true) returns bitfield where the corresponding bits are
          set for all active invocations in the sub-group.

        * ballotARB(false) returns zero.

    Syntax:

        genType readInvocationARB(genType value, uint invocationIndex);
        genIType readInvocationARB(genIType value, uint invocationIndex);
        genUType readInvocationARB(genUType value, uint invocationIndex);

        genType readFirstInvocationARB(genType value);
        genIType readFirstInvocationARB(genIType value);
        genUType readFirstInvocationARB(genUType value);

    The function readInvocationARB() returns the <value> from a given
    <invocationIndex> to all active invocations in the sub-group.
    The <invocationIndex> must be the same for all active invocations
    in the sub-group otherwise results are undefined.

    The function readFirstInvocationARB() returns the <value> from the first
    active invocation to all active invocations in the sub-group.

Issues

    1) How are the values of gl_SubGroup??MaskARB defined?

    RESOLVED.  Earlier versions of this specification defined a bitmask
    such as "LtMask" ("less than mask") as having bits set if
    "gl_SubGroupInvocationARB <  bit index".  However, this was reversed
    from the definition in GL_NV_shader_thread_group that these built-ins
    were derived from, and also mismatched a recent Vulkan/SPIR-V extension.

    Fortunately, all known implementations of this extension had implemented
    "wrong" behavior (matching the sense of the original built-ins in
    GL_NV_shader_thread_group), so the best thing to do is change the
    definition in the spec.

Revision History

    Rev  Date        Author    Changes
    ---  ----------  --------  ---------------------------------------------
      8  03/18/2017  jbolz     Reversed the sense of the comparison in the
                               definition of gl_SubGroup??MaskARB.
      7  08/25/2015  nhenning  Add ARB suffix on documentation for
                               readInvocation and readFirstInvocation
                               functions.
      6  07/31/2015  pdaniell  Add ARB suffix on the readInvocation and
                               readFirstInvocation functions.
      5  07/30/2015  pdaniell  Update the function definition syntax to use
                               our standard gen*Type conventions.
      4  06/23/2015  tlottes   More precise spec language.
      3  06/22/2015  tlottes   Deferred GPU processor another spec.
                               Cleaned up spec language.
      2  04/20/2015  tlottes   Updated spec language.
      1  03/09/2015  tlottes   Initial revision based on AMD_gcn_shader and
                               NV_shader_thread_group.