133 lines
4.1 KiB
Plaintext
133 lines
4.1 KiB
Plaintext
include::meta/VK_EXT_shader_subgroup_vote.txt[]
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*Last Modified Date*::
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2016-11-28
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*IP Status*::
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No known IP claims.
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*Interactions and External Dependencies*::
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- This extension requires the
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https://www.khronos.org/registry/spir-v/extensions/KHR/SPV_KHR_subgroup_vote.html[+SPV_KHR_subgroup_vote+]
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SPIR-V extension.
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- This extension requires the
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https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_shader_group_vote.txt[+GL_ARB_shader_group_vote+]
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extension for GLSL source languages.
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*Contributors*::
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- Neil Henning, Codeplay
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- Daniel Koch, NVIDIA Corporation
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This extension adds support for the following SPIR-V extension in Vulkan:
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* +SPV_KHR_subgroup_vote+
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This extension provides new SPIR-V instructions:
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* code:OpSubgroupAllKHR,
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* code:OpSubgroupAnyKHR, and
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* code:OpSubgroupAllEqualKHR.
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to compute the composite of a set of boolean conditions across a group of
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shader invocations that are running concurrently (a _subgroup_).
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These composite results may be used to execute shaders more efficiently on a
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slink:VkPhysicalDevice.
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When using GLSL source-based shader languages, the following shader
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functions from GL_ARB_shader_group_vote can map to these SPIR-V
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instructions:
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* code:anyInvocationARB() -> code:OpSubgroupAnyKHR,
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* code:allInvocationsARB() -> code:OpSubgroupAllKHR, and
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* code:allInvocationsEqualARB() -> code:OpSubgroupAllEqualKHR.
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The subgroup across which the boolean conditions are evaluated is
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implementation-dependent, and this extension provides no guarantee over how
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individual shader invocations are assigned to subgroups.
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In particular, a subgroup has no necessary relationship with the compute
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shader _local workgroup_ -- any pair of shader invocations in a compute
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local workgroup may execute in different subgroups as used by these
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instructions.
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Compute shaders operate on an explicitly specified group of threads (a local
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workgroup), but many implementations will also group non-compute shader
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invocations and execute them concurrently.
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When executing code like
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[source,c++]
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----------------------------------------
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if (condition) {
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result = do_fast_path();
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} else {
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result = do_general_path();
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}
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----------------------------------------
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where code:condition diverges between invocations, an implementation might
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first execute code:do_fast_path() for the invocations where code:condition
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is true and leave the other invocations dormant.
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Once code:do_fast_path() returns, it might call code:do_general_path() for
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invocations where code:condition is code:false and leave the other
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invocations dormant.
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In this case, the shader executes *both* the fast and the general path and
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might be better off just using the general path for all invocations.
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This extension provides the ability to avoid divergent execution by
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evaluating a condition across an entire subgroup using code like:
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[source,c++]
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----------------------------------------
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if (allInvocationsARB(condition)) {
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result = do_fast_path();
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} else {
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result = do_general_path();
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}
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----------------------------------------
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The built-in function code:allInvocationsARB() will return the same value
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for all invocations in the group, so the group will either execute
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code:do_fast_path() or code:do_general_path(), but never both.
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For example, shader code might want to evaluate a complex function
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iteratively by starting with an approximation of the result and then
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refining the approximation.
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Some input values may require a small number of iterations to generate an
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accurate result (code:do_fast_path) while others require a larger number
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(code:do_general_path).
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In another example, shader code might want to evaluate a complex function
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(code:do_general_path) that can be greatly simplified when assuming a
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specific value for one of its inputs (code:do_fast_path).
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=== New Object Types
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None.
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=== New Enum Constants
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None.
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=== New Enums
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None.
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=== New Structures
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None.
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=== New Functions
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None.
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=== New Built-In Variables
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None.
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=== New SPIR-V Capabilities
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* <<spirvenv-capabilities-table-subgroupvote,SubgroupVoteKHR>>
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=== Issues
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None.
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=== Version History
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* Revision 1, 2016-11-28 (Daniel Koch)
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- Initial draft
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