[BUG] Eager copying of struct even though no mutation occurs

[Brian-M-J]

Bug description

Mojo aims to provide copy-on-write semantics, so programmers do not have to pay a price to reassign data if no mutation occurs. However, in the snippet linked below, even though no mutation is performed, the value of demo_obj_1 is copied eagerly as demonstrated by the output:

Called constructor for DemoStruct(List(1, 2, 3, 4, 5))
Called copy constructor for DemoStruct(List(1, 2, 3, 4, 5))
DemoStruct(List(1, 2, 3, 4, 5))
Calling destructor for DemoStruct(List(1, 2, 3, 4, 5))
DemoStruct(List(1, 2, 3, 4, 5))
Calling destructor for DemoStruct(List(1, 2, 3, 4, 5))

This introduces unnecessary run-time costs into the program.

Steps to reproduce

Run this snippet in the playground: https://gist.github.com/modularbot/6aed759930420cd70f38795dbcb874fe

System information

The Mojo playground as of 26-04-2024

[ematejska]

This is working as intended per @jpamer

[Brian-M-J]

This is working as intended per @jpamer

I don't fully understand what you mean. It's mentioned that:

Because String implements a copy constructor, it's able to be moved into bar and leave behind a copy. Under the hood this is still passing by reference for maximum efficiency, it'll only create a copy if foo is mutated.

...but as demonstrated here, a copy is in fact created as demonstrated by the output (even though no mutation occurs):

Called __init__ for DemoStruct(String("foo"))
Called __copyinit__ for DemoStruct(String("foo"))
Called __del__ for DemoStruct(String("foo"))
Called bar with argument: DemoStruct(String("foo"))
DemoStruct(String("foo"))
Called __del__ for DemoStruct(String("foo"))

__copyinit__ is in fact called, and __del__ is called twice. This seems to contradict the statement in the blog post that Mojo enforces copy-on-write semantics.

I say "Mojo enforces COW" and not "String enforces COW" because it's not like String has some special magic code to enforce COW. Its __copyinit__ just calls List's __copyinit__, which is nothing special. So I can only assume it's the compiler that reasons about mutation and eliminates copies as necessary. Type authors don't have to worry about it.

[soraros]

Mojo enforces COW

Citation needed.

[Brian-M-J]

Mojo enforces COW

Citation needed.

I already gave it:

it'll only create a copy if foo is mutated

it's not like String has some special magic code to enforce COW. Its __copyinit__ just calls List's __copyinit__, which is nothing special. So I can only assume it's the compiler that reasons about mutation and eliminates copies as necessary. Type authors don't have to worry about it.

[soraros]

I think you misunderstood what Jack wrote in the blog, he was talking about owned argument and move. String doesn't have COW semantics, and COW is most certainly not enforced by the language. I guess we could expect the compiler to eliminate more unnecessary copy, but I don't think that optimisation is/can be guaranteed (consider when __copyinit__ has other side effect).

fn main():
  var a: String = "a"
  var b = a  # a copy is made here

  ...