Name

    ARB_gl_spirv

Name Strings

    GL_ARB_gl_spirv

Contributors

    John Kessenich, Google
    Daniel Koch, NVIDIA
    Christophe Riccio, Unity
    Graham Sellers, AMD

Contact Point for Bug Reports

    https://www.khronos.org/bugzilla/enter_bug.cgi?product=OpenGL

Notice

    Copyright (c) 2015-2016 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete.
    Ratified by the Khronos Board of Promoters on July 22, 2016.

Version

    Last Modified Date: 21-Jul-2016
    Revision: 31

Number

    ARB Extension #190

Dependencies

    This extension requires version 3.3 or later of The OpenGL Graphics System.
    (It is not written for OpenGL ES.)

    This extension is written against the following specifications:
      - The GL_KHR_vulkan_glsl extension, Version 30, April 12, 2016.
      - The OpenGL Graphics System, Version 4.5, Core Profile, May 28, 2015.
      - The OpenGL Shading Language, Version 4.50, Revision 6, April 14, 2016.
      - SPIR-V Specification, Version 1.00, Revision 5

    This extension interacts with ARB_parallel_shader_compile.

    This extension interacts with ARB_separate_shader_objects.

    This extension interacts with ARB_program_interface_query.

Overview

    This is version 100 of the GL_ARB_gl_spirv extension.

    This extension does two things:

    1. Allows a SPIR-V module to be specified as containing a programmable
       shader stage, rather than using GLSL, whatever the source language
       was used to create the SPIR-V module.

    2. Modifies GLSL to be a source language for creating SPIR-V modules
       for OpenGL consumption. Such GLSL can be used to create such SPIR-V
       modules, outside of the OpenGL runtime.

    Enabling GLSL SPIR-V Features
    -----------------------------

    This extension is not enabled with a #extension as other extensions are.
    It is also not enabled through use of a profile or #version.  The intended
    level of GLSL features comes from the traditional use of #version, profile,
    and #extension.

    Instead, use of this extension is an effect of using a GLSL front-end in a
    mode that has it generate SPIR-V for OpenGL.  Such tool use is outside the
    scope of using the OpenGL API and outside the definition of GLSL and this
    extension. See the documentation of the compiler to see how to request
    generation of SPIR-V for OpenGL.

    When a front-end is used to accept the GLSL features in this extension, it
    must error check and reject shaders not adhering to this specification, and
    accept those that do.  Implementation-dependent maximums and capabilities
    are supplied to, or part of, the front-end, so it can do error checking
    against them.

    A shader can query the level of GLSL SPIR-V support available, using the
    predefined

        #define GL_SPIRV 100

    This allows shader code to say, for example,

        #ifdef GL_SPIRV
            layout(constant_id = 11) const int count = 4;
            #if GL_SPIRV > 100
                ...
            #endif
        #else
            const int count = 6;
        #endif

    SPIR-V Modules
    --------------

    An entire stage is held in a single SPIR-V module.  The SPIR-V model is
    that multiple (e.g., GLSL) compilation units forming a single stage
    in a high-level language are all compiled and linked together to form a
    single entry point (and its call tree) in a SPIR-V module.  This would be
    done prior to using the OpenGL API to create a program object containing the
    stage.

    The OpenGL API expects the SPIR-V module to have already been validated,
    and can return an error if it discovers anything invalid in
    the module.  An invalid SPIR-V module is allowed to result in undefined
    behavior.

    Specialization Constants
    ------------------------

    SPIR-V specialization constants, which can be set later by the client API,
    can be declared using "layout(constant_id=...)". For example, to make a
    specialization constant with a default value of 12:

        layout(constant_id = 17) const int arraySize = 12;

    Above, "17" is the ID by which the API or other tools can later refer to
    this specific specialization constant.  The API or an intermediate tool can
    then change its value to another constant integer before it is fully
    lowered to executable code.  If it is never changed before final lowering,
    it will retain the value of 12.

    Specialization constants have const semantics, except they don't fold.
    Hence, an array can be declared with 'arraySize' from above:

        vec4 data[arraySize];  // legal, even though arraySize might change

    Specialization constants can be in expressions:

        vec4 data2[arraySize + 2];

    This will make data2 be sized by 2 more than whatever constant value
    'arraySize' has when it is time to lower the shader to executable code.

    An expression formed with specialization constants also behaves in the
    shader like a specialization constant, not a like a constant.

        arraySize + 2       // a specialization constant (with no constant_id)

    Such expressions can be used in the same places as a constant.

    The constant_id can only be applied to a scalar *int*, a scalar *float*
    or a scalar *bool*.

    Only basic operators and constructors can be applied to a specialization
    constant and still result in a specialization constant:

        layout(constant_id = 17) const int arraySize = 12;
        sin(float(arraySize));    // result is not a specialization constant

    While SPIR-V specialization constants are only for scalars, a vector
    can be made by operations on scalars:

        layout(constant_id = 18) const int scX = 1;
        layout(constant_id = 19) const int scZ = 1;
        const vec3 scVec = vec3(scX, 1, scZ);  // partially specialized vector

    A built-in variable can have a 'constant_id' attached to it:

        layout(constant_id = 18) gl_MaxImageUnits;

    This makes it behave as a specialization constant.  It is not a full
    redeclaration; all other characteristics are left intact from the
    original built-in declaration.

    The built-in vector gl_WorkGroupSize can be specialized using special
    layout local_size_{xyz}_id's applied to the "in" qualifier.  For example:

        layout(local_size_x_id = 18, local_size_z_id = 19) in;

    This leaves gl_WorkGroupSize.y as a non-specialization constant, with
    gl_WorkGroupSize being a partially specialized vector.  Its x and z
    components can be later specialized using the ID's 18 and 19.

    gl_FragColor
    ------------

    The fragment-stage built-in gl_FragColor, which implies a broadcast to all
    outputs, is not present in SPIR-V. Shaders where writing to gl_FragColor
    is allowed can still write to it, but it only means to write to an output:
     - of the same type as gl_FragColor
     - decorated with location 0
     - not decorated as a built-in variable.
    There is no implicit broadcast.

    Mapping to SPIR-V
    -----------------

    For informational purposes (non-specification), the following is an
    expected way for an implementation to map GLSL constructs to SPIR-V
    constructs:

    Mapping of Storage Classes:

      uniform sampler2D...;        -> UniformConstant
      uniform variable (non-block) -> UniformConstant
      uniform blockN { ... } ...;  -> Uniform, with Block decoration
      in / out variable            -> Input/Output, possibly with block (below)
      in / out block...            -> Input/Output, with Block decoration
      buffer  blockN { ... } ...;  -> Uniform, with BufferBlock decoration
      ... uniform atomic_uint ...  -> AtomicCounter
      shared                       -> Workgroup
      <normal global>              -> Private

    Mapping of input/output blocks or variables is the same for all versions
    of GLSL. To the extent variables or members are available in a
    version and a stage, its location is as follows:

      These are mapped to SPIR-V individual variables, with similarly spelled
      built-in decorations (except as noted):

        General:

            in gl_VertexIndex
            in gl_InstanceIndex
            in gl_InvocationID
            in gl_PatchVerticesIn      (PatchVertices)
            in gl_PrimitiveIDIn        (PrimitiveID)
            in/out gl_PrimitiveID      (in/out based only on storage qualifier)
            in gl_TessCoord

            in/out gl_Layer
            in/out gl_ViewportIndex

            patch in/out gl_TessLevelOuter  (uses Patch decoration)
            patch in/out gl_TessLevelInner  (uses Patch decoration)

        Compute stage only:

            in gl_NumWorkGroups
            in gl_WorkGroupSize
            in gl_WorkGroupID
            in gl_LocalInvocationID
            in gl_GlobalInvocationID
            in gl_LocalInvocationIndex

        Fragment stage only:

            in gl_FragCoord
            in gl_FrontFacing
            in gl_ClipDistance
            in gl_CullDistance
            in gl_PointCoord
            in gl_SampleID
            in gl_SamplePosition
            in gl_HelperInvocation
            out gl_FragDepth
            in gl_SampleMaskIn        (SampleMask)
            out gl_SampleMask         (in/out based only on storage qualifier)

      These are mapped to SPIR-V blocks, as implied by the pseudo code, with
      the members decorated with similarly spelled built-in decorations:

        Non-fragment stage:

            in/out gl_PerVertex {
                gl_Position
                gl_PointSize
                gl_ClipDistance
                gl_CullDistance
            }                        (name of block is for debug only)

      There is at most one input and one output block per stage in SPIR-V.

    Mapping of precise:

      precise -> NoContraction

    Mapping of atomic_uint /offset/ layout qualifier

      offset         ->  Offset (decoration)

    Mapping of images

      imageLoad()   -> OpImageRead
      imageStore()  -> OpImageWrite
      texelFetch()  -> OpImageFetch

      imageAtomicXXX(params, data)  -> %ptr = OpImageTexelPointer params
                                              OpAtomicXXX %ptr, data

      XXXQueryXXX(combined) -> %image = OpImage combined
                                        OpXXXQueryXXX %image

    Mapping of layouts

      std140/std430  ->  explicit *Offset*, *ArrayStride*, and *MatrixStride*
                         Decoration on struct members
      shared/packed  ->  not allowed
      <default>      ->  not shared, but std140 or std430
      xfb_offset     ->  *Offset* Decoration on the object or struct member
      xfb_buffer     ->  *XfbBuffer* Decoration on the object
      xfb_stride     ->  *XfbStride* Decoration on the object
      any xfb_*      ->  the *Xfb* Execution Mode is set
      captured XFB   ->  has both *XfbBuffer* and *Offset*
      non-captured   ->  lacking *XfbBuffer* or *Offset*

      max_vertices   ->  OutputVertices

    Mapping of other instructions

      %     -> OpUMod/OpSMod
      mod() -> OpFMod
      NA    -> OpSRem/OpFRem

      atomicExchange()      -> OpAtomicExchange
      imageAtomicExchange() -> OpAtomicExchange
      atomicCompSwap()      -> OpAtomicCompareExchange
      imageAtomicCompSwap() -> OpAtomicCompareExchange
      NA                    -> OpAtomicCompareExchangeWeak

      pack/unpack (conversion)    -> pack/unpack in GLSL extended instructions
      pack/unpack (no conversion) -> OpBitcast

    Differences Relative to Other Specifications
    --------------------------------------------

    The following summarizes the set of differences to existing specifications.

