Name

    NV_shader_atomic_counters

Name Strings

    GL_NV_shader_atomic_counters

Contact

    Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com)

Status

    Complete

Version

    Last Modified Date:         June 23, 2011
    NVIDIA Revision:            1

Number

    423

Dependencies

    ARB_shader_atomic_counters and NV_gpu_program5 are required.

    This extension is written against the NV_gpu_program4 specification, as
    extended by NV_gpu_program5.

Overview

    This extension builds upon the ARB_shader_atomic_counters and
    NV_gpu_program5 extensions to provide assembly language support for
    incrementing, decrementing, and querying the values of atomic counters
    stored in buffer object memory.

    The extension uses the same set of atomic counter buffer binding points as
    the ARB_shader_atomic_counters extension; applications using this
    extension should use the APIs specified there to bind buffers.

New Procedures and Functions

    None.

New Tokens

    None.

Additions to Chapter 2 of the OpenGL 2.0 Specification (OpenGL Operation)

    (All modifications are relative to Section 2.X, GPU Programs, from the
     NV_gpu_program4 specification.)

    Modify Section 2.X.2, Program Grammar

    (add after the long list of grammar rules) If a program specifies the
    NV_shader_atomic_counters program option, the following rules are added
    to the NV_gpu_program4 base program grammar:

    <instruction>           ::= <AtomCtrInstruction>

    <AtomCtrInstruction>    ::= <ATOMCTRop_instruction>

    <ATOMCTRop_instuction>  ::= <ATOMCTRop> <opModifiers> <instResult> ","
                                  <counterUseV>

    <ATOMCop>               ::= "ATOMCTR"

    <opModifier>            ::= "INCR"
                              | "DECR"
                              | "GET"

    <namingStatement>       ::= <COUNTER_statement>

    <COUNTER_statement>     ::= "COUNTER" <establishName> <counterSingleInit>
    <COUNTER_statement>     ::= "COUNTER" <establishName> <optArraySize>
                                  <counterMultipleInit>

    <counterSingleInit>     ::= <counterUseDS>

    <counterMultipleInit>   ::= "{" <counterItemList> "}"

    <counterItemList>       ::= <counterUseDM>
    <counterItemList>       ::= <counterUseDM> "," <counterItemList>

    <counterUseV>           ::= <counterVarName> <optArrayMem>

    <counterUseDS>          ::= <counterBufBinding> <arrayMemAbs>

    <counterUseDM>          ::= <counterUseDS>
    <counterUseDM>          ::= <counterBufBinding>
    <counterUseDM>          ::= <counterBufBinding> <arrayRange>

    <counterBufBinding>     ::= "program" "." "counter" <arrayMemAbs>


    Add New Section 2.X.3.Y, Atomic Counter Buffers, after Section 2.X.3.6,
    Program Parameter Buffers

    Atomic counter buffers are arrays consisting of single-component unsigned
    integer values stored in a buffer object.  The GL provides an
    implementation-dependent number of atomic counter buffer object binding
    points to which buffer objects can be attached.  Atomic counter variables
    can be changed either by updating the contents of bound buffer objects, by
    changing the buffer object attached to a binding point, or by using the
    ATOMCTR instruction in a shader to increment or decrement the current
    value.

    Atomic counter buffer bindings are established by calling BindBufferBase,
    BindBufferOffsetEXT, or BindBufferRange with a target of
    ATOMIC_COUNTER_BUFFER, as documented in the ARB_shader_atomic_counters
    extension.  The number of atomic counter buffer object binding points is
    given by the value of the constant MAX_ATOMIC_COUNTER_BUFFER_BINDINGS.
    There is a limit on the maximum number of words of a buffer object that
    can be accessed using any single parameter buffer binding point, given by
    the implementation-dependent constant MAX_ATOMIC_COUNTER_BUFFER_SIZE.
    Buffer objects larger than this size may be used, but the results of
    accessing portions of the buffer object beyond the limit are undefined.

    Atomic counter variables may only be used as operands in the ATOMCTR
    instruction; they may not be used by used as results or operands in
    general instructions.  Atomic counter variables must be declared
    explicitly via the <COUNTER_statement> grammar rule.  Atomic counter
    buffer bindings can not be used directly in executable instructions.

    Atomic counter variables may be declared as arrays, but all bindings
    assigned to the array must use the same binding point and must increase
    consecutively.

      Binding                        Components  Underlying State
      -----------------------------  ----------  -----------------------------
      program.counter[a][b]          (x,x,x,x)   atomic counter buffer a,
                                                   element b
      program.counter[a][b..c]        (x,x,x,x)  atomic counter buffer a,
                                                   elements b through c
      program.counter[a]              (x,x,x,x)  atomic counter buffer a,
                                                   all elements

      Table X.Y: Atomic Counter Buffer Bindings.  <a> indicates a buffer
      number, <b> and <c> indicate individual elements.

