[jganetsk]

I don't know that synchronizing with acquire-release is enough to make LLVM happy in this case. Mutable references are tagged with the LLVM noalias attribute. Rust language folks are very adamant about how wrong it is to end up with multiple aliases tagged with noalias. This is playing with fire.

[Arnavion]

There are no multiple aliases in the code presented in the blog post.

[jganetsk]

Ok you are right.

[steveklabnik]

Also UnsafeCell’s purpose in life is to make sure noalias is removed where appropriate.

[jganetsk]

Can you explain that more? Because the requirement I pasted about having unique aliases does come from the UnsafeCell documentation. My understanding is that &mut still needs to be a unique alias even when it comes from an UnsafeCell.

[Arnavion]

>My understanding is that &mut still needs to be a unique alias even when it comes from an UnsafeCell.

That is correct. steveklabnik was not talking about this.

UnsafeCell needs to prevent optimizations that reorder accesses to the `&mut` derived from its `*mut`. If reordering was allowed it could be possible that you write code that creates two `&mut` (or one `&mut` and one `&`) that appear to have distinct lifetimes (which ought to be well-defined) but are nevertheless reordered by the optimizer to overlap (which is aliasing, and thus UB).

It also needs to disable the assumption that a `&Foo` is immutable if `Foo` is `UnsafeCell` or an aggregate that transitively contains `UnsafeCell`.

This special behavior is why UnsafeCell is a lang item.

[steveklabnik]

Yes, the latter was what I was thinking of:

  use std::cell::UnsafeCell;

  pub fn no_unTsafecell(x: &i32) {
    
  }

  pub fn unsafecell(x: &UnsafeCell<i32>) {
    
  }

gives

  define void @_ZN10playground14no_unTsafecell17hd5b10dbf031749b4E(i32* noalias nocapture readonly align 4 dereferenceable(4) %x) unnamed_addr #0 {

  define void @_ZN10playground10unsafecell17h3318292255a4201bE(i32* nocapture align 4 dereferenceable(4) %x) unnamed_addr #0 {

thanks :)

[jganetsk]

Does it also help that the code doesn't actually cast the raw pointer to references? It only uses the read and write methods.