    Additional use of existing SPIR-V features, relative to Vulkan:
      + The *UniformConstant* Storage Class can be used on individual
        variables at global scope. (That is, uniforms don't have to be in a
        block, unless they are built-in members that are in block in GLSL
        version 4.5.)
      + *AtomicCounter* Storage Class can use the *Offset* decoration
      + OriginLowerLeft

    Corresponding features that GLSL keeps, despite GL_KHR_vulkan_glsl removal:
      . default uniforms (those not inside a uniform block)
      . atomic-counter bindings have the /offset/ layout qualifier
      . special rules for locations for input doubles in the vertex shader
      . origin_lower_left

    Addition of features to GLSL:
      + specialization constants, as per GL_KHR_vulkan_glsl
      + #define GL_SPIRV 100, when compiled for OpenGL and SPIR-V generation

    Non-acceptance of SPIR-V features, relative to Vulkan:
      - VertexIndex and InstanceIndex built-in decorations cannot be used
      - Push-constant buffers cannot be used
      - *DescriptorSet* must always be 0, if present
      - input targets and input attachments
      - OpTypeSampler

    Corresponding features not added to GLSL that GL_KHR_vulkan_glsl added:
      . gl_VertexIndex and gl_InstanceIndex
          (gl_VertexID and gl_InstanceID have same semantics as in GLSL)
      . push_constant buffers
      . descriptor sets
      . input_attachment_index and accessing input targets
      . shader-combining of textures and samplers

    Removal of features from GLSL, as removed by GL_KHR_vulkan_glsl:
      - subroutines
      - the already deprecated texturing functions (e.g., texture2D())
      - compatibility profile features
      - DepthRangeParameters
      - *shared* and *packed* block layouts

    Addition of features to The OpenGL Graphics System:
      + a command to associate a SPIR-V module with a program (ShaderBinary)
      + a command to select a SPIR-V entry point and set specialization
        constants in a SPIR-V module (SpecializeShaderARB)
      + a new appendix for SPIR-V validation rules, precision, etc.

    Changes of system features, relative to Vulkan:
      . Vulkan uses only one binding point for a resource array, while OpenGL
        still uses multiple binding points, so binding numbers are counted
        differently for SPIR-V used in Vulkan and OpenGL
      . gl_FragColor can be written to, but it won't broadcast, for versions of
        OpenGL that support gl_FragColor

    Additions to the SPIR-V specification:
      + *Offset* can also apply to an object, for transform feedback.
      + *Offset* can also apply to a default uniform, for atomic_uint offset.

New Procedures and Functions

    void SpecializeShaderARB(uint shader,
                             const char* pEntryPoint,
                             uint numSpecializationConstants,
                             const uint* pConstantIndex,
                             const uint* pConstantValue);

New Tokens

    Accepted by the <binaryformat> parameter of ShaderBinary:

        SHADER_BINARY_FORMAT_SPIR_V_ARB         0x9551

    Accepted by the <pname> parameter of GetShaderiv:

        SPIR_V_BINARY_ARB                       0x9552

Modifications to Chapter 7 of the OpenGL 4.5 (Core Profile) Specification
(Programs and Shaders)

  Additions to overview of Chapter 7, "Programs and Shaders"

    Modify the 4th paragraph beginning with "Alternatively, pre-compiled
    shader binary code..." as follows:

    "Alternatively, pre-compiled shader binary code can be loaded
    into a shader object. A SPIR-V module can also be associated with a
    shader and then _specialized_. An implementation must..."

  Additions to Section 7.1, "Shader Objects":

    Add the following to the end of the description of ShaderSource:

    "If <shader> was previously associated with a SPIR-V module (via the
    ShaderBinary command), that association is broken. Upon successful
    completion of this command the SPIR_V_BINARY_ARB state of <shader>
    is set to FALSE."

    Add a new error for the CompileShader command:

    "An INVALID_OPERATION error is generated if the SPIR_V_BINARY_ARB
    state of <shader> is TRUE."

    [[ if ARB_parallel_shader_compile is supported: ]]
    Modify the preamble to the MaxShaderCompilerThreadsARB command to read:

    "Applications may use the following to hint to the driver the maximum
    number of background threads it would like to be used in the process
    of compiling or specializing shaders, or linking programs, ..."

  Additions to Section 7.2, "Shader Binaries":

    In the description of ShaderBinary, remove the text stating that GL
    defines no specific binary formats and replace it with the following:

    "GL defines an execution environment for shaders created from SPIR-V
    modules. To load a SPIR-V binary into GL, set <binaryformat> to
    SHADER_BINARY_FORMAT_SPIR_V_ARB. <binary> should point to the start
    of a valid SPIR-V module binary and <length> should contain the length
    of that binary, in bytes. Upon successful consumption of the SPIR-V
    module:
    * each entry of <shaders> will be associated with that SPIR-V module,
    * the SPIR_V_BINARY_ARB state of each shader is set to TRUE,
    * the COMPILE_STATUS of each of these shaders is set to FALSE,
    * any existing source string (specified by ShaderSource) is removed, and
    * any information about a previous compile is lost.
    Shaders associated with SPIR-V modules must be finalized by calling
    SpecializeShaderARB as described in Section 7.2.1.

    GL also provides a mechanism to obtain token values for additional
    binary formats provided by extensions. The number of binary formats..."

    Replace the error condition for ShaderBinary which states:

    "An INVALID_OPERATION error is generated if more than one of the
    handles in shaders refers to the same type of shader object."

    with the following:

    "An INVALID_OPERATION error is generated if <binaryformat> is not
    SHADER_BINARY_FORMAT_SPIR_V_ARB and more than one of the
    handles in shaders refers to the same type of shader object."

  Insert Subsection 7.2.1, "Shader Specialization":

    "Shaders associated with SPIR-V modules must be specialized before they
    can be linked into a program object. It is not necessary to specialize
    the shader before it is attached to a program object. Once specialized,
    a shader may not be specialized again without first re-associating the
    original SPIR-V module with it, through ShaderBinary.

    Specialization does two things:

        * Selects the name of the entry point, for that shader's stage, from
          the SPIR-V module.
        * Sets the values of all, or a subset of, the specialization constants
          in the SPIR-V module.

    To specialize a shader created from a SPIR-V module, call:

        void SpecializeShaderARB(uint shader,
                                 const char* pEntryPoint,
                                 uint numSpecializationConstants,
                                 const uint* pConstantIndex,
                                 const uint* pConstantValue);

    <shader> is the name of a shader object containing unspecialized SPIR-V
    as created from a successful call to ShaderBinary to which a SPIR-V
    module was passed. <pEntryPoint> is a pointer to a NUL-terminated UTF-8
    string specifying the name of the entry point in the SPIR-V module to
    use for this shader. <numSpecializationConstants> is the number
    of specialization constants whose values to set in this call.
    <pConstantIndex> is a pointer to an array of
    <numSpecializationConstants> unsigned integers, each holding the index
    of a specialization constant in the SPIR-V module whose value to set.
    The corresponding entry in <pConstantValue> is used to set the value of
    the specialization constant indexed by the entry in <pConstantIndex>.
    Although this array is of unsigned integer, each entry is bitcast to the
    appropriate type for the module, and therefore, floating-point constants
    may be set by including their IEEE-754 bit representation in the
    <pConstantValue> array. Specialization constants not referenced by
    <pConstantIndex> retain their default values as specified in the SPIR-V
    module.

    On successful shader specialization, the compile status for <shader>
    is set to TRUE. On failure, the compile status for <shader> is set to
    FALSE and additional information about the cause of the failure may
    be available in the shader compilation log. Specialization can fail
    if the SPIR-V module fails to meet the requirements listed in Appendix
    "The OpenGL SPIR-V Execution Environment".

        Errors

        An INVALID_VALUE error is generated if <shader> is not the name
        of either a program or shader object.

        An INVALID_OPERATION error is generated if <shader> is the name
        of a program object.

        INVALID_OPERATION is generated if the value of SPIR_V_BINARY_ARB
        for <shader> is not TRUE, or if the shader has already been
        specialized.

        INVALID_VALUE is generated if <pEntryPoint> does not name a valid
        entry point for <shader>.

        INVALID_VALUE is generated if any element of <pConstantIndex>
        refers to a specialization constant that does not exist in the
        shader module contained in <shader>."

  Additions to Section 7.3, "Program Objects":

    Modify the first paragraph after the AttachShader command to read:

    "Shader objects may be attached to program objects before source code
    or shader binary has been loaded into the shader object, or before the
    shader object has been compiled or specialized. Multiple shader ..."

    Modify the first paragraph after the LinkProgram command to read:

    "Linking can fail for a variety of reasons as specified in the OpenGL
    Shading language specification for source shaders, or if the requirements
    in the Appendix "The OpenGL SPIR-V Execution Environment" are not met
    for SPIR-V shaders, as well as any of the following reasons..."

    Modify the second bullet point in the list of reasons LinkProgram can
    fail:

    "One or more of the shader objects attached to <program> are not compiled
    or specialized successfully."

