link-time constants in reflection API

[PierreEVEN]

Hi,

As discussed in #5664 (comment), I think it would be nice to be able to get a list of available link-time constants 'parameters', for example to generate shader permutations.

I'm working on a toy game engine, and I was searching for a convenient way to automatically handle shaders permutations.
Looking at the slang syntax, I thought that using link-time constants would be a good idea as it doesn't requires to compile the modules again like the examples provided in the user guide. I would only need to inject a module that set the value for each constant parameter.

Short example :

Slang code : PlayGround Link

Cpp-side :

// STEP ONE : use the reflection API to get a list of link-time constant parameters => we store this in a map named "static_switches"

// Example of how I would use the reflection API to populate the static_switches map. This doesn't exists in the current version of slang.
std::unordered_map<std::string, bool> static_switches;
for (const auto& param : module->getLayout()->getLinkTimeConstants())
    static_switches->emplace(param->get_name(), param->get_default_value());


// STEP TWO : each time we need a different combination of static_switches values, we inject the desired values as follows :

// This currently works as expected, no change are required here.
std::vector<slang::IComponentType*> components;
if (!static_switches.keys().empty())
{
    std::string generated_module;
    for (const auto& sw: static_switches)
        generated_module += std::format("export static const bool {} = {};", sw.first, sw.second ? "true" : "false");

    auto permutation_values_src = UnownedRawBlob::create(generated_module.c_str(), generated_module.size());
    components.emplace_back(session->loadModuleFromSource("permutation-values", "permutation-values.slang", permutation_values_src));
}
components.emplace_back(entry_point);
Slang::ComPtr<slang::IComponentType> program;
session->createCompositeComponentType(components.data(), components.size(), program.writeRef());

// then link + getEntryPointCode

Currently there is no way to get a list of link-time constants, so I have to manually parse the shader file to retrieve them, which is not a very clean approach.
Also optionally, it would be nice if we can get the default value (which is probable harder to implement)

[bmillsNV]

Feature request. We'll evaluate this next quarter when figuring out our priorities. In the meantime, we welcome contributions if anyone wants to take this one up.

[jjiangweilan]

I found a way to get the name of link-time constants

    auto moduleReflection = module->getModuleReflection();
    for (auto child : moduleReflection->getChildren())
    {
        auto type = child->getKind();
        if (type == slang::DeclReflection::Kind::Variable)
        {
            auto asVariable = child->asVariable();
            bool hasExtern = asVariable->findModifier(slang::Modifier::Extern) != nullptr;
            bool hasStatic = asVariable->findModifier(slang::Modifier::Static) != nullptr;
            bool hasConst = asVariable->findModifier(slang::Modifier::Const) != nullptr;

            // bool hasDefault = asVariable->hasDefaultValue(); // you can know if it has default value, but I haven't find a way to know the exact value assigned

            if (hasExtern && hasStatic && hasConst)
            {
                std::string name = child->getName(); // name of the link-time constants
            }
        }
    }

[PierreEVEN]

I found a way to get the name of link-time constants

    auto moduleReflection = module->getModuleReflection();
    for (auto child : moduleReflection->getChildren())
    {
        auto type = child->getKind();
        if (type == slang::DeclReflection::Kind::Variable)
        {
            auto asVariable = child->asVariable();
            bool hasExtern = asVariable->findModifier(slang::Modifier::Extern) != nullptr;
            bool hasStatic = asVariable->findModifier(slang::Modifier::Static) != nullptr;
            bool hasConst = asVariable->findModifier(slang::Modifier::Const) != nullptr;

            // bool hasDefault = asVariable->hasDefaultValue(); // you can know if it has default value, but I haven't find a way to know the exact value assigned

            if (hasExtern && hasStatic && hasConst)
            {
                std::string name = child->getName(); // name of the link-time constants
            }
        }
    }

Works as expected

Thanks !