Reflection API : How to know if push constants are used by an entry point (using HLSL syntax)

[PierreEVEN]

Hi,

I would like to know if a push constant buffer is used by a given entry point.

I was initially using the hlsl syntax to declare my push constants:

struct PushConstsStruct
{
    float4x4 model;
};
[[vk::push_constant]] PushConstsStruct pc;

(Note : I replaced it with the slang syntax later, which led me to another issue : #5676)

With available reflection functions, the only way I found to know if a given parameter is used is :

slang::IMetadata* metadata;
program->getEntryPointMetadata(0, 0, &metadata, diagnostics.writeRef());
if (diagnostics)
    return result.push_error({static_cast<const char*>(diagnostics->getBufferPointer())});

StageData data;
slang::ProgramLayout* program_lay = program->getLayout();
for (unsigned par_i = 0; par_i < program_lay ->getParameterCount(); par_i++)
{
   // will always be set to false if it is a push constant
    bool b_is_used = true;
    slang::VariableLayoutReflection* parameter = program_lay ->getParameterByIndex(par_i);
    metadata->isParameterLocationUsed(static_cast<SlangParameterCategory>(parameter->getCategory()), 0, parameter->getBindingIndex(), b_is_used);
    ...
}

which doesn't works as it always return false.

I'm not sure that isParameterLocationUsed() is expected to works with push constants, but it's the only available method so I guess there is something missing.

here is the full shader : playground-link

And how I use the reflection Api :

slang::IModule* module = {...} // target is spirv_1.5

for (SlangInt32 ep_i = 0; ep_i < module->getDefinedEntryPointCount(); ++ep_i)
{
    Slang::ComPtr<slang::IEntryPoint> entry_point;
    if (SLANG_FAILED(module->getDefinedEntryPoint(ep_i, entry_point.writeRef())))
        return "error";

    std::vector<slang::IComponentType*> components;
    components.emplace_back(entry_point);
    Slang::ComPtr<slang::IComponentType> program;
    if (SLANG_FAILED(session->createCompositeComponentType(components.data(), components.size(), program.writeRef())))
        return "error";

    Slang::ComPtr<slang::IComponentType> linkedProgram;
    Slang::ComPtr<slang::IBlob>          diagnostics;
    program->link(linkedProgram.writeRef(), diagnostics.writeRef());
    if (diagnostics)
        return "error";

    slang::IMetadata* metadata;
    linkedProgram->getEntryPointMetadata(0, 0, &metadata, diagnostics.writeRef());
    if (diagnostics)
        return "error";

    StageData             data;
    slang::ProgramLayout* shaderReflection = linkedProgram->getLayout();
    for (unsigned par_i = 0; par_i < shaderReflection->getParameterCount(); par_i++)
    {
        bool                             b_is_used = true;
        slang::VariableLayoutReflection* parameter = shaderReflection->getParameterByIndex(par_i);
        metadata->isParameterLocationUsed(static_cast<SlangParameterCategory>(parameter->getCategory()), 0, parameter->getBindingIndex(), b_is_used);

        if (parameter->getCategory() == slang::PushConstantBuffer && !b_is_used)
                std::cout << std::format("ERROR : UNUSED PC !! {} \n", parameter->getName());
    }
}