    Add a new bullet point to this list:

    "All the shader objects attached to <program> do not have the same value
    for the SPIR_V_BINARY_ARB state."

  Modifications to Section 7.3.1, "Program Interfaces"

    Add the following paragraph after the list of rules describing how the
    name string is generated, before the paragraph that begins "The order
    of the active resource list...":

    "For shaders constructed from SPIR-V binaries (that is with a state of
    SPIR_V_BINARY_ARB equal to TRUE), variables may not have names associated
    with them as the OpName and OpMemberName debug instructions are optional
    and may not be present in a SPIR-V module. When the Op*Name instructions
    are present, it is implementation-dependent if these are reported via
    the name reflection APIs. If no _name reflection information_ is
    available, the name string associated with each active variable is the
    empty string (""). In this case, any queries that operate with a name
    as input, will return INVALID_INDEX or -1 as appropriate, and any queries
    that return information about the name of a resource will report a
    name length of one (for the null character) and return an empty string
    with a length of zero."

  Add a new subjection "7.4.spv, SPIR-V Shader Interface Matching" after
  section 7.4.1 "Shader Interface Matching":

    "7.4.spv, SPIR-V Shader Interface Matching

    SPIR-V shaders must also follow the rules in this section, whether they
    add to or override those given in section 7.4.1 "Shader Interface
    Matching."  Most importantly, SPIR-V variables and structure members
    do not have names and so no interface matching is done by name strings.

    All variables forming the input or output interfaces of shader stages
    must be listed as operands to the *OpEntryPoint* instruction and are
    declared with the *Input* or *Output* Storage Classes, respectively,
    in the SPIR-V module.

    Shader built-in variables meeting the following requirements define the
    built-in interface block. They must:
      - be explicitly declared (there are no implicit built-ins),
      - be decorated with a *BuiltIn* variable,
      - be declared in a block whose top-level members are the built-ins.
      - They must not have any *Location* or *Component* decorations.
    Built-ins only participate in interface matching if they are declared in
    such a block. There must be no more than one built-in interface block
    per shader per interface.

    User-defined interface variables must be decorated with a *Location*
    and can also be decorated with a *Component*. These correspond to the
    location and component discussed in section 7.4.1 "Shader Interface
    Matching".

    A user-defined output variable is considered to match an input variable
    in the subsequent stage only if the two variables are declared with the
    same *Location* and *Component* decoration and match in type and
    decoration, except that interpolation decorations are not required to
    match.

    Variables or block members declared as structures are considered to
    match in type if and only if the structure members match in type,
    decoration, number, and declaration order. Variables or block members
    declared as arrays are considered to match in type only if both
    declarations specify the same element type and size.

    At an interface between two non-fragment shader stages, the built-in
    interface block must match exactly, as described above. At an interface
    involving the fragment shader inputs, the presence or absence of any
    built-in output does not affect the interface matching.

    At an interface between two shader stages, the user-defined variable
    interface must match exactly. Additionally, scalar and vector inputs
    are well-defined if there is a corresponding output satisfying all of
    the following conditions:
     - the input and output match exactly in decoration,
     - the output is a vector with the same basic type and has at least as
       many components as the input, and
     - the common component type of the input and output is 32-bit integer
       or floating-point (64-bit component types are excluded).
    In this case, the components of the input will be taken from the first
    components of the output, and any extra components of the output will
    be ignored."

  Add a new subsection 7.6.2.spv "SPIR-V Uniform Offsets and Strides" after
  section 7.6.2.2 "Standard Uniform Block Layout":

    "7.6.2.spv SPIR-V Uniform Offsets and Strides

    The SPIR-V decorations *GLSLShared* or *GLSLPacked* must not be used. A
    variable in the *Uniform* Storage Class decorated as a *Block* must be
    explicitly laid out using the *Offset*, *ArrayStride*, and *MatrixStride*
    decorations. If the variable is decorated as a *BufferBlock*, its offsets
    and strides must not contradict std430 alignment and minimum offset
    requirements.  Otherwise, its offsets and strides must not contradict
    std140 alignment and minimum offset requirements."

  Modifications to Section 7.13 "Shader, Program, and Program Pipeline Queries"

    Add the following to the list of <pname> that are accepted by GetShaderiv:

    "If <pname> is SPIR_V_BINARY_ARB, TRUE is returned if the shader has been
    successfully associated with a SPIR-V binary module by the ShaderBinary
    command and FALSE is returned otherwise."

    Modify the description of COMPILE_STATUS to be as follows:

    "If <pname> is COMPILE_STATUS, TRUE is returned if the shader was last
    compiled or specialized successfully, and FALSE is returned otherwise."

    [[ if ARB_parallel_shader_compile is supported: ]]
    Modify the description of COMPLETION_STATUS_ARB to be as follows:

    "If <pname> is COMPLETION_STATUS_ARB, FALSE is returned if the shader is
     currently being compiled or specialized by a background thread, TRUE
     otherwise."

    Modify the Errors list for GetShaderiv and add SPIR_V_BINARY_ARB to the
    list of <pname> that are accepted.

    In the description of the GetProgramiv command, modify the definitions of
    the ACTIVE_ATTRIBUTE_MAX_LENGTH, ACTIVE_UNIFORM_MAX_LENGTH,
    TRANSFORM_FEEDBACK_VARYING_MAX_LENGTH, and
    ACTIVE_UNIFORM_BLOCK_MAX_NAME_LENGTH <pname> values to be as follows:

    "If pname is ACTIVE_ATTRIBUTE_MAX_LENGTH, the length of the longest
    active attribute name, including a null terminator, is returned.
    If no active attributes exist, zero is returned. If no name reflection
    information is available, one is returned."

    "If pname is ACTIVE_UNIFORM_MAX_LENGTH, the length of the longest active
    uniform name, including a null terminator, is returned. If no active
    uniforms exist, zero is returned. If no name reflection information is
    available, one is returned."

    "If pname is TRANSFORM_FEEDBACK_VARYING_MAX_LENGTH, the length of the
    longest output variable name specified to be used for transform feedback,
    including a null terminator, is returned. If no outputs are used for
    transform feedback, zero is returned. If no name reflection information
    is available, one is returned."

    "If pname is ACTIVE_UNIFORM_BLOCK_MAX_NAME_LENGTH, the length of the
    longest active uniform block name, including the null terminator, is
    returned. If no active uniform blocks exist, zero is returned. If no
    name reflection information is available, one is returned."

    Modify the description of GetShaderInfoLog to read as follows:

    "If <shader> is a shader object, GetShaderInfoLog will return either
    an empty string or information about the last compilation or
    specialization attempt for that object."

  Modifications to Section 7.14 "Required State":

    Add to the list of state per shader object:

    "A boolean holding the SPIR-V binary status, initial FALSE."

Modifications to Chapter 11 of the OpenGL 4.5 (Core Profile) Specification
(Programmable Vertex Processing)

  Modify Section 11.1.1 "Vertex Attributes"

    Replace the last sentence in the first paragraph ("This binding can be..")
    with the following:

    "This binding can be specified in the shader text using the location
    qualifier, in a SPIR-V shader using the *Location* decoration, by the
    application before the program is linked, or automatically assigned by
    the GL when the program is linked."

    In the second paragraph of the definition of BindAttribLocation, replace
    occurrences of "shader text" with "shader text or SPIR-V binary".

    Add to the end of the third paragraph of the definition of
    BindAttribLocation:

    "BindAttribLocation has no effect on SPIR-V shaders as the locations
    must always be fully specified in the SPIR-V shader as described in
    section 11.1.1.1 (SPIR-V Vertex Input Interface)."

  Add the following new subsection to the end of section 11.1.1
  "Vertex Attributes":

    "Section 11.1.1.1 SPIR-V Vertex Input interface

    When a SPIR-V vertex stage is present, the vertex shader
    variables listed by *OpEntryPoint* with the *Input* Storage Class
    form the vertex input attribute interface. These inputs must be
    decorated with a *Location* and can also be decorated with a
    *Component* decoration. These correspond to the location and
    component layout discussed in section  11.1.1 "Vertex Attributes."

    The vertex shader input variables are matched only by the
    *Location* and *Component* decorations, and must have a corresponding
    attribute and binding in the pipeline."

  Modify section 11.1.2.1 "Output Variables"

    Replace the first sentence of the second paragraph describing transform
    feedback mode ("The set of variables to record...") with the following:

    "The set of variables to record can be specified in shader text using
    xfb_buffer, xfb_offset, or xfb_stride layout qualifiers. For SPIR-V
    shaders, these are specified by the *XfbBuffer*, *Offset*, and *XfbStride*
    decorations, respectively. When a SPIR-V entry point is using any of these
    for transform feedback, it must declare the *Xfb* Execution Mode."

    Modify the last paragraph of the definition of TransformFeedbackVaryings
    ("If the shader is used to record...") as follows:

    "If the shader used to record output variables for transform feedback
    varyings uses the xfb_buffer, xfb_offset, or xfb_stride layout qualifiers,
    or its SPIR-V entry point declares the *Xfb* Execution Mode, the values
    specified by TransformFeedbackVaryings are ignored, and the set of
    variables captured for transform feedback is instead derived from the
    specified layout qualifiers or SPIR-V decorations: outputs specifying both
    an *XfbBuffer* and an *Offset* are captured, while outputs not specifying
    both of these are not captured. Values are captured each time the shader
    writes to such a decorated object."

Modifications to Chapter 15 of the OpenGL 4.5 (Core Profile) Specification
(Programmable Fragment Processing)

  Modify section 15.2.3 "Shader Outputs"

    Replace the sentence leading up to the definition of the
    BindFragDataLocationIndexed command with the following:

    "The binding of a user-defined output variable to components of a
    fragment color number can be specified explicitly in the shader text,
    SPIR-V shader, or using the command ..."

