SubgroupSize(3) Manual Page

Decorating a variable with the SubgroupSize builtin decoration will make that variable contain the implementation-dependent number of invocations in a subgroup. This value must be a power-of-two integer.

If the pipeline was created with the VK_PIPELINE_SHADER_STAGE_CREATE_ALLOW_VARYING_SUBGROUP_SIZE_BIT flag set, the SubgroupSize decorated variable will contain the subgroup size for each subgroup that gets dispatched. This value must be between minSubgroupSize and maxSubgroupSize and must be uniform with subgroup scope. The value may vary across a single draw call, and for fragment shaders may vary across a single primitive. In compute dispatches, SubgroupSize must be uniform with command scope.

If the pipeline was created with a chained VkPipelineShaderStageRequiredSubgroupSizeCreateInfo structure, the SubgroupSize decorated variable will match requiredSubgroupSize.

If the pipeline was not created with the VK_PIPELINE_SHADER_STAGE_CREATE_ALLOW_VARYING_SUBGROUP_SIZE_BIT flag set and no VkPipelineShaderStageRequiredSubgroupSizeCreateInfo structure was chained, the variable decorated with SubgroupSize will match subgroupSize.

The maximum number of invocations that an implementation can support per subgroup is 128.