VulkanHub

VK_EXT_shader_subgroup_vote

Other Extension Metadata

Last Modified Date

2016-11-28

IP Status

No known IP claims.

Interactions and External Dependencies
Contributors
  • Neil Henning, Codeplay
  • Daniel Koch, NVIDIA Corporation

Description

This extension adds support for the following SPIR-V extension in Vulkan:

  • SPV_KHR_subgroup_vote

This extension provides new SPIR-V instructions:

  • OpSubgroupAllKHR,
  • OpSubgroupAnyKHR, and
  • OpSubgroupAllEqualKHR.

to compute the composite of a set of boolean conditions across a group of shader invocations that are running concurrently (a subgroup). These composite results may be used to execute shaders more efficiently on a VkPhysicalDevice.

When using GLSL source-based shader languages, the following shader functions from GL_ARB_shader_group_vote can map to these SPIR-V instructions:

  • anyInvocationARB() → OpSubgroupAnyKHR,
  • allInvocationsARB() → OpSubgroupAllKHR, and
  • allInvocationsEqualARB() → OpSubgroupAllEqualKHR.

The subgroup across which the boolean conditions are evaluated is implementation-dependent, and this extension provides no guarantee over how individual shader invocations are assigned to subgroups. In particular, a subgroup has no necessary relationship with the compute shader local workgroup — any pair of shader invocations in a compute local workgroup may execute in different subgroups as used by these instructions.

Compute shaders operate on an explicitly specified group of threads (a local workgroup), but many implementations will also group non-compute shader invocations and execute them concurrently. When executing code like

if (condition) {
  result = do_fast_path();
} else {
  result = do_general_path();
}

where condition diverges between invocations, an implementation might first execute do_fast_path() for the invocations where condition is true and leave the other invocations dormant. Once do_fast_path() returns, it might call do_general_path() for invocations where condition is false and leave the other invocations dormant. In this case, the shader executes both the fast and the general path and might be better off just using the general path for all invocations.

This extension provides the ability to avoid divergent execution by evaluating a condition across an entire subgroup using code like:

if (allInvocationsARB(condition)) {
  result = do_fast_path();
} else {
  result = do_general_path();
}

The built-in function allInvocationsARB() will return the same value for all invocations in the group, so the group will either execute do_fast_path() or do_general_path(), but never both. For example, shader code might want to evaluate a complex function iteratively by starting with an approximation of the result and then refining the approximation. Some input values may require a small number of iterations to generate an accurate result (do_fast_path) while others require a larger number (do_general_path). In another example, shader code might want to evaluate a complex function (do_general_path) that can be greatly simplified when assuming a specific value for one of its inputs (do_fast_path).

Deprecated by Vulkan 1.1

All functionality in this extension is superseded by the core Vulkan 1.1 subgroup operations.

New Enum Constants

  • VK_EXT_SHADER_SUBGROUP_VOTE_EXTENSION_NAME
  • VK_EXT_SHADER_SUBGROUP_VOTE_SPEC_VERSION

New SPIR-V Capabilities

Version History

  • Revision 1, 2016-11-28 (Daniel Koch)
    • Initial draft