Specialization constant not considered constant for subgroupBroadcast

[FunMiles]

Compilation of the following code gives an error stating that the argument to subgroupBroadcast should be a compile-time constant. As far as I understand it, a specialization constant is a compilation constant and should be acceptable.

#version 450
#extension GL_KHR_shader_subgroup_basic : enable
#extension GL_KHR_shader_subgroup_ballot : enable
#extension GL_KHR_shader_subgroup_arithmetic : enable
#extension GL_KHR_memory_scope_semantics : enable

layout(local_size_x_id = 0) in;

layout(constant_id = 1) const int subGroupSize = 64;

layout(set = 0, binding = 0) readonly buffer InBuf {
    uint[] in_buf;
};

layout(set = 0, binding = 1) buffer OutBuf {
    uint[] out_buf;
};


void main() {

    uint v = in_buf[gl_GlobalInvocationID.x];
    uint total = subgroupInclusiveAdd(v);
    total += subgroupBroadcast(total, subGroupSize-1);
    out_buf[gl_GlobalInvocationID.x] = total;
}

Error diagnostic:

ERROR: /Users/michel/CLionProjects/GPUGraph/shaders/cms_example.comp:24: 'id' : argument must be compile-time constant 
ERROR: /Users/michel/CLionProjects/GPUGraph/shaders/cms_example.comp:24: '' : compilation terminated 
ERROR: 2 compilation errors.  No code generated.

There was a similar issue for texture load but that has been resolved #2178

Using a specialization constant as the broadcast id is useful in prefix sum operations as stated in #1591 where @jekstrand suggests the use of a specialization constant with subgroupBroadcast (which will not work as long as glslang marks this as an error.

[greg-lunarg]

Yes, subGroupSize-1 should be considered a compile-time constant by glslang.

[greg-lunarg]

I am not seeing the Error diagnostic with the latest glslang. Here is the SPIR-V that is generated, which looks to be correct:

               OpCapability Shader
               OpCapability GroupNonUniform
               OpCapability GroupNonUniformArithmetic
               OpCapability GroupNonUniformBallot
          %1 = OpExtInstImport "GLSL.std.450"
               OpMemoryModel Logical GLSL450
               OpEntryPoint GLCompute %main "main" %_ %gl_GlobalInvocationID %__0
               OpExecutionMode %main LocalSize 1 1 1
               OpSource GLSL 450
               OpSourceExtension "GL_KHR_memory_scope_semantics"
               OpSourceExtension "GL_KHR_shader_subgroup_arithmetic"
               OpSourceExtension "GL_KHR_shader_subgroup_ballot"
               OpSourceExtension "GL_KHR_shader_subgroup_basic"
               OpName %main "main"
               OpName %v "v"
               OpName %InBuf "InBuf"
               OpMemberName %InBuf 0 "in_buf"
               OpName %_ ""
               OpName %gl_GlobalInvocationID "gl_GlobalInvocationID"
               OpName %total "total"
               OpName %subGroupSize "subGroupSize"
               OpName %OutBuf "OutBuf"
               OpMemberName %OutBuf 0 "out_buf"
               OpName %__0 ""
               OpDecorate %_runtimearr_uint ArrayStride 4
               OpMemberDecorate %InBuf 0 NonWritable
               OpMemberDecorate %InBuf 0 Offset 0
               OpDecorate %InBuf Block
               OpDecorate %_ DescriptorSet 0
               OpDecorate %_ Binding 0
               OpDecorate %gl_GlobalInvocationID BuiltIn GlobalInvocationId

               OpDecorate %subGroupSize SpecId 1
               OpDecorate %_runtimearr_uint_0 ArrayStride 4
               OpMemberDecorate %OutBuf 0 Offset 0
               OpDecorate %OutBuf Block
               OpDecorate %__0 DescriptorSet 0
               OpDecorate %__0 Binding 1
               OpDecorate %45 SpecId 0
               OpDecorate %gl_WorkGroupSize BuiltIn WorkgroupSize
       %void = OpTypeVoid
          %3 = OpTypeFunction %void
       %uint = OpTypeInt 32 0
%_ptr_Function_uint = OpTypePointer Function %uint
%_runtimearr_uint = OpTypeRuntimeArray %uint
      %InBuf = OpTypeStruct %_runtimearr_uint
%_ptr_StorageBuffer_InBuf = OpTypePointer StorageBuffer %InBuf
          %_ = OpVariable %_ptr_StorageBuffer_InBuf StorageBuffer
        %int = OpTypeInt 32 1
      %int_0 = OpConstant %int 0
     %v3uint = OpTypeVector %uint 3
%_ptr_Input_v3uint = OpTypePointer Input %v3uint
%gl_GlobalInvocationID = OpVariable %_ptr_Input_v3uint Input
     %uint_0 = OpConstant %uint 0
%_ptr_Input_uint = OpTypePointer Input %uint
%_ptr_StorageBuffer_uint = OpTypePointer StorageBuffer %uint
     %uint_3 = OpConstant %uint 3
%subGroupSize = OpSpecConstant %int 64
      %int_1 = OpConstant %int 1
         %32 = OpSpecConstantOp %int ISub %subGroupSize %int_1
         %33 = OpSpecConstantOp %uint IAdd %32 %uint_0
%_runtimearr_uint_0 = OpTypeRuntimeArray %uint
     %OutBuf = OpTypeStruct %_runtimearr_uint_0
%_ptr_StorageBuffer_OutBuf = OpTypePointer StorageBuffer %OutBuf
        %__0 = OpVariable %_ptr_StorageBuffer_OutBuf StorageBuffer
         %45 = OpSpecConstant %uint 1
     %uint_1 = OpConstant %uint 1
%gl_WorkGroupSize = OpSpecConstantComposite %v3uint %45 %uint_1 %uint_1
       %main = OpFunction %void None %3
          %5 = OpLabel
          %v = OpVariable %_ptr_Function_uint Function
      %total = OpVariable %_ptr_Function_uint Function
         %20 = OpAccessChain %_ptr_Input_uint %gl_GlobalInvocationID %uint_0
         %21 = OpLoad %uint %20
         %23 = OpAccessChain %_ptr_StorageBuffer_uint %_ %int_0 %21
         %24 = OpLoad %uint %23
               OpStore %v %24
         %26 = OpLoad %uint %v
         %28 = OpGroupNonUniformIAdd %uint %uint_3 InclusiveScan %26
               OpStore %total %28
         %29 = OpLoad %uint %total
         %34 = OpGroupNonUniformBroadcast %uint %uint_3 %29 %33
         %35 = OpLoad %uint %total
         %36 = OpIAdd %uint %35 %34
               OpStore %total %36
         %41 = OpAccessChain %_ptr_Input_uint %gl_GlobalInvocationID %uint_0
         %42 = OpLoad %uint %41
         %43 = OpLoad %uint %total
         %44 = OpAccessChain %_ptr_StorageBuffer_uint %__0 %int_0 %42
               OpStore %44 %43
               OpReturn
               OpFunctionEnd

I am going to close this. If you don't see a similar result with the latest glslang, please comment and I will re-open.