Name

    INTEL_fragment_shader_ordering

Name Strings

    GL_INTEL_fragment_shader_ordering

Contact

    Slawomir Grajewski, Intel  (slawomir.grajewski 'at' intel.com)

Contributors

    Tim Foley, Intel
    Brent Insko, Intel 
    Tomasz Janczak, Intel
    Marco Salvi, Intel 
    Larry Seiler, Intel
    Tomasz Poniecki, Intel

Status

    Draft.

Version

    Last Modified Date:        08/29/2013
    INTEL Revision:            3

Number

    OpenGL Extension #441

Dependencies

    This extension is written against the OpenGL 4.4 (Core Profile) 
    Specification.

    This extension is written against version 4.40 (revision 6) of the OpenGL
    Shading Language Specification.

    OpenGL 4.2 or ARB_shader_image_load_store is required;GLSL 4.20 is
    required.

Overview

    Graphics devices may execute in parallel fragment shaders referring to the
    same window xy coordinates. Framebuffer writes are guaranteed to be
    processed in primitive rasterization order, but there is no order guarantee
    for other instructions and image or buffer object accesses in
    particular.

    The extension introduces a new GLSL built-in function, 
    beginFragmentShaderOrderingINTEL(), which blocks execution of a fragment 
    shader invocation until invocations from previous primitives that map to 
    the same xy window coordinates (and same sample when per-sample shading
    is active) complete their execution. All memory transactions from previous 
    fragment shader invocations are made visible to the fragment shader 
    invocation that called beginFragmentShaderOrderingINTEL() when the function 
    returns. 

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

      #extension GL_INTEL_fragment_shader_ordering : <behavior>

    where <behavior> is as specified in section 3.3.

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

      #define GL_INTEL_fragment_shader_ordering 1

New Procedures and Functions

    None.

New Tokens

    None.

Additions to the OpenGL 4.4 (Core Profile) Specification 

       Modify section 2.11.13 Shader Memory Access
       (add new text in last bullet on p. 143)
       
       The exception is when the built-in function 
       beginFragmentShaderOrderingINTEL() is used. This function creates a 
       region in a fragment shader in which memory operations complete and are 
       made visible in rasterization order for fragments that overlap in xy 
       window coordinates.
       
       (add new paragraph after second paragraph on p. 144)
       
       The built-in function beginFragmentShaderOrderingINTEL() may be used to 
       provide ordering of reads and writes performed by fragment shader 
       invocations that overlap in xy window coordinates. Calling 
       beginFragmentShaderOrderingINTEL() guarantees that any memory 
       transactions issued by shader invocations from previous primitives, 
       mapped to same xy window coordinates (and same sample when per-sample 
       shading is active), complete and are visible to the shader invocation 
       that called beginFragmentShaderOrderingINTEL().


Modifications to the OpenGL Shading Language Specification, Version 4.40
(revision 6)


    Modify Section 8.17 Shader Memory Control Functions

    (add to the table, p. 178)

    Syntax
    ------

    void beginFragmentShaderOrderingINTEL()


    This function is available only in fragment shaders.

    Description
    -----------

    The beginFragmentShaderOrderingINTEL() function blocks fragment shader 
    execution until completion of all shader invocations from previous 
    primitives that map to the same window xy coordinates (and same sample 
    if appropriate). All memory transactions from previous fragment shader
    invocations mapped to same xy window coordinates are made visible to 
    the current fragment shader invocation when this function returns. 
    When per-fragment shading is active, the ordering guarantee applies 
    regardless of whether previous and current invocations cover overlapping 
    or disjoint sets of samples under same fragment of window xy coordinates. 
    When per-sample shading is active, the ordering guarantee applies to 
    shader invocations mapped to the same sample number under same window xy
    coordinates. This function has no effect on shader execution for fragments
    with non-overlapping window xy coordinates.

    
    The beginFragmentShaderOrderingINTEL() function can be called under control
    flow, including non-uniform control flow. Synchronization and ordering
    guarantees are valid only if the conditional branch is taken during 
    execution. It is valid for a fragment shader to dynamically execute 
    multiple calls to beginFragmentShaderOrderingINTEL(). In such cases, the 
    first executed call will block as described above, but subsequent calls 
    will never block.


    Note there is no explicit built-in function to signal the end of the region 
    that should be ordered. Instead, the region that will be ordered logically 
    extends to the end of fragment shader execution.


    GLSL code example
    -----------------

    layout(binding = 0, rgba8) uniform image2D image;

    vec4 main()
    {
        ... compute output color 
        if (color.w > 0)        // potential non-uniform control flow
        {
            beginFragmentShaderOrderingINTEL();
            ... read/modify/write image         // ordered access guaranteed 
        }
        ... no ordering guarantees (as varying branch might not be taken) 
 
        beginFragmentShaderOrderingINTEL();

        ... update image again                  // ordered access guaranteed 
    }

Errors

    None.

New State

    None.

New Implementation Dependent State

    None.

Issues

    None. 

Revision History

    Rev.    Date    Author        Changes
    ----  --------  --------     -----------------------------------------
     1    07/30/13  S.Grajewski  Initial revision.
     2    08/26/13  T.Janczak    removed per-image synchronization 
     3    08/29/13  T.Janczak    incorporate internal review feedback