SPIR-V Built-In

SubgroupLocalInvocationId

ID of the invocation within a subgroup
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, 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].

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.

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