    Modify the paragraph following the definition of BindFragDataLocation,
    replacing the second and third sentences with the following:

    "Output variables declared with location, component, or index layout
    qualifiers will use the values specified in the shader text. For SPIR-V
    shaders, these are specified by the *Location*, *Component*, and *Index*
    decorations. Output variables without such layout or decoration qualifiers
    will use the bindings specified by BindFragDataLocationIndexed or
    BindFragDataLocation, if any. BindFragDataLocation* has no effect on
    SPIR-V shaders as the locations must always be fully specified as
    described in section 15.2.3.1 (SPIR-V Fragment Output Interface)."

  Add the following new subsection to the end of 15.2.3 "Shader Outputs"

    "Section 15.2.3.1 "SPIR-V Fragment Output Interface"

    When a SPIR-V fragment stage is present, the variables listed by
    *OpEntryPoint* with the *Output* Storage Class form the fragment
    output interface. These variables must be decorated with a *Location*
    They can also be decorated with a *Component* and/or an *Index*.

    User-defined fragment shader output variables are matched only by their
    *Location*, *Component*, and *Index* decorations. If two outputs
    are placed within the same location, they must have the same underlying
    type (floating-point or integer). No component aliasing of output
    variables is allowed.  That is, there must not be two output variables
    which have the same location, component, and index, either explicitly
    declared or implied. Output values written by a fragment shader must
    be declared with either *OpTypeFloat* or *OpTypeInt*, and a *Width* of
    32. Composites of these types are also permitted."


Add Appendix A.spv "The OpenGL SPIR-V Execution Environment" to the OpenGL 4.5
(Core Profile) Specification

    "A.spv.1 (Required Versions and Formats)

    An implementation of this extension must support the 1.0 version of
    SPIR-V and the 1.0 version of the SPIR-V Extended Instructions for GLSL.

    A SPIR-V module passed into ShaderBinary is interpreted as a series of
    32-bit words in host endianness, with literal strings packed as described
    in section 2.2 of the SPIR-V Specification. The first few words of the
    SPIR-V module must be a magic number and a SPIR-V version number, as
    described in section 2.3 of the SPIR-V Specification.

    A.spv.2 (Valid SPIR-V Built-In Variable Decorations)

    Implementations supporting GL 4.5 must support the following
    built-in variable decorations. When used with earlier versions
    of GL, this list may be subset to the functionality in that
    version. Each built-in below lists the minimum OpenGL version
    (or extension) that is required to use the built-in variable
    decoration.


        Built-in Variable Decoration    Minimum GL version (Extension)
        ----------------------------    -----------------------------
        NumWorkgroups                   GL 4.3 (ARB_compute_shader)
        WorkgroupSize                   GL 4.3 (ARB_compute_shader)
        WorkgroupId                     GL 4.3 (ARB_compute_shader)
        LocalInvocationId               GL 4.3 (ARB_compute_shader)
        GlobalInvocationId              GL 4.3 (ARB_compute_shader)
        LocalInvocationIndex            GL 4.3 (ARB_compute_shader)

        VertexId
        InstanceId

        Position
        PointSize
        ClipDistance
        CullDistance                    GL 4.5 (ARB_cull_distance)

        PrimitiveId
        InvocationId                    GL 4.0 (ARB_tessellation_shader)

        Layer
        ViewportIndex                   GL 4.1 (ARB_viewport_array)

        PatchVertices                   GL 4.0 (ARB_tessellation_shader)
        TessLevelOuter                  GL 4.0 (ARB_tessellation_shader)
        TessLevelInner                  GL 4.0 (ARB_tessellation_shader)
        TessCoord                       GL 4.0 (ARB_tessellation_shader)

        FragCoord
        FrontFacing
        PointCoord
        SampleId                        GL 4.0 (ARB_sample_shading)
        SamplePosition                  GL 4.0 (ARB_sample_shading)
        SampleMask                      GL 4.0 (ARB_sample_shading)
        HelperInvocation                GL 4.5 (ARB_ES3_1_compatibility)
        FragDepth

    A.spv.3 (Valid SPIR-V Capabilities)

    Implementations supporting OpenGL 4.5 must support the following
    capability operands declared by OpCapability.

    When used against earlier versions of GL, this list may be subset
    to the functionality in that version. Each capability below lists
    the OpenGL version (or extension) that is required to use the
    OpCapability.

        OpCapability                        Minimum GL version (Extension)
        ------------------------------      -----------------------------
        Matrix
        Shader
        Geometry
        Tessellation                        GL 4.0 (ARB_tessellation_shader)
        Float64                             GL 4.0 (ARB_gpu_shader_fp64)
        AtomicStorage                       GL 4.2 (ARB_shader_atomic_counters)
        TessellationPointSize               GL 4.0 (ARB_tessellation_shader)
        GeometryPointSize
        ImageGatherExtended                 GL 4.0 (ARB_gpu_shader5)
        StorageImageMultisample             GL 4.2 (ARB_shader_image_load_store)
        UniformBufferArrayDynamicIndexing   GL 4.0 (ARB_gpu_shader5)
        SampledImageArrayDynamicIndexing    GL 4.0 (ARB_gpu_shader5)
        StorageBufferArrayDynamicIndexing   GL 4.3 (ARB_shader_storage_buffer_object)
        StorageImageArrayDynamicIndexing    GL 4.2 (ARB_shader_image_load_store)
        ClipDistance
        CullDistance                        GL 4.5 (ARB_cull_distance)
        ImageCubeArray                      GL 4.0 (ARB_texture_cube_map_array)
        SampleRateShading                   GL 4.0 (ARB_sample_shading)
        ImageRect
        SampledRect
        Sampled1D
        Image1D
        SampledCubeArray                    GL 4.0 (ARB_texture_cube_map_array)
        SampledBuffer
        ImageBuffer
        ImageMSArray
        StorageImageExtendedFormats         GL 4.2 (ARB_shader_image_load_store)
        ImageQuery
        DerivativeControl                   GL 4.5 (ARB_derivative_control)
        InterpolationFunction               GL 4.0 (ARB_gpu_shader5)
        TransformFeedback
        GeometryStreams                     GL 4.0 (ARB_gpu_shader5)
        StorageImageWriteWithoutFormat      GL 4.2 (ARB_shader_image_load_store)
        MultiViewport                       GL 4.1 (ARB_viewport_array)

    Implementations supporting ARB_gpu_shader_int64 must support the
    following operand declared by OpCapability:

        Int64

    Implementations supporting ARB_sparse_texture2 must support the
    following operand declared by OpCapability:

        SparseResidency

    Implementations supporting ARB_sparse_texture_clamp must support the
    following operand declared by OpCapability:

        MinLod

    Implementations supporting EXT_shader_image_load_formatted must
    support the following operand declared by OpCapability:

        StorageImageReadWithoutFormat

    Implementations supporting NV_shader_atomic_int64 must support the
    following operand declared by OpCapability:

        Int64Atomics

    A.spv.4 (Validation rules)

    (in addition to what's allowed/disallowed above)

    Every entry point must have no return value and accept no arguments.

    The *Logical* addressing model must be selected.

    *Scope* for execution must be limited to:
      - Workgroup
      - Subgroup

    *Scope* for memory must be limited to:
      - Device
      - Workgroup
      - Invocation

    Images:
      - OpTypeImage must declare a scalar 32-bit float or 32-bit integer
        type for the /Sampled Type/.
      - OpSampledImage must only consume an /Image/ operand whose type has
        its /Sampled/ operand set to 1.
      - The /Depth/ operand of OpTypeImage is ignored.

    Decorations:
      - The Flat, NoPerspective, Sample, and Centroid decorations must not
        be used on variables with Storage Class other than *Input* or on
        variables used in the interface of non-fragment shader entry points.
      - The Patch decoration must not be used on variables in the interface
        of a vertex, geometry, or fragment shader stage's entry point.
      - OpTypeRuntimeArray must only be used for the last member of an
        OpTypeStruct in the Uniform Storage Class.
      - If the *Xfb* Execution Mode is set, any output variable that is at
        least partially captured:
        * must be decorated with an *XfbBuffer*, declaring the capturing buffer
        * must have at least one captured output variable in the capturing
          buffer decorated with an *XfbStride* (and all such *XfbStride* values
          for the capturing buffer must be equal)
      - If the *Xfb* Execution Mode is set, any captured output:
        * must be a non-structure decorated with *Offset* or a member of a
          structure whose type member is decorated with *Offset*
          (not all members of such a struct need be captured)
        * must be a numerical type scalar, vector, matrix, or array of these
        * must have an *Offset* that is a multiple of its first component
        * must have an *Offset* that is a multiple of 8 if any captured output
          in the capturing buffer is a 64-bit type, in which case the
          corresponding *XfbStride* must also be a multiple of 8
        * must not overlap (i.e., alias in any capturing buffer) any other
          captured output
        * must not have an *Offset* that causes overflow of the *XfbStride*

    Compute Shaders
      - For each compute shader entry point, either a LocalSize execution
        mode or an object decorated with the WorkgroupSize decoration must
        be specified.

    Recursion:
     - For all entry points, the static function-call graph rooted at that
       entry point must not contain cycles.

    A.spv.5 (Precision and Operation of SPIR-V Instructions)

    The following rules apply to both single and double-precision floating
    point instructions:
     - Positive and negative infinities and positive and negative zeros are
       generated as dictated by [IEEE 754], but subject to the precisions
       allowed in the following table.
     - Dividing a non-zero by a zero results in the appropriately signed
       IEEE 754 infinity.
     - Any denormalized value input into a shader or potentially generated
       by any instruction in a shader may be flushed to 0.
     - The rounding mode cannot be set and is undefined.
     - NaNs are not required to be generated. Instructions that operate on
       a NaN are not required to return a NaN as the result.
     - Support for signaling NaNs is optional and exceptions are never
       raised.

