SPIR-V output fails VUID-StandaloneSpirv-Flat-06202, VUID-StandaloneSpirv-Flat-04744

[jimblandy]

Using naga e7ddd35 to compile wgpu test and example shaders to SPIR-V produces code that fails the standalone validation rules VUID-StandaloneSpirv-Flat-06202 and VUID-StandaloneSpirv-Flat-04744.

For example:

$ cd wgpu/wgpu/tests/shader_primitive_index
$ naga primitive_index.wgsl primitive_index.spv
$ spirv-val --target-env vulkan1.0 primitive_index.spv 
error: line 29: [VUID-StandaloneSpirv-Flat-04744] Fragment OpEntryPoint operand 27 with Input interfaces with integer or float type must have a Flat decoration for Entry Point id 31.
  %gl_PrimitiveID = OpVariable %_ptr_Input_uint Input

$ cd ../../examples/water/
$ naga water.wgsl water.spv
$ spirv-val --target-env vulkan1.0 water.spv 
error: line 139: [VUID-StandaloneSpirv-Flat-06202] OpEntryPoint interfaces variable must not be vertex execution model with an input storage class for Entry Point id 479.
  %465 = OpVariable %_ptr_Input_v2int Input

$ git rev-parse --short HEAD
2cd08a1c

[nical]

The primitive_index test case can be reduced down to:

@fragment
fn fs_main(@builtin(primitive_index) index: u32) -> @location(0) vec4<f32> {
    return vec4<f32>(0.0, 0.0, 0.0, 0.0);
}

$ naga reduced.wgsl reduced.spv && spirv-val --target-env vulkan1.0 reduced.spv
error: line 15: [VUID-StandaloneSpirv-Flat-04744] Fragment OpEntryPoint operand 8 with Input interfaces with integer or float type must have a Flat decoration for Entry Point id 13.
  %gl_PrimitiveID = OpVariable %_ptr_Input_uint Input

$ spirv-dis ./reduced.spv                     
; SPIR-V
; Version: 1.0
; Generator: Khronos; 28
; Bound: 17
; Schema: 0
               OpCapability Shader
               OpCapability Geometry
          %1 = OpExtInstImport "GLSL.std.450"
               OpMemoryModel Logical GLSL450
               OpEntryPoint Fragment %13 "fs_main" %gl_PrimitiveID %11
               OpExecutionMode %13 OriginUpperLeft
               OpDecorate %gl_PrimitiveID BuiltIn PrimitiveId
               OpDecorate %11 Location 0
       %void = OpTypeVoid
      %float = OpTypeFloat 32
    %float_0 = OpConstant %float 0
       %uint = OpTypeInt 32 0
    %v4float = OpTypeVector %float 4
%_ptr_Input_uint = OpTypePointer Input %uint
%gl_PrimitiveID = OpVariable %_ptr_Input_uint Input
%_ptr_Output_v4float = OpTypePointer Output %v4float
         %11 = OpVariable %_ptr_Output_v4float Output
         %14 = OpTypeFunction %void
         %13 = OpFunction %void None %14
          %7 = OpLabel
         %10 = OpLoad %uint %gl_PrimitiveID
               OpBranch %15
         %15 = OpLabel
         %16 = OpCompositeConstruct %v4float %float_0 %float_0 %float_0 %float_0
               OpStore %11 %16
               OpReturn
               OpFunctionEnd

The issue appears to be with what naga generates for the argument@builtin(primitive_index) index: u32.

[nical]

Inserting OpDecorate %gl_PrimitiveID Flat in the text version near the other OpDecorate instructions appears to fix the problem.

I tested that using:

$ spirv-dis ./reduced.spv > spvtext

then edited the spvtext file and validated it with:

$ spirv-as --target-env vulkan1.0 spvtext && spirv-val --target-env vulkan1.0 out.spv

[nical]

Moving the investigation of the water example to #2036.

[jimblandy]

I got some background on what's going on here. These are rules that have long been in the Vulkan spec, but the validation layers and spirv-val have not caught up to the spec. Recently there's been some contributor who's been going around submitting PRs that implement more validation, and the SPIRV-Tools maintainers have been merging them, since they're correctly following the spec.

So it's just coincidence that we were getting away with it before, and we can expect more changes in the future raising the standard of the actual validation tools closer to the spec.