[SPIR-V] ShaderViewportIndexLayerEXT is now Core 1.2

[turanszkij]

Hello, the SPIR-V compiler adds the ShaderViewportIndexLayerEXT extension requirement to some of my shaders, but I believe they should be using the ShaderViewportIndex or ShaderLayer features instead: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/html/vkspec.html#spirvenv-capabilities-table-ShaderViewportIndex

I am using the -fspv-target-env=vulkan1.2 flag when compiling shaders. I am using the latest official DXC release.

[jaebaek]

@turanszkij could you please give us more information?

the SPIR-V compiler adds the ShaderViewportIndexLayerEXT extension requirement

Do you have validation errors?

If you provided us with a minimal reproducible shader, command to build the shader, and the error message, it would be easier to understand this issue.

[turanszkij]

Sure, this is a vertex shader that writes to a render target slice:

struct VSOut
{
	float4 position : SV_Position;
	uint rtindex : SV_RenderTargetArrayIndex;
};

VSOut main(float4 pos : POSITION, uint inst : SV_InstanceID)
{
	VSOut ret;
	ret.position = pos;
	ret.rtindex = inst;
	return ret;
}

Compiled with the DXC release found here: https://github.com/microsoft/DirectXShaderCompiler/releases/tag/v1.6.2106 With the following arguments:

dxc vs.hlsl -T vs_6_0 -spirv -fspv-target-env=vulkan1.2

Produces this code:

; SPIR-V
; Version: 1.5
; Generator: Google spiregg; 0
; Bound: 18
; Schema: 0
               OpCapability Shader
               OpCapability ShaderViewportIndexLayerEXT
               OpExtension "SPV_EXT_shader_viewport_index_layer"
               OpMemoryModel Logical GLSL450
               OpEntryPoint Vertex %main "main" %gl_Position %gl_InstanceIndex %gl_Layer %in_var_POSITION
               OpSource HLSL 600
               OpName %in_var_POSITION "in.var.POSITION"
               OpName %main "main"
               OpDecorate %gl_InstanceIndex BuiltIn InstanceIndex
               OpDecorate %gl_Position BuiltIn Position
               OpDecorate %gl_Layer BuiltIn Layer
               OpDecorate %gl_Layer Flat
               OpDecorate %in_var_POSITION Location 0
      %float = OpTypeFloat 32
    %v4float = OpTypeVector %float 4
%_ptr_Input_v4float = OpTypePointer Input %v4float
       %uint = OpTypeInt 32 0
%_ptr_Input_uint = OpTypePointer Input %uint
%_ptr_Output_v4float = OpTypePointer Output %v4float
%_ptr_Output_uint = OpTypePointer Output %uint
       %void = OpTypeVoid
         %14 = OpTypeFunction %void
%in_var_POSITION = OpVariable %_ptr_Input_v4float Input
%gl_InstanceIndex = OpVariable %_ptr_Input_uint Input
%gl_Position = OpVariable %_ptr_Output_v4float Output
   %gl_Layer = OpVariable %_ptr_Output_uint Output
       %main = OpFunction %void None %14
         %15 = OpLabel
         %16 = OpLoad %v4float %in_var_POSITION
         %17 = OpLoad %uint %gl_InstanceIndex
               OpStore %gl_Position %16
               OpStore %gl_Layer %17
               OpReturn
               OpFunctionEnd

Notice that it states the capability: OpCapability ShaderViewportIndexLayerEXT I'd expect that this extension wouldn't be required with Vulkan 1.2, as it is now in the core as ShaderViewportIndex and ShaderLayer This is described here: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VK_EXT_shader_viewport_index_layer.html

[Keenuts]

Hello!

I can reproduce this one, I'll take a look into it 🙂

[Keenuts]

Hello! Update on this: found the reason: DXC doesn't check the SPV version and requests the extension all the time. Fix seemed trivial, but now DXC just drops the generated SPV without errors. Trying to understand why.