[SPIR-V] Arrays of empty structs in cbuffers produce validation errors when optimizations are disabled (asks to file a bug)

[Dredhog]

Title

Having an array of empty structs in a cbuffer produces a validation error when compiling without optimizations. Error message asks to report issue.

Functional impact

Compilation fails only with optimization level 0 i.e. when using compilation flag -O0 or -Od

Minimal repro steps

1. Compile the following pixel shader with dxc.exe -T ps_6_0 -E Frag repro_shader.hlsl -spirv -O0

struct PSInput
{
	float4 color : COLOR;
};

cbuffer UnityInstancing_PerDraw1 {
    struct {
	} unity_Builtins1Array[2];
}

float4 PSMain(PSInput input) : SV_TARGET
{
	return input.color;
}

Observe printed error message:

fatal error: generated SPIR-V is invalid: Structure id 165 decorated as Block for variable in Uniform storage class must follow relaxed uniform buffer layout rules: member 0 contains an array with stride 0
  %type_UnityInstancing_PerDraw1 = OpTypeStruct %_arr__struct_167_uint_2

note: please file a bug report on https://github.com/Microsoft/DirectXShaderCompiler/issues with source code if possible

2. Note: the issue was discovered while using scalar block layout which is more relevant to us at Unity:
   the error produced is appropriately different:

fatal error: generated SPIR-V is invalid: Structure id 165 decorated as Block for variable in Uniform storage class must follow scalar uniform buffer layout rules: member 0 contains an array with stride 0
  %type_UnityInstancing_PerDraw1 = OpTypeStruct %_arr__struct_167_uint_2

note: please file a bug report on https://github.com/Microsoft/DirectXShaderCompiler/issues with source code if possible

Here's a quick reproduction on shader-playground

Expected result

Compilation succeeds and works since the DXIL backend works with -O0. In DXIL the empty cbuffer seems to be removed if unused (which might not be an option) but even if the empty struct array is put into a used cbuffer it is actually correctly identified (marked with /* empty struct */ see example)

Actual result

Compilation fails with error seen in the reproduction steps. Error message asks to report the issue.

Further technical details

Reproduces also on most recent source builds of DXC 530958e

[jaebaek]

I am not sure we have a clear solution for this issue because it happens because of the limitation of SPIR-V.
To avoid the validation error message, we have to emit the following SPIR-V:

               OpMemberDecorate %type_UnityInstancing_PerDraw1 0 Offset 0
               OpDecorate %type_UnityInstancing_PerDraw1 Block
               OpDecorate %_arr__struct_13_uint_2 ArrayStride 16
...
%type_UnityInstancing_PerDraw1 = OpTypeStruct %_arr__struct_13_uint_2

If we do not set the ArrayStride of %_arr__struct_13_uint_2 as 16, spirv-val reports the same error, because it is a part of an object with Block decoration.

However, the ArrayStride of %_arr__struct_13_uint_2 is not 16. Since unity_Builtins1Array[2] is an empty array, its array stride must be 0. It is more clear with the following code:

struct PSInput
{
	float4 color : COLOR;
};

cbuffer UnityInstancing_PerDraw1 {
    struct {
	} unity_Builtins1Array[2];
  float4 additionalData;
}

float4 PSMain(PSInput input) : SV_TARGET
{
	return input.color;
}

If we set the ArrayStride of %_arr__struct_13_uint_2 as 16, the offset of additionalData must be 32 not 0, which is not true because unity_Builtins1Array[2] is empty.

The DXIL result sets the array size and stride as 0. We also do the same thing as 0 for the SPIR-V result and it results in the spirv-val error.

In my opinion, the best thing we can do is just disabling the spirv-val check when it has a cbuffer with an empty array and the legalization is disabled.