Luau Compiler Compliance

When compiling bytecode using Luau::compile, the bytecode emitted will contain the GETIMPORTS bytecode instruction. This is generally fine on tables that are immutable (meaning they do not change), however globals such as workspace, game and a few others such as script cannot suffer from this behaviour.

Why is this an issue?

If we inspect luau_load, we will see that when it is deserializing and loading the bytecode into memory, they do something tricky on the constants, they have a specific clase called LBC_CONSTANT_IMPORT. This is a Luau optimization that attempts to obtain everything a function may need that is known to not change, in order to improve performance. This is great! However, this indirectly inlines properties from globals such as game, workspace and others. This behaviour is bad.

ROBLOX themselves know this is an issue, and have made internally a small list for the compiler to not optimize these on the CompilerOptions::mutableGlobals field of the compiler options, which is the following:

const char *mutableGlobals[] = {
        "Game", "Workspace", "game", "plugin", "script", "shared", "workspace",
        nullptr
};

However, rSUNC has its own set of mutable globals which must also be defined on this array to prevent issues, which are the following _ENV, _GENV and _RENV.

This also includes Luau optimizations for Vector types, Luau optimizes them properly on Roblox-compiled code as they have already resolved this issue, however we do not have this resolved! In order to allow the Vector3 optimizations to shine we must set the CompilerOptions::vectorLib, CompilerOptions::vectorCtor and CompilerOptions::vectorType to the appropriate values, being Vector3, new and Vector3.

We also need to note that you must make use of CompilerOptions::optimizationLevel to be of 1 and the CompilerOptions::debugLevel to be of 1.

In summary, we must make our compiler play nice with ROBLOX, as well as with our types. This concludes on a simple CompileLuau function.

std::string CompileLuau(std::string_view szLuauCode) {
    const char *mutableGlobals[] = {
            "Game", "Workspace", "game", "plugin", "script", "shared", "workspace",
            "_ENV", "_GENV", "_RENV", // rSUNC globals.
            nullptr
    };

    auto compileOpts = Luau::CompileOptions { };
    compileOpts.optimizationLevel = 1;
    compileOpts.debugLevel = 1;
    compileOpts.vectorLib = "Vector3";
    compileOpts.vectorCtor = "new";
    compileOpts.vectorType = "Vector3";
    compileOpts.mutableGlobals = mutableGlobals;
    return Luau::compile(szLuauCode.data(), compileOpts, {}, BYTECODE_ENCODER /* Studio requires no bytecode encoder! */);
}

This function contains all elements required to have your compiler be rSUNC compliant.