    If an atomic counter buffer binding matches "program.counter[a][b]", the
    atomic counter variable is associated with element <b> of the buffer
    object bound to binding point <a>.  If no buffer object is bound to
    binding point <a>, or the bound buffer object is not large enough to hold
    an element <b>, reads from the binding return zero and writes to the
    binding have no effect.  The binding point <a> must be a nonnegative
    integer constant.

    For atomic counter array declarations, "program.counter[a][b..c]" is
    equivalent to specifying elements <b> through <c> of the buffer object
    bound to binding point <a> in order.

    For atomic counter array declarations, "program.counter[a]" is equivalent
    to specifying the entire buffer -- elements 0 through <n>-1, where <n> is
    either the size of the array (if declared) or the implementation-dependent
    maximum atomic counter buffer object size limit (if no size is declared).


    Modify Section 2.X.4, Program Execution Environment

    (Add to the set of opcodes in Table X.13)

                  Modifiers 
      Instruction F I C S H D  Out Inputs    Description
      ----------- - - - - - -  --- --------  --------------------------------
      ATOMCTR     - X X - - U  s   -         atomic counter operation


    Modify Section 2.X.4.1, Program Instruction Modifiers

    (Add to Table X.14, Instruction Modifiers, and to the corresponding
    description following the table)

      Modifier  Description
      --------  -----------------------------------------------
      INCR      Increment an atomic counter variable
      DECR      Decrement an atomic counter variable
      GET       Query the value of an atomic counter variable

    For atomic counter instructions, the "INCR", "DECR", and "GET" atomic
    counter instruction modifiers specify an operation to perform on an atomic
    counter variable.  Atomic counter instruction modifiers are supported by
    the ATOMCTR instruction and are covered in more detail in the description
    of that instruction.

    Modify Section 2.X.6, Program Options

    + Atomic Counter Support (NV_shader_atomic_counters)

    If a program specifies the "NV_shader_atomic_counters" option, it may use
    the COUNTER statement to declare atomic counter variables and the ATOMCTR
    instruction to increment, decrement, or query the values of atomic counter
    variables.


    Modify Section 2.X.8, Program Instruction Set

    Section 2.X.8.Z, ATOMCTR:  Atomic Counter Operation

    The ATOMCTR instruction performs an atomic global counter operation by
    reading from memory corresponding to the atomic counter variable specified
    in the instruction, optionally modifying that memory, and then returning
    an scalar unsigned integer result.  The memory transaction is atomic,
    guaranteeing that no other write to the memory accessed will occur between
    the time it is read and written by the ATOMCTR instruction.

    The ATOMCTR instruction requires an atomic counter instruction modifier,
    as enumerated in Table X.Y.  The instruction modifier specifies the type
    of operation to perform.

      atomic
      modifier   operation
      --------   -------------------------------------------------
       INCR      increment counter (returns non-incremented value)
       DECR      decrement counter (returns decremented value)
       GET       query counter

     Table X.Y, Supported atomic and storage modifiers for the ATOM
     instruction.

    ATOMCTR performs a scalar atomic operation and returns an atomic counter
    value in the <x> component of the result vector.  The <y>, <z>, and <w>
    components of the result vector are undefined.
      
      address = AtomicCounterAddress(op0);
      result = BufferMemoryLoad(address, U32);
      switch (atomicModifier) {
      case INCR:
        writeval = result + 1;
        break;
      case DECR:
        writeval = result - 1;
        result = writeval;   // return decremented value
        break;
      case GET:
        return result;  // no memory store
      BufferMemoryStore(address, writeval, U32);

    If an incremented or decremented counter value overflows the representable
    range of counter values, it will wrap.

    ATOMCTR supports only signed and unsigned integer data type modifiers, but
    no storage modifers.  The memory holding the atomic counter and the result
    vector are treated as scalar signed or unsigned integers, according to the
    data type modifier.


Additions to Chapter 3 of the OpenGL 2.0 Specification (Rasterization)

    None.

Additions to Chapter 4 of the OpenGL 2.0 Specification (Per-Fragment
Operations and the Frame Buffer)

    None.
Additions to Chapter 5 of the OpenGL 2.0 Specification (Special Functions)

    None.

Additions to Chapter 6 of the OpenGL 2.0 Specification (State and
State Requests)

    None.

Additions to Appendix A of the OpenGL 2.0 Specification (Invariance)

    None.

Additions to the AGL/GLX/WGL Specifications

    None.

Errors

    None.

New State

    None.

New Implementation Dependent State

    None.

Issues

    (1) Should this extension have its own extension string entry, or should
        its existence be implied by the presence of the
        ARB_shader_atomic_counters and NV_gpu_program5 extensions?

      RESOLVED:  Provide a separate extension string entry.

Revision History

    Rev.    Date    Author    Changes
    ----  --------- --------  --------------------------------------------
     1    20-Jun-11 pbrown    Internal spec development.