    The precision of double-precision instructions is at least that of
    single precision. For single precision (32 bit) instructions,
    precisions are required to be at least as follows:

       Instruction                                           Precision
       -----------                                           ---------
       OpFAdd                                            Correctly rounded
       OpFSub                                            Correctly rounded
       OpFMul                                            Correctly rounded
       OpFOrdEqual, OpFUnordEqual                          Correct result
       OpFOrdLessThan, OpFUnordLessThan                    Correct result
       OpFOrdGreaterThan, OpFUnordGreaterThan              Correct result
       OpFOrdLessThanEqual, OpFUnordLessThanEqual          Correct result
       OpFOrdGreaterThanEqual, OpFUnordGreaterThanEqual    Correct result
       OpFDiv                                          2.5 ULP for b in the
                                                  range [2^(-126), 2^(126)]
       conversions between types                         Correctly rounded

    A.spv.6 (Precision of GLSL.std.450 Instructions)

       Instruction                                           Precision
       -----------                                           ---------
       fma()             Inherited from OpFMul followed by OpFAdd.
       exp(x), exp2(x)   (3+2*|x|)(3+2*|x|) ULP
       log(), log2()     3 ULP outside the range [0.5, 2.0]
                         absolute error < 2^(-21) inside the range [0.5, 2.0]
       pow(x, y)         Inherited from exp2(y * log2(x))
       sqrt()            Inherited from 1.0 / inversesqrt()
       inversesqrt()     2 ULP

    GLSL.std.450 extended instructions specifically defined in terms of
    the above instructions inherit the above errors. GLSL.std.450
    extended instructions not listed above and not defined in terms of the
    above have undefined precision. These include, for example, the
    trigonometric functions and determinant.

    For the OpSRem and OpSMod instructions, if either operand is negative
    the result is undefined.

Changes to The OpenGL Shading Language Specification, Version 4.50

  Changes to Chapter 1 of the OpenGL Shading Language 4.50 Specification
  (Introduction)

    Add a paragraph: "The OpenGL API will specify the OpenGL commands used to
    manipulate SPIR-V shaders.  Independent offline tool chains will compile
    GLSL down to the SPIR-V intermediate language. SPIR-V generation is not
    enabled with a #extension, #version, or a profile.  Instead, use of GLSL
    for SPIR-V is determined by offline tool-chain use. See the documentation
    of such tools to see how to request generation of SPIR-V for OpenGL."

    "GLSL -> SPIR-V compilers must be directed as to what SPIR-V *Capabilities*
    are legal at run-time and give errors for GLSL feature use outside those
    capabilities.  This is also true for implementation-dependent limits that
    can be error checked by the front-end against constants present in the
    GLSL source: the front-end can be informed of such limits, and report
    errors when they are exceeded."

  Changes to Chapter 3 of the OpenGL Shading Language 4.50 Specification
  (Basics)

    Add a new paragraph at the end of section "3.3 Preprocessor":  "When
    shaders are compiled for OpenGL SPIR-V, the following predefined macro is
    available:

        #define GL_SPIRV 100

  Changes to Chapter 4 of the OpenGL Shading Language 4.50 Specification
  (Variables and Types)

    Change section 4.3.3 Constant Expressions:

      Add a new very first sentence to this section:

        "SPIR-V specialization constants are expressed in GLSL as const, with
        a layout qualifier identifier of constant_id, as described in section
        4.4.x Specialization-Constant Qualifier."

      Add to this bullet

        "A constant expression is one of...
          * a variable declared with the const qualifier and an initializer,
            where the initializer is a constant expression"

      to make it say:

        "A constant expression is one of...
          * a variable declared with the const qualifier and an initializer,
            where the initializer is a constant expression; this includes both
            const declared with a specialization-constant layout qualifier,
            e.g., 'layout(constant_id = ...)' and those declared without a
            specialization-constant layout qualifier"

      Add to "including getting an element of a constant array," that

        "an array access with a specialization constant as an index does
        not result in a constant expression"

      Add to this bullet

        "A constant expression is one of...
          * the value returned by a built-in function..."

      to make it say:

        "A constant expression is one of...
          * for non-specialization-constants only: the value returned by a
            built-in function... (when any function is called with an argument
            that is a specialization constant, the result is not a constant
            expression)"

      Rewrite the last half of the last paragraph to be its own paragraph
      saying:

        "Non-specialization constant expressions may be evaluated by the
        compiler's host platform, and are therefore not required ...
        [rest of paragraph stays the same]"

      Add a paragraph

        "Specialization constant expressions are never evaluated by the
        front-end, but instead retain the operations needed to evaluate them
        later on the host."

    Add to the table in section 4.4 Layout Qualifiers:

                             | Individual Variable | Block | Allowed Interface
      ------------------------------------------------------------------------
      constant_id =          |     scalar only     |       |      const
      ------------------------------------------------------------------------

    (The other columns remain blank.)

    Also add to this table:

                         | Qualifier Only | Allowed Interface
      -------------------------------------------------------
      local_size_x_id =  |        X       |       in
      local_size_y_id =  |        X       |       in
      local_size_z_id =  |        X       |       in

    (The other columns remain blank.)

    Expand this sentence in section 4.4.1 Input Layout Qualifiers

      "Where integral-constant-expression is defined in section 4.3.3 Constant
      Expressions as 'integral constant expression'"

    to include the following:

      ", with it being a compile-time error for integer-constant-expression to
      be a specialization constant:  The constant used to set a layout
      identifier X in layout(layout-qualifier-name = X) must evaluate to a
      front-end constant containing no specialization constants."

    Add a new subsection at the end of section 4.4:

      "4.4.x Specialization-Constant Qualifier

      "Specialization constants are declared using "layout(constant_id=...)".
      For example:

        layout(constant_id = 17) const int arraySize = 12;

      "The above makes a specialization constant with a default value of 12.
      17 is the ID by which the API or other tools can later refer to
      this specific specialization constant. If it is never changed before
      final lowering, it will retain the value of 12. It is a compile-time
      error to use the constant_id qualifier on anything but a scalar bool,
      int, uint, float, or double.

      "Built-in constants can be declared to be specialization constants.
      For example,

        layout(constant_id = 31) gl_MaxClipDistances;  // add specialization id

      "The declaration uses just the name of the previously declared built-in
      variable, with a constant_id layout declaration.  It is a compile-time
      error to do this after the constant has been used: Constants are strictly
      either non-specialization constants or specialization constants, not
      both.

      "The built-in constant vector gl_WorkGroupSize can be specialized using
      the local_size_{xyz}_id qualifiers, to individually give the components
      an id. For example:

          layout(local_size_x_id = 18, local_size_z_id = 19) in;

      "This leaves gl_WorkGroupSize.y as a non-specialization constant, with
      gl_WorkGroupSize being a partially specialized vector.  Its x and z
      components can be later specialized using the ids 18 and 19.  These ids
      are declared independently from declaring the work-group size:

        layout(local_size_x = 32, local_size_y = 32) in;   // size is (32,32,1)
        layout(local_size_x_id = 18) in;                   // constant_id for x
        layout(local_size_z_id = 19) in;                   // constant_id for z

      "Existing rules for declaring local_size_x, local_size_y, and
      local_size_z are not changed by this extension. For the local-size ids,
      it is a compile-time error to provide different id values for the same
      local-size id, or to provide them after any use.  Otherwise, order,
      placement, number of statements, and replication do not cause errors.

      "Two arrays sized with specialization constants are the same type only if
      sized with the same symbol, involving no operations.

        layout(constant_id = 51) const int aSize = 20;
        const int pad = 2;
        const int total = aSize + pad; // specialization constant
        int a[total], b[total];        // a and b have the same type
        int c[22];                     // different type than a or b
        int d[aSize + pad];            // different type than a, b, or c
        int e[aSize + 2];              // different type than a, b, c, or d

      "Types containing arrays sized with a specialization constant cannot be
      compared, assigned as aggregates, declared with an initializer, or used
      as an initializer.  They can, however, be passed as arguments to
      functions having formal parameters of the same type.

      "Arrays inside a block may be sized with a specialization constant, but
      the block will have a static layout.  Changing the specialized size will
      not re-layout the block. In the absence of explicit offsets, the layout
      will be based on the default size of the array."

    Remove all mention of the 'shared' and 'packed' layout qualifiers.

    Change section 4.4.5 Uniform and Shader Storage Block Layout Qualifiers

      "The initial state of compilation is as if the following were declared:

        layout(std140, column_major) uniform;
        layout(std430, column_major) buffer;

  Changes to Chapter 5 of the OpenGL Shading Language 4.50 Specification
  (Operators and Expressions)

    Add a section at the end of section 5

      "5.x Specialization Constant Operations"

      Only some operations discussed in this section may be applied to a
      specialization constant and still yield a result that is as
      specialization constant.  The operations allowed are listed below.
      When a specialization constant is operated on with one of these
      operators and with another constant or specialization constant, the
      result is implicitly a specialization constant.

