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KhronosGroup / MoltenVK

MoltenVK is a Vulkan Portability implementation. It layers a subset of the high-performance, industry-standard Vulkan graphics and compute API over Apple's Metal graphics framework, enabling Vulkan applications to run on macOS, iOS and tvOS.
Apache License 2.0
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Why requiredSubgroupSizeStages is 0? #2218

Closed stripe2933 closed 2 months ago

stripe2933 commented 2 months ago

MoltenVK advertises that supporting VK_EXT_subgroup_size_control and minSubgroupSize...maxSubgroupSIze=4...32, however requiredSubgroupSizeStages is 0. Does it means none of the shader stage supports the subgroup size control? I can't understand why this value is 0.

The validation layer complains me:

VUID-VkPipelineShaderStageCreateInfo-pNext-02755(ERROR / SPEC): msgNum: -2142406189 - Validation Error: [ VUID-VkPipelineShaderStageCreateInfo-pNext-02755 ] Object 0: handle = 0x2e2941000000001f, type = VK_OBJECT_TYPE_SHADER_MODULE; | MessageID = 0x804d79d3 | vkCreateComputePipelines(): pCreateInfos[0].stage SPIR-V (VK_SHADER_STAGE_COMPUTE_BIT) is not in requiredSubgroupSizeStages (VkShaderStageFlags(0)). The Vulkan spec states: If a VkPipelineShaderStageRequiredSubgroupSizeCreateInfo structure is included in the pNext chain, the subgroupSizeControl feature must be enabled, and stage must be a valid bit specified in requiredSubgroupSizeStages (https://vulkan.lunarg.com/doc/view/1.3.275.0/mac/1.3-extensions/vkspec.html#VUID-VkPipelineShaderStageCreateInfo-pNext-02755)

cdavis5e commented 2 months ago

Does it means none of the shader stage supports the subgroup size control?

Yes, that's correct. AFAIK Metal provides no way to declare that a shader function must run with a particular SIMD-group[1] size. MoltenVK, being an implementation of Vulkan on top of Metal, mirrors the capabilities provided by Metal to the application.

[1]: The Metal term for what in Vulkan is called a subgroup.

stripe2933 commented 2 months ago

Thank you for explanation.