Modular•2mo 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•2mo 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

blblblblOP•2mo 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•2mo ago

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

Darkmatter•2mo ago

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

blblblblOP•2mo ago

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

Darkmatter•2mo 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•2mo ago

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

Darkmatter•2mo ago

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

blblblblOP•2mo ago

What makes smth an Alias?

ModularBot•2mo ago

Congrats @blblblbl, you just advanced to level 1!

Darkmatter•2mo 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.

blblblblOP•2mo 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•2mo 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.

blblblblOP•2mo 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•2mo ago

__lifetime_of(self) is the correct behavior

blblblblOP•2mo ago

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

Darkmatter•2mo ago

Yes

blblblblOP•2mo 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•2mo 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.

blblblblOP•2mo ago

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

Darkmatter•2mo ago

Please do, I was about to start filing it.

blblblblOP•2mo 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•2mo ago

Thank you

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How do safe Reference and List work together?