SYNC-HAZARD-READ_AFTER_WRITE: false positive with VK_EXT_fragment_shader_interlock

[csmartdalton]

Describe the Issue

I'm hitting SYNC-HAZARD-READ_AFTER_WRITE in the validation layers for VK_LAYER_KHRONOS_validation, while using the VK_EXT_fragment_shader_interlock interlock extension.

Specific use case:

1. Issue a draw that stores to an image from inside the interlock
2. Issue a second draw that then loads the same image from inside the interlock

..\..\src\tests\test_utils\ANGLETest.cpp(65): error: RendererVk.cpp:834 (DebugUtilsMessenger): [ SYNC-HAZARD-READ_AFTER_WRITE ] Validation Error: [ SYNC-HAZARD-READ_AFTER_WRITE ] Object 0: handle = 0x7f000000007f, type = VK_OBJECT_TYPE_IMAGE_VIEW; | MessageID = 0x4cb75b2d | vkCmdDraw: Hazard READ_AFTER_WRITE for VkImageView 0x7f000000007f[], in VkCommandBuffer 0x1ff0a57eca0[], and VkPipeline 0x880000000088[], VkDescriptorSet 0x8a000000008a[], type: VK_DESCRIPTOR_TYPE_STORAGE_IMAGE, imageLayout: VK_IMAGE_LAYOUT_GENERAL, binding #1, index 0. Access info (usage: SYNC_FRAGMENT_SHADER_SHADER_STORAGE_READ, prior_usage: SYNC_FRAGMENT_SHADER_SHADER_STORAGE_WRITE, write_barriers: 0, command: vkCmdDraw, seq_no: 13, reset_no: 1).
                            Object: 0x7f000000007f (type = Image View(14))

One workaround is to also store to the image during draw #(2), after it has been loaded. This can even be done in a branch that never executes:

imageLoad(...);

// Inserting the following block fixes the validation error.
if (myUniformThatEquals0 != 0.0) // Never executes
    imageStore(...);

This appears to be a false positive because the interlock provides the necessary synchronization and ordering guarantees.

Other experiments:

• All tests pass (on NVIDIA and Intel both) if I just turn off validation layers
• All tests fail again (on NVIDIA and Intel both) if I turn off validation layers and don't use the interlock

Environment:

• OS: Windows 11
• GPU: NVIDIA NVIDIA GeForce RTX 3050 Laptop GPU (0x000025A2)), NVIDIA-512.78.0.0
• SDK or header version if building from repo: https://github.com/KhronosGroup/Vulkan-ValidationLayers/tree/aa7ee26f15bfbe8bf5564e6af3d499e07da8e776
• Options enabled (synchronization, best practices, etc.): spv::CapabilityFragmentShaderPixelInterlockEXT, SPIRVExtensions::FragmentShaderInterlockEXT, spv::ExecutionMode::ExecutionModePixelInterlockOrderedEXT

[ncesario-lunarg]

@jeremyg-lunarg @jzulauf-lunarg any thoughts? I don't have a good grasp as to why this modification to the shader would have this effect. I also didn't realize sync validation was hooking into the shader module state at all?

[jzulauf-lunarg]

Sync Val is labeled as "limited extension support"... add this to the list of extension needing support.

[ncesario-lunarg]

Thanks for taking a look @jzulauf-lunarg, are we tracking this somewhere already (i.e., "missing extension support in sync validation")? I wasn't able to find a related issue or project.

[jzulauf-lunarg]

It would probably be easier list what we do support than what we don't. I don't see a clear list of it anywhere.