Name ARB_shader_ballot Name Strings GL_ARB_shader_ballot Contact Timothy Lottes (timothy.lottes 'at' amd.com) Contributors Timothy Lottes, AMD Graham Sellers, AMD Daniel Rakos, AMD Jeannot Breton, NVIDIA Pat Brown, NVIDIA Eric Werness, NVIDIA Mark Kilgard, NVIDIA Jeff Bolz, NVIDIA Notice Copyright (c) 2015 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Specification Update Policy Khronos-approved extension specifications are updated in response to issues and bugs prioritized by the Khronos OpenGL Working Group. For extensions which have been promoted to a core Specification, fixes will first appear in the latest version of that core Specification, and will eventually be backported to the extension document. This policy is described in more detail at https://www.khronos.org/registry/OpenGL/docs/update_policy.php Status Complete. Approved by the ARB on June 26, 2015. Ratified by the Khronos Board of Promoters on August 7, 2015. Version Last Modified Date: 03/18/2017 Revision: 8 Number ARB Extension #183 Dependencies This extension is written against Revision 5 of the version 4.50 of the OpenGL Shading Language Specification, dated January 30, 2015. This extension requires GL_ARB_gpu_shader_int64. Overview This extension provides the ability for a group of invocations which execute in lockstep to do limited forms of cross-invocation communication via a group broadcast of a invocation value, or broadcast of a bitarray representing a predicate value from each invocation in the group. New Procedures and Functions None. New Tokens None. IP Status None. Modifications to the OpenGL Shading Language Specification, Version 4.50 Including the following line in a shader can be used to control the language features described in this extension: #extension GL_ARB_shader_ballot : where is as specified in section 3.3. New preprocessor #defines are added to the OpenGL Shading Language: #define GL_ARB_shader_ballot 1 Additions to Chapter 7 of the OpenGL Shading Language Specification (Built-in Variables) Modify Section 7.4, Built-In Uniform State, p. 133 (Add to the list of built-in uniform variable declaration) uniform uint gl_SubGroupSizeARB; (Add this paragraph at the end of this section) A sub-group is a collection of invocations which execute in lockstep. The variable is the maximum number of invocations in a sub-group. The maximum supported in this extension is 64. Modify Section 7.1, Built-in Languages Variable, p. 110 (Add to the list of built-in variables for the compute, vertex, geometry, tessellation control, tessellation evaluation and fragment languages) in uint gl_SubGroupInvocationARB; in uint64_t gl_SubGroupEqMaskARB; in uint64_t gl_SubGroupGeMaskARB; in uint64_t gl_SubGroupGtMaskARB; in uint64_t gl_SubGroupLeMaskARB; in uint64_t gl_SubGroupLtMaskARB; (Add those paragraphs at the end of this section) The variable holds the index of the invocation within sub-group. This variable is in the range 0 to -1, where is the total number of invocations in a sub-group. The variables provide a bitmask for all invocations, with one bit per invocation starting with the least significant bit, according to the following table, variable equation for bit values -------------------- ------------------------------------ gl_SubGroupEqMaskARB bit index == gl_SubGroupInvocationARB gl_SubGroupGeMaskARB bit index >= gl_SubGroupInvocationARB gl_SubGroupGtMaskARB bit index > gl_SubGroupInvocationARB gl_SubGroupLeMaskARB bit index <= gl_SubGroupInvocationARB gl_SubGroupLtMaskARB bit index < gl_SubGroupInvocationARB Additions to Chapter 8 of the OpenGL Shading Language Specification (Built-in Functions) Add Section 8.18, Shader Invocation Group Functions Syntax: uint64_t ballotARB(bool value); The function ballotARB() returns a bitfield containing the result of evaluating the expression in all active invocations in the sub-group. An active invocation is one that is executing the ballotARB() call. The sub-group may have inactive invocations for example due to exit of the shader, or divergent branching. Sub-groups of up to 64 invocations may be represented by the return value of ballotARB(). Bits for each invocation are packed in least significant bit ordering. If evaluates to true for an active invocation then the corresponding bit is set to one in the result, otherwise it is zero. Bits corresponding to invocations that are not active or that do not exist in the sub group (because, for example, they are at bit positions beyond the sub-group size) are set to zero. The following trivial assumptions can be made: * ballotARB(true) returns bitfield where the corresponding bits are set for all active invocations in the sub-group. * ballotARB(false) returns zero. Syntax: genType readInvocationARB(genType value, uint invocationIndex); genIType readInvocationARB(genIType value, uint invocationIndex); genUType readInvocationARB(genUType value, uint invocationIndex); genType readFirstInvocationARB(genType value); genIType readFirstInvocationARB(genIType value); genUType readFirstInvocationARB(genUType value); The function readInvocationARB() returns the from a given to all active invocations in the sub-group. The must be the same for all active invocations in the sub-group otherwise results are undefined. The function readFirstInvocationARB() returns the from the first active invocation to all active invocations in the sub-group. Issues 1) How are the values of gl_SubGroup??MaskARB defined? RESOLVED. Earlier versions of this specification defined a bitmask such as "LtMask" ("less than mask") as having bits set if "gl_SubGroupInvocationARB < bit index". However, this was reversed from the definition in GL_NV_shader_thread_group that these built-ins were derived from, and also mismatched a recent Vulkan/SPIR-V extension. Fortunately, all known implementations of this extension had implemented "wrong" behavior (matching the sense of the original built-ins in GL_NV_shader_thread_group), so the best thing to do is change the definition in the spec. Revision History Rev Date Author Changes --- ---------- -------- --------------------------------------------- 8 03/18/2017 jbolz Reversed the sense of the comparison in the definition of gl_SubGroup??MaskARB. 7 08/25/2015 nhenning Add ARB suffix on documentation for readInvocation and readFirstInvocation functions. 6 07/31/2015 pdaniell Add ARB suffix on the readInvocation and readFirstInvocation functions. 5 07/30/2015 pdaniell Update the function definition syntax to use our standard gen*Type conventions. 4 06/23/2015 tlottes More precise spec language. 3 06/22/2015 tlottes Deferred GPU processor another spec. Cleaned up spec language. 2 04/20/2015 tlottes Updated spec language. 1 03/09/2015 tlottes Initial revision based on AMD_gcn_shader and NV_shader_thread_group.