Confusion with compiler options

[PonasKovas]

Hello, I'm having trouble understanding some things related to the compilation of Luau scripts and I would really appreciate some clarification regarding these lua_CompileOptions:

vector

// alternative global builtin to construct vectors, in addition to default builtin 'vector.create'
const char* vectorLib;
const char* vectorCtor;

// alternative vector type name for type tables, in addition to default type 'vector'
const char* vectorType;

Why is vector separated like this? What special behavior does it enable? And just why would you want to have "alternative" builtin for vectors? Is this like an alias? I just dont really get this part at all.

mutable globals optimization

// null-terminated array of globals that are mutable; disables the import optimization for fields accessed through these
const char* const* mutableGlobals;

Maybe I misunderstand what exactly globals are in lua, but couldn't you add more (mutable) globals to a VM after some code is already compiled? What happens then? You have to recompile everything with that mutable global added to this list?

Thank you in advance!

[vegorov-rbx]

Hello!

vector library is a relatively new addition and we previously have used the vector options to define the typename to be recognized as well as a way to enable built-in vector constructor.
If you haven't used those options before, they can be ignored, but if you happened to have vector library or constructor under a different name that you want to keep using (like Vector3.new with Vector3 type or a short-hand vector constructor instead of vector.create), you can set those up.

For the second setting, Luau uses an optimization for global table accesses that we have detailed here https://luau.org/performance#importing-global-access-chains
However, if you have globals with table entries that switch values dynamically, the value they have at script load time (not compile time) will be captured. To avoid that, you put mutable globals in this list.

[PonasKovas]

Thank your for your quick reply!

I'm still not sure if I understand the mutable globals thing, so for instance in a situation like this:

1. I compile and run a luau script that creates a global variable a = 5 and reads b global variable which is nil for now
2. In the same VM I compile and run another script that creates b = 10 and reads the a global.

• When compiling the second script do I need to add a to the mutable globals list? (because i might run both of these scripts more times)
• After running the second script, should I recompile the first one with b also added to the mutable globals?

I'm just a bit confused how all of this would look in practice, do you ever need to recompile something to add more stuff to the mutable globals list?

[PonasKovas]

I also attempted to compile a simple script with and without the mutable globals list, but I seem to be getting identical bytecode.

function test()
    print(a)
    a = 5
end

print(a)
a = 4

I tried compiling this without and with a in mutableGlobals, yet I get the same bytecode.

Am I just completely misunderstanding something?