[SPIR-V] [outputcontrolpoints(0)] or missing outputcontrolpoints Hull stage attribute crash the compiler (works with DXIL backend)

[Dredhog]

Title

[SPIR-V] [outputcontrolpoints(0)] or missing outputcontrolpoints Hull stage attribute crash the compiler (works with DXIL backend)

Functional impact

Cannot produce hull shaders that don't have any control point output (e.g. when only patch data is needed for the domain stage)

Minimal repro steps

1. Compile the following shader with dxc.exe -T hs_6_0 -E Hull -spirv repro_shader.txt:
   repro_shader.txt
   or do dxc.exe -T hs_6_0 -E Hull -spirv repro_shader_no_attribute.txt
   repro_shader_no_attribute.txt

Expected result

Compilation succeeds with control point output count being zero

Actual result

Compiler crashes with output:
Internal compiler error: access violation. Attempted to read from address 0x0000000000000020

Further technical details

Used DXC release v1.6.2104 (e09a454)

[jaebaek]

@Dredhog, Thank you for reporting this issue. We will take a look.

[sudonatalie]

@Dredhog Do you happen to have any additional context on how you intend to use zero control points in hull shaders, or is this simply an issue to report a crash (never correct) and mismatch with the DXIL backend (undesirable)?

Given that Vulkan doesn't allow zero as the number of output vertices (https://docs.vulkan.org/spec/latest/chapters/pipelines.html#VUID-VkPipelineShaderStageCreateInfo-stage-00713) and it looks like the D3D docs might also suggest it's invalid (https://learn.microsoft.com/en-us/windows/win32/direct3d11/direct3d-11-advanced-stages-tessellation#hull-shader-stage) I'm thinking the correct solution might be to fail gracefully for both backends.