Description

SubgroupLocalInvocationId

Decorating a variable with the SubgroupLocalInvocationId builtin decoration will make that variable contain the index of the invocation within the subgroup. This variable is in range [0,SubgroupSize-1].

If VK_PIPELINE_SHADER_STAGE_CREATE_REQUIRE_FULL_SUBGROUPS_BIT is specified, or if module declares SPIR-V version 1.6 or higher, and the local workgroup size in the X dimension of the stage is a multiple of SubgroupSize, full subgroups are enabled for that pipeline stage. When full subgroups are enabled, subgroups must be launched with all invocations active, i.e., there is an active invocation with SubgroupLocalInvocationId for each value in range [0,SubgroupSize-1].

Note

There is no direct relationship between SubgroupLocalInvocationId and LocalInvocationId or LocalInvocationIndex. If the pipeline or shader object was created with full subgroups applications can compute their own local invocation index to serve the same purpose:

index = SubgroupLocalInvocationId + SubgroupId × SubgroupSize

If full subgroups are not enabled, some subgroups may be dispatched with inactive invocations that do not correspond to a local workgroup invocation, making the value of index unreliable.
Note

VK_PIPELINE_SHADER_STAGE_CREATE_REQUIRE_FULL_SUBGROUPS_BIT and VK_SHADER_CREATE_REQUIRE_FULL_SUBGROUPS_BIT_EXT are effectively deprecated when compiling SPIR-V 1.6 shaders, as this behavior is the default for Vulkan with SPIR-V 1.6. This is more aligned with developer expectations, and avoids applications unexpectedly breaking in the future.
Valid Usage

  • VUID-SubgroupLocalInvocationId-SubgroupLocalInvocationId-04380
    The variable decorated with SubgroupLocalInvocationId must be declared using the Input Storage Class

  • VUID-SubgroupLocalInvocationId-SubgroupLocalInvocationId-04381
    The variable decorated with SubgroupLocalInvocationId must be declared as a scalar 32-bit integer value

See Also

No cross-references are available

Document Notes

For more information, see the Vulkan Specification

This page is extracted from the Vulkan Specification. Fixes and changes should be made to the Specification, not directly.

Copyright

Copyright 2014-2024 The Khronos Group Inc.

SPDX-License-Identifier: CC-BY-4.0