       - int(), uint(), and bool() constructors for type conversions
         from any of the following types to any of the following types:
           * int
           * uint
           * bool
       - vector versions of the above conversion constructors
       - allowed implicit conversions of the above
       - swizzles (e.g., foo.yx)
       - The following when applied to integer or unsigned integer types:
           * unary negative ( - )
           * binary operations ( + , - , * , / , % )
           * shift ( <<, >> )
           * bitwise operations ( & , | , ^ )
       - The following when applied to integer or unsigned integer scalar types:
           * comparison ( == , != , > , >= , < , <= )
       - The following when applied to the Boolean scalar type:
           * not ( ! )
           * logical operations ( && , || , ^^ )
           * comparison ( == , != )

  Changes to the Chapter 9 of the OpenGL Shading Language 4.50 Specification
  (Shading Language Grammar for Core Profile)

    Arrays can no longer require the size to be a compile-time folded constant
    expression.  Change

      | LEFT_BRACKET constant_expression RIGHT_BRACKET

    to

      | LEFT_BRACKET conditional_expression RIGHT_BRACKET

    and change

      | array_specifier LEFT_BRACKET constant_expression RIGHT_BRACKET

    to

      | array_specifier LEFT_BRACKET conditional_expression RIGHT_BRACKET

Changes to the SPIR-V specification

  Section 3.20. Decoration

    *Offset* can also apply to an object, for transform feedback.

    *Offset* can also apply to an object in AtomicCounter storage for an
    atomic counter.

Dependencies on ARB_parallel_shader_compile

    If ARB_parallel_shader_compile is supported, the shader specialization
    may occur on a background thread in the same manner that compiling and
    linking does.

Dependencies on ARB_separate_shader_objects

    If ARB_separate_shader_objects is not supported, ignore all references
    to separable program objects.

Dependencies on ARB_program_interface_query

    If ARB_prorgram_interface_query is not supported, ignore references
    to commands added by this extension, however other commands defined
    in terms of these functions still operate as specified before the
    addition of the program interface query extension.

New State

Add the following to Table 23.30 "Shader Object State"

Get Value         Type  Get Command  Initial Value Description           Sec
----------------- ----- ------------ ------------- --------------------- ---
SPIR_V_BINARY_ARB  B    GetShaderiv  FALSE         Shader is associated  7.2
                                                   with a SPIR-V module.

Usage Example

    const uint* pSpirVModule;   // This points to the SPIR-V code in memory
    uint spirVLength;           // Length of pSpirVModule in bytes.

    // Create the shader object.
    GLuint shader = glCreateShader(GL_FRAGMENT_SHADER);

    // Load the SPIR-V module into the shader object
    glShaderBinary(1, &shader,
                   GL_SHADER_BINARY_FORMAT_SPIR_V_ARB,
                   pSpirVModule, spirVLength);

    // This will now return FALSE
    GLint status;
    glGetShaderiv(shader, GL_COMPILE_STATUS, &status);

    // Specialize the shader
    const GLuint constantIndices[] = { 0, 7, 3 };
    const GLuint constantValues[] = { 1, 42, 0x3F800000 };
    glSpecializeShaderARB(shader,
                          "main",
                          3,
                          constantIndices,
                          constantValues);

    // This should now return TRUE
    glGetShaderiv(shader, GL_COMPILE_STATUS, &status);

    // Create a program, attach our shader to it, and link
    GLuint program = glCreateProgram();

    glAttachShader(program, shader);

    glLinkProgram(program);


Issues

1. Do we need separate extensions to expose legacy GLSL and ESSL legacy
   capabilities in SPIR-V?

    DISCUSSION:

    When GLSL was created, multiple extensions were simultaneously introduced
    to separate the API mechanism (ARB_shader_objects), the shading language
    (GL_ARB_shading_language_100) and the shader stages (GL_ARB_vertex_shader,
    GL_ARB_fragment_shader).

    We can expect that future versions of OpenGL will support SPIR-V
    as well but with a different set of capabilities.

    Additionally, it's probably enough for the ARB to only define a SPIR-V
    capabilities for OpenGL 4.5 but SPIR-V is not only going to be consumed by
    OpenGL implementation but also by engine tool chains which can't afford to
    only support the last version of OpenGL. As a result Unity would define its
    own private extensions at least to clearly define the capability sets our
    engine supports at time T, e.g.: GL_UNITY_spirv_webgl,
    GL_UNITY_spirv_base_level, GL_UNITY_spirv_base_compute,
    GL_UNITY_spirv_max_level, etc to target older versions of OpenGL without
    being forced to use a specific GLSL version.

    For example, GL_UNITY_spirv_base_compute has a feature subset between
    GLSL 420 + GL_ARB_shader_storage_buffer_object + GL_ARB_compute_shader +
    GL_ARB_shader_image_load_store and GLES 310 would correspond to what we
    call an OpenGL device level in Unity 5.1.

    We should except that many engines would have the need of such ISV
    specific capability sets. This said it's important for the Khronos Group
    to set future convergence point but OpenGL 4.5 is not realistic today, which
    is the domain where ISVs must work.

    Possible Resolution:
       Two extensions:
        - GL_ARB_program_binary_spirv to specify the mechanism.
        - GL_ARB_spir_v_gl_450 to specify the required SPIR-V capabilities for
        OpenGL 4.5.

    Key data needed: What will IHVs actually support?

    RESOLVED:

    Minimum: 3.3 for desktop
             3.1 for ES [ however, ES is not covered by this extension ]

    Need to add capabilities needed to distinguish between 3.3 and 4.5.

    See bug 14520 for data on fine-grained capabilities and other
    relevant details.

2. Do we want to look at SPIR-V as a shading language or a binary format?

    DISCUSSION:

    Practically, this issue is about whether we want to load SPIR-V
    through the shader object API, the program binary API or do we need
    a new construct. Unfortunately, there is a lot of issues with the
    two first options and the third might be a little too complex.

    Support through program binary API:
    - SPIR-V is the standard binary format
    - It makes SPIR-V a post LinkProgram construct
    - No location remapping is possible (glBindAttribLocation,
      glBindFragDataLocation[Indexed])
    - Can't support multiple OpEntryPoint instructions
    - Can't provide program arguments (alternative to #pragma)
    - Can't retrieve SPIR-V source. GL_NUM_PROGRAM_BINARY_FORMATS can
    enumerate multiple format but glGetProgramBinary doesn't allow us to
    select the format we want to query back.
    - Currently SPIR-V doesn't support multiple shader stages per module
    but this would be useful for linked programs.

    Support through shader object API:
    - OpenGL API allows to attach multiple shader objects but SPIR-V modules
    are fully contained: A single module define the entire stage.
    - SPIR-V modules don't have to support multiple stage per module but we
    lose the represent linked program and perform offline cross shader stage
    dead code elimination.
    - Need a new glCompileShader variant to support multiple entry-points
    (OpEntryPoint) or compilation arguments (OpCompileFlag)
    - glShaderSource doesn't allow to specify the shading language. Is looking
    at the first bytes to check the magic string okay?
    - Reflection may be striped from SPIR-V modules

    Support through new API, "modules":
    // The difference between a module and a shader:
    // - Can contain multiple entry points
    // - Could specify the language, support both GLSL and SPIR-V (maybe)
    // - Can contain multiple shader stages
    // - Cross shader stage dead code elimination could be perform offline. (How to hint the compiler? OpCompileFlag?)
    // - Could be faster to compile shader variants?
    // - Programs created with glLinkProgramModule don't need GL_PROGRAM_SEPARABLE
    // - No need to specify the shader stage at object creation, it's embedded in the SPIR-V module

    GLint length = 0;
    GLbyte* binary = LoadBinary(Path, &length);

    GLuint ModuleName = 0;
    glCreateModules(1, &ModuleName);
    glCompileModule(ModuleName, GL_SHADING_LANGUAGE_SPIR_V, &binary, &length);

    GLboolean HasReflection = GL_FALSE;
    glGetModuleiv(ModuleName, GL_MODULE_REFLECTION, &HasReflection);

    GLuint ProgramName[2];
    glCreatePrograms(2, &ProgramName[0]);

    assert(HasReflection == GL_TRUE);
    GLchar const * Strings[] = {"gl_Position", "block.Color"};
    glTransformFeedbackVaryings(ProgramName[0], 2, Strings, GL_INTERLEAVED_ATTRIBS);
    glBindAttribLocation(ProgramName[0], 0, "Position");
    glBindAttribLocation(ProgramName[0], 1, "Color");
    glBindFragDataLocation(ProgramName[0], 0, "Color");
    glLinkProgramModule(ProgramName[0], ModuleName, "mainA", "-pragma optimize(on)");

    glLinkProgramModule(ProgramName[1], ModuleName, "mainB", "-pragma optimize(on)");

    Other notes:

     - API doesn't require GLSL -> SPIR-V followed by getProgramBinary()
       * need an ID for specifying you want a SPIR-V binary?
        (christophe) GetProgramBinary doesn't do conversion, currently we would retrieve a SPIR-V
        is the program binary was a SPIR-V binary. However, that would prevent query a of native
        binary cache. I guess we could add a program parameter or specific that if we use
        GL_PROGRAM_BINARY_RETRIEVABLE_HINT GetProgramBinary return a native program binary cache,
        otherwise we get a SPIR-V binary back.

     - LoadProgramBinary() needs a flag to link by SSO or by name?
        * or does API spec. just say this comes from using SSO mode or...?
        (christophe) GL_PROGRAM_SEPARABLE doesn't change linking by name or by location.
        In GLSL 450, when locations are present, we link by location. Otherwise, we link by name.
        GL_PROGRAM_SEPARABLE protects the shader code against a backward compatibility change regarding
        gl_PerVertex block. With separated shader stages, the block must be declared in the shader will
        before it was optional. This behavior was requested by AMD during the separated program
        ratification process.

    Neither the program binary API or the shader object API
    seem to match SPIR-V features. Creating a new module API has potential but maybe
    we could build something around the shader object.

    RESOLVED: Use the existing ShaderBinary() and a new entry point,
    SpecializeShaderARB() to set specialization constants.

