SPIRV shader debugger: `OpDebugValue` does not update the value of an vector element.

[Yong He]

Description

When debugging SPIRV shader that uses OpDebugValue to update an element of a OpDebugTypeVector debug variable, the variable value does not change in the debug watch window.

Steps to reproduce

repro.zip

• Open the attached repro file in top-of-tree render doc.
• Select the vkCmdDispatch command from event browser.
• Goto "Pipeline State" tab, click "Debug" button to debug the compute shader.
• Step through line 75 of the shader source.
• Should expect t.memberB to have updated vector value, but it is not updated.

Environment

• RenderDoc commit: 70b79d7
• Operating System: Windows 11
• Graphics API: Vulkan 1.2/SPIRV 1.5

[Jake Turner]

The construction of using DebugValue to index into a vector and using OpAccessChain through a pointer to a structure member to modify the y-component is not currently handled by the RenderDoc source level debugging.

Source

    t.memberB = float3(1, 2, 3);
    t.memberB.y = 4.0;

SPIRV slang

%167 = OpAccessChain %_ptr_Function_v3float %t %int_1
       OpStore %167 %168
%175 = OpAccessChain %_ptr_Function_float %167 %int_1
       OpStore %175 %float_4

For reference: SPIRV glslang construction which does work with RenderDoc source debugging

%86 = OpAccessChain %_ptr_Function_v3float %t %int_1
      OpStore %86 %84
%89 = OpAccessChain %_ptr_Function_float %t %int_1 %uint_1
      OpStore %89 %float_4

[Yong He]

The difference with glslang and slang's own spirv backend (enabled with -emit-spirv-directly in slangc) is that Slang uses more SSA registers than explicit OpVars and is relying on OpDebugValue to update a debug variable. Therefore we don't use OpDebugDeclare to link a debug variable with an actual variable as done in glslang, but rely more heavily on OpDebugValue.