Modular•6d ago

How do safe Reference and List work together?

From my understanding Reference is just a pointer in memory with associated lifetime. But I don't understand how the following code doesn't crash

    var list: List[Int] = List(10,11,12)
    var element = Reference(list[0])
    list.resize(0)
    for i in range(10000):
         list.append(i)
    print(element[])

    var list: List[Int] = List(10,11,12)
    var element = Reference(list[0])
    list.resize(0)
    for i in range(10000):
         list.append(i)
    print(element[])

Certainly reallocation should have happened here and therefore element should point to invalid memory location, which should cause segfault. But it prints 0. What am I missing?

23 Replies

Helehex•6d ago

On latest nightly:

fn main():
    var list: List[Int] = List(10,11,12)
    var element = Pointer.address_of(list[0])
    print(element[]) # prints 10
    for i in range(10000):
         list.append(i)
    print(element[]) # prints 0

fn main():
    var list: List[Int] = List(10,11,12)
    var element = Pointer.address_of(list[0])
    print(element[]) # prints 10
    for i in range(10000):
         list.append(i)
    print(element[]) # prints 0

blblblbl•6d ago

But why doesn't it segfault? Or maybe even more precise questions are: 1. Does this code has well defined behaviour? 2. If yes, how does this work? If not, then how safe is safe Reference?

Melody Daniel•6d ago

I'm interested in understanding this as well. @Owen Hilyard can you help?

Darkmatter•6d ago

This looks like a compiler bug, the existence of element should prevent calling list.append.

blblblbl•6d ago

Why should it prevent it? I haven't found anything about it in docs

Darkmatter•6d ago

Mutable xor Alias. You need to mutate the list to append to it, but doing there is an alias to a list element (element), which means mutation should not be allowed until element is dropped.

ModularBot•6d ago

Congrats @Owen Hilyard, you just advanced to level 24!

Darkmatter•6d ago

That's the property that provides memory safety and data race safety to both Mojo and Rust.

blblblbl•6d ago

What makes smth an Alias?

ModularBot•6d ago

Congrats @blblblbl, you just advanced to level 1!

Darkmatter•6d ago

If there are multiple references to it. Calling any method takes a reference to self, so should only be able to call methods which can't change the list in any way until you no longer have a pointer into the list. The Pointer[is_mutable=False, T] type is closer to const &T in C++. UnsafePointer is like T*. Pointer[is_mutable=True, T] is &T.

blblblbl•6d ago

But we have mutable reference to a value inside UnsafePointer inside a List. Should compiler understand that this reference is also an alias for List?

Darkmatter•6d ago

Collections are supposed to return a type which has a reference tied to self. It looks like Pointer is not actually handing that information correctly.

blblblbl•6d ago

In my original question I used Reference(list[0]). __getitem__ of list returns a ref [__lifetime_of(self)] T, so I guess there is no room to have reference to self So, should __getitem__ return something else more complex, that has reference to self?

Darkmatter•6d ago

__lifetime_of(self) is the correct behavior

blblblbl•6d ago

So __lifetime_of(self) should already provide information to compiler that Reference(list[0]) is an Alias to list?

Darkmatter•6d ago

Yes

blblblbl•6d ago

Got you, thx! Probably it is a good idea to write in docs about aliases and the fact that lifetime not only manages value lifetime but also controls aliasing

Darkmatter•6d ago

There was a big rewrite of a bunch of these semantics a week or two ago, so docs are waiting for experimentation to end.

blblblbl•6d ago

On a side note, should I make an issue in Github about this?

Darkmatter•6d ago

Please do, I was about to start filing it.

blblblbl•6d ago

Done https://github.com/modularml/mojo/issues/3682

GitHub

[BUG] Compiler doesn't prevent mutation of an aliased variable · Is...

Bug description Compiler doesn't prevent mutation of a List variable using append method, when a Reference to a list element exists Link to discussion in Discord: link Steps to reproduce fn mai...

Darkmatter•6d ago

Thank you

Gaming

Programming

How do safe Reference and List work together?