3. Allow more than one stage per SPIR-V module?

    DISCUSSION:

    SPIR-V supports multiple entry points per module, of any stage, or any
    number of instances of the same stage.  However, the knowledge of which
    of those might be used together lies outside the module.

    OpenGL supports both linked programs and separated programs. If we
    consider strict separated programs with a program per shader stage, we can
    rely on a SPIR-V module per program. However, linked programs allows
    combining multiple shader stages into a single program. Furthermore,
    linked programs are more efficient than separated programs as the
    implementation can perform cross shader stages optimizations and packing of
    inputs and outputs with linked programs.

    Allowing multiple shader stage in a module would profile a semantic for
    linked programs and would allow performing dead code elimination and input
    and output location packing offline.

    RESOLVED: Allow full generality of what SPIR-V supports, and have
    the new entry points in the API select the subset of a module that it
    cares about.

4. Do we want to expose multiple entry points?

    DISCUSSION:

    SPIR-V supports multiple entry points through the OpEntryPoint
    instruction. Do we want to expose this?

    A way to expose it would be to introduce a variant of glLinkProgram
    which takes the entry point main as an argument.

    Typically, engines such as UE4 and Unity are rebuilding a big shared shader
    library over and over again for each shader variation.

    While orthogonal to GL_ARB_program_instance, this feature would work
    extremely well with program instance for an application.

    We could defer but I'll rather have all the SPIR-V interactions
    mechanisms defined in one extension from start to provide first class
    citizenship to SPIR-V.

    RESOLVED: This is mostly a duplicate of Issue 3, and a single resolution
    for both is needed there in that one place.

5. Do we want to add support for "constantId" in OpenGL?

    RESOLVED: Have the new entry points in the API support specializing
    constants, as in Vulkan.

6. Do we want to support multiple modules per program

    DISCUSSION:

    A use case could be to compile subroutines into different modules
    and link a set of subroutines we want to use into a program canvas.
    In another use case, using multiple entry points we could build multiple
    code paths sharing the same shader library.

    SPIR-V for Vulkan currently expects fully linked stages.

    This could be part of a much needed rework or the subroutines.
    Okay to defer for now.

    RESOLVED: Fully linked modules only, no direct "subroutine" support.

7. Do we want to be able to provide compilation arguments?

    DISCUSSION:

    SPIR-V supports the OpCompileFlag instructions to store
    compilation arguments however OpenGL doesn't support compilation
    arguments. If we add a new variant of glLinkProgram, it could be an
    opportunity to add a compilation arguments parameter to glLinkProgram.

    Some use cases:
    - Compiling the shaders with different optimizations: It's currently
    supported with #pragma but SPIR-V doesn't contain preprocessor
    information
    - Vendors refused to fix GLSL bugs and there are a lot of them. The
    reason is that fixing these bugs could cause to break shader compilation
    hence applications. If we want to start a virtue circle, we need to
    give developers the opportunity to enforce conformant compilation.
    - Display on or off warnings in the compilation log.
    - All the cool stuff C++ compilers are using compilation arguments for.

    Note that the instruction OpCompileFlag was removed from SPIR-V, and
    a Bug 13418 "Need a way to control target-compiler behavior from the API"
    was deferred to a future release.

    RESOLVED: No flags in first release.

8. Do we want to build a reflection API on SPIR-V modules specifics?

    - Retrieve compilation arguments
    - Retrieve shader stages included
    - Retrieve the list of entry points
    - Retrieve the list of required capabilities

    Arguably, it's trivial to build a SPIR-V parser and figure this out
    ourselves.

    DISCUSSION:

    If drivers implement SPIR-V support by lowering it to the same
    representation that GLSL is lowered to, and that representation is the
    source of reflection information, much reflection would naturally be
    present.

    RESOLVED: First start without reflection, then add later if needed.
    In the meantime, we have to document how the existing API operates
    with no reflection information. See Issue 22.

9. Separate programs require gl_PerVertex output blocks to be declared.
   How do we enforce this requirement in SPIR-V modules?

    DISCUSSION: Force compiler to generate a gl_PerVertex block if the
    shader code writes vertex-shader output built-ins?

    RESOLVED: (In the validation rules): SPIR-V should contain the
    same form of input/output block as expected by recent versions of GLSL.
    All in/out built-in show up in SPIR-V in blocks, as per Vulkan rules.
    GLSL can stay the same, front ends need to block non-blocked built-ins.

10. Do we want to support glBindAttribLocation and glBindFragDataLocation[Indexed]
    with SPIR-V?  Also, what about glTransformFeedbackVaryings?

    DISCUSSION: Locations can be explicitly put into the shader, and
    then the application uses the right location.

    Note that SPIR-V 1.0 does not allow using specialization constants for

        layout(location=X) ...  // X must be a literal, not specialized

    Alternatively, add a SPIR-V instruction that assigns a string name to an
    input which the API can use to link with.  This would initially be an
    extension to SPIR-V as part of this.

    RESOLVED: Don't support any of glBindAttribLocation,
    glBindFragDataLocation[Index], or glTransformFeedbackVaryings.

11. What capabilities do we need for GL 4.5?

    RESOLVED: There is a Appendix A.spv for this.

12. How is user-linkage done between stages?

    DISCUSSION: SPIR-V linkage is by location number and BuiltIn decoration.
    Does OpenGL need linkage by name? Is SSO required to work with SPIR-V?
    Require OpName can't be stripped out for linkage objects.  If we use the
    program binary API, yes but this is not desirable. See issue 2.

    RESOLVED: Link the Vulkan way: built-in decorations, location numbers,
    etc., never by name.  No reflection required.  This is a lot like SSO.

13. Can we really handle separate textures and samplers?

    DISCUSSION: AMD: No, can't do this because OpenGL has features that need
    cross validation that can't be done.

    RESOLVED: Remove separate textures and samplers.

14. Are there differences in binding counting between Vulkan and OpenGL?

    DISCUSSION: Yes, OpenGL uses multiple binding points for a resource array
    while Vulkan uses just one.

    RESOLVED: This leaves OpenGL as is, but state in the overview what this
    difference is.

15. What GL version(s) should be required for the ImageQuery OpCapability?

    DISCUSSION: The GL features it corresponds with are:
        - textureSize - EXT_gpu_shader4 (GL 3.0)
        - textureQueryLod - ARB_texture_query_lod (GL 4.0)
        - textureQueryLevels - ARB_texture_query_levels (GL 4.3)
        - imageSize - ARB_shader_image_size (GL 4.3)
        - textureSamples, imageSamples - ARB_shader_texture_image_samples (GL 4.5)
    The belief is that these are largely software features and while some
    of them were not added until later API versions, it was not because of
    hardware reasons.

    RESOLVED: Require ImageQuery to be supported for all versions of GL,
    as it is required for all Vulkan implementations.

16. At what point should an error due to an invalid SPIR-V module/capability
    be reported? ShaderBinary, SpecializeShaderARB, LinkProgram time?
    ShaderBinary says a "valid SPIR-V module binary" is required, but you
    can have a valid module that uses capabilities or extensions not
    supported by an implementation.

    RESOLVED. ShaderBinary is expected to form an association between the
    SPIR-V module and likely would not parse the module as would be required
    to detect unsupported capabilities or other validation failures. In order
    to avoid requiring the implementation to parse the module multiples times,
    we allow this analysis to happen at either SpecializeShaderARB or
    LinkProgram time, similar to how many errors for source shaders are
    detected at either compile or link time.

17. Should we allow program objects to contain both source shaders and
    SPIR-V binary shaders, or should this be a link time failure?

    RESOLVED. No. This would be needlessly complex to specify. Make this
    a link-time error. This can be determined by examining the
    SPIR_V_BINARY_ARB state of each shader. They must either: all be TRUE
    (SPIR-V shader) or all be FALSE (source shaders). If an application
    really wishes to mix source and SPIR-V binary shaders, this can be
    done at program object boundaries by using separable program objects
    (if supported).

18. Do we need a section for "SPIR-V Transform Feedback Interface"?
    This would discuss any requirements for the Xfb related decorations
    in SPIR-V.

    RESOLVED. Yes.

19. How should the program interface query operations behave for program
    objects created from SPIR-V shaders?

    DISCUSSION: we previously said we didn't need reflection to work
    for SPIR-V shaders (at least for the first version), however we
    are left with specifying how it should "not work". The primary issue
    is that SPIR-V binaries are not required to have names associated
    with variables. They can be associated in debug information, but there
    is no requirement for that to be present, and it should not be relied
    upon.

    Options:
    A) Make it work.  If debug names are present they are enumerated
    correctly with associated variables. If the names aren't present,
    the compiler or driver gets to make them up. Alternatively, we could
    augment SPIR-V to have a mode where all interface variables need to
    have names which must not be stripped.

    B) Completely disallow it. All/Most such operations return an error.
    This may result in some implementation-dependent behavior which
    is impossible to know (although this problem may exist anyhow due
    to the offline-compilation step).  This would likely include not
    being able to tell how many active resources there are and related
    problems.

    C) Allow as much as possible to work "naturally". You can query for
    the number of active resources, and for details about them. Anything
    that doesn't query by name will work as expected. Queries for maximum
    length of names return one. Queries for anything "by name" return
    INVALID_INDEX (or -1). Querying the name property of a resource
    returns an empty string. This may allow many queries to work, but it's
    not clear how useful it would be if you can't actually know which
    specific variable you are retrieving information on. If everything
    is specified a-priori by location/binding/offset/index/component
    in the shader, this may be sufficient.

    RESOLVED.  Pick (c), but also allow debug names to be returned if an
    implementation wants to.

20. How should we deal with implementation-dependent behavior that
    must normally be queried after linking? Examples include:
     - set of active resources
     - offsets and strides of GLSLShared and GLSLPacked UBO/SSBOs
     - MATRIX_STRIDE, UNIFORM_ARRAY_STRIDE for UBOs (only relevant for
       packed/shared?)
     - UNIFORM_ARRAY_STRIDE for arrays of atomic counter buffers.

    DISCUSSION:
    - Option (c) from issue 19 allows determination of active resources (by
      shader specified layouts, but not name).
    - GLSLShared and GLSLPacked should not be allowed in this
      extension as there is no way to sensibly support, short of Option (a)
      from Issue 19.
    - For arrays of atomic counters, Option (c) from Issue 19 may be enough
      to figure this out, but I'm not sure that will work for offline
      compilation. Do we need to define a standard "layout" (stride) for
      arrays of atomic counters?

    RESOLVED:
    Picked (c) in issue 19, allowing determination of the number and types
    of active resources.
    Remove the shared and packed layouts and have the same behavior as in
    Vulkan.
    Atomic counter buffers don't have an associated name string already so
    there should be no issue with figuring out the UNIFORM_ARRAY_STRIDE for
    them.

21. What do we need to say about various linking rules related to "named
    uniform blocks" and "named shader storage blocks"?

    RESOLVED. We don't need to say anything, as they won't have
    names in SPIR-V shaders, so they aren't really "named". Instead of
    being identified by name they are identified (and matched) by
    uniform block index and/or binding.

22. How do the program interface query APIs work when no name reflection
    information is available?

    RESOLVED: The following naturally follows from the specification:

    GetProgramInterfaceiv(.., pname=MAX_NAME_LENGTH, ..) -> params = 1
    GetProgramResourceIndex(.., name) -> INVALID_INDEX
    GetProgramResourceName(.., length, name) -> length=0, name = ""
    GetProgramResourceiv(.., props=NAME_LENGTH, ..) -> params = 1
    GetProgramResourceLocation(.., name) -> -1
    GetProgramResourceLocationIndex(.., name) -> -1
    GetUniformLocation(.., name) -> -1
    GetActiveUniformName(.., length, uniformName) -> length=0, uniformName = ""
    GetUniformIndices(..,uniformNames, uniformIndices) -> uniformIndices=INVALID_INDEX
    GetActiveUniform(..,length,.., name) -> length = 0, name = ""
    GetActiveUniformsiv(..,pname=UNIFORM_NAME_LENGTH,..) -> params = 1
    GetUniformBlockIndex(.., uniformBlockName) -> INVALID_INDEX
    GetActiveUniformBlockName(..,length,uniformBlockName) -> length=0, uniformBlockName=""
    GetActiveUniformBlockiv(.., pname=UNIFORM_BLOCK_NAME_LENGTH, ..) -> params = 1
    GetActiveAttrib(..) -> length = 0, name = ""
    GetAttribLocation(.., name) -> -1
    GetTransformFeedbackVarying(..) -> length = 0, name = ""
    GetFragDatatLocation(.., name) -> -1
    GetFragDataIndex(.., name) -> -1
    GetProgramiv(.., pname=ACTIVE_ATTRIBUTE_MAX_LENGTH, ..) -> params = 1
    GetProgramiv(.., pname=ACTIVE_UNIFORM_MAX_LENGTH, ..) -> params = 1
    GetProgramiv(.., pname=TRANSFORM_FEEDBACK_VARYING_MAX_LENGTH, ..) -> params = 1
    GetProgramiv(.., pname=ACTIVE_UNIFORM_BLOCK_MAX_NAME_LENGTH, ..) -> params = 1

23. How does setting uniforms work if we can't query for locations based on
    names?

    RESOLVED. The shaders need to specify explicit locations for all uniform
    variables. The application can use offline reflection (or a reflection
    library) to know which variables are at which location, and then
    use that to specify the values of uniforms by location, as normal
    (e.g. Uniform*, ProgramUniform*).

24. What about UniformBlockBinding and ShaderStorageBlockBinding?

    RESOLVED. Because it is not possible to obtain the uniform block index
    or storage block index from an unnamed block, the binding value remains
    as specified in the shader by the layout qualifier or decoration.
    If you are feeling lucky you could just guess and remap use index values
    [0, #active blocks), but you won't really know what is going
    where, so just treat it as an immutable binding, similar to the atomic
    counter buffer binding point. Really.

25. What about subroutine queries based on names?

    RESOLVED. SPIR-V does not currently support subroutines, so it is not
    possibly to have any active subroutines from a SPIR-V based shader,
    and thus there is never anything to report.

26. How do we change the location of samplers and images?

    RESOLVED. You don't. Well you can using Uniform1i as usual, but you have
    no way of knowing which location corresponds to which sampler or image
    variable as GetUniformLocation won't work without a named variable. Best
    to just treat them as immutable bindings which are specified in the
    shader source or binary.

27. Appendix A.spv.3 gives a list of required capabilities that correspond
    to the core GL 4.5 features as well as for *some* of the ARB extensions
    that require shading language support. What about the rest of the
    ARB and vendor extensions that require shading language support?

    RESOLVED: Extensions that can be represented in terms of core
    SPIR-V 1.0 functionality don't need any specific consideration.
    Other ARB or vendor extensions that require SPIR-V functionality that
    can't be expressed in terms of SPIR-V 1.0 functionality will need to
    have SPIR-V extensions defined to add the required functionality.
    Further GL extensions can be defined to advertise support for
    consuming such SPIR-V capabilities once they have been defined.

    A (partial) list of extensions that are expected to need some form
    of modification to SPIR-V follows.

        ARB_shader_viewport_layer_array
            - need to allow ViewportIndex and Layer to be output by VS
              and TES.
        ARB_shader_clock
            - need clock() builtin function
        ARB_shader_ballot
            - use subgroup capability and add other builtins
        ARB_shader_atomic_counter_ops
            - need atomic counter builtin functions
        ARB_post_depth_coverage
            - need post_depth_coverage layout
        ARB_fragment_shader_interlock
            - need new layouts and interlock builtin functions
        KHR_blend_equation_advanced
            - need new layouts
        ARB_shader_group_vote
            - need new builtin functions
        ARB_shader_draw_parameters
            - need new builtin variables
        ARB_bindless_texture
            - need new layout qualifiers and probably other semantics
        ARB_compute_variable_group_size
            - need new layout qualifiers and builtin variables

Revision History

    Rev.    Date         Author         Changes
    ----  -----------    ------------   ---------------------------------
    31    21-Jul-2016    dkoch          Clarify string length on queries
    30    13-Jul-2016    JohnK          SPIR-V Offset can also apply to an
                                        atomic_uint offset.
    29    04-Jul-2015    dkoch          Allow handles of the same shader types
                                        for ShaderBinary when using SPIRV.
    28    09-Jun-2016    dkoch          Move future extensions to Issue 27.
    27    08-Jun-2016    dkoch          Assign enums, add a couple missing
                                        errors. Editorial fixes.
                                        Specify required version and format
                                        of SPIRV. Add Issue 27.
    26    02-Jun-2016    JohnK          Completion of draft. Pack/unpack, TBD,
                                        and resolutions.
    25    02-Jun-2016    JohnK          Include XFB linking/interface rules
    24    02-Jun-2016    JohnK          Remove use of GLSL shared/packed
                                        and SPIR-V GLSLShared/GLSLPacked,
                                        bringing in KHR_vulkan_glsl rules for
                                        std140 and std430
    23    01-Jun-2016    dkoch          Finish API edits. Cleanup editing
                                        pass on formatting and issue resolutions.
                                        Add issues 22-26 and resolve the rest.
    22    26-May-2016    dkoch          Add ARB suffix to SpecializeShader
                                        Add SPIR_V_BINARY_ARB shader state
                                        (and some but not all related rules)
                                        Add a bunch of API edits alluding to
                                        SPIR-V shaders. Lots of comments about
                                        outstanding API areas that still need
                                        to be addressed.
                                        Add unresolved issues 16-21.
    21    25-May-2016    JohnK          Add interface matching rules
    20    19-May-2016    dkoch          Remove language about 'default entry point'
                                        Recast features in terms of GL version.
    19    19-May-2016    dkoch          Add min GLSL version required for
                                        built-in variable decorations and
                                        OpCapabilities. Fix various dates.
    18    13-May-2016    JohnK          Bring in the actual correct subset of
                                        GL_KHR_vulkan_glsl, rather than refer
                                        to it with differences
    17    12-May-2016    dkoch          Verify capabilities for GLSL 4.50
                                        Add capabilities for non-core ARB
                                        extensions, and extensions that
                                        already have SPIR-V correspondence.
    16    12-May-2016    dkoch          grammatical fixes, replace non-ascii
                                        chars, formatting
    15    11-May-2016    JohnK          Clear up misc. TBDs throughout
    14    11-May-2016    JohnK          Flesh out GL_KHR_vulkan_glsl changes
    13    11-May-2016    JohnK          Move to final organization to flesh out
    12    10-May-2016    JohnK          Add the Vulkan validation rules that
                                        apply and the TBD from the f2f
    11    21-Apr-2016    JohnK          Editorial update to API description
    10    11-Feb-2016    gsellers       Add prototype API language.
     9    31-Jan-2016    JohnK          Issues 13 & 14
     8    04-Dec-2015    JohnK          Resolve issue 10
     7    05-Nov-2015    JohnK          Remove gl_FragColor, update issue 10
     6    22-Oct-2015    JohnK          Resolutions from Houston
     5    21-Oct-2015    JohnK          Make into a consistent format
     4    16-Oct-2015    JohnK          Added dependencies with GL_KHR_vulkan_glsl
     3    08-Oct-2015    JohnK          Added exec. env. detail, updated issues
     2    22-Apr-2015    Christophe     Added issues
     1    26-Mar-2015    JohnK          Initial revision