Now the experimental miri stacked borrows consider this as Undefined Behavior <div

To implement it, we need: <li class="task-list-ite

Any code in Rust is always thread-safe, otherwise it's <a href="https://doc.rust-lang.

Does it give any improvement in speed by itself? <p dir

Develompent in <a href="https://github.com/linksplatform/doublets-rs/tree/stacked-borr

Stacked borrows Initiative about doublets-rs HOT 9 OPEN

uselessgoddess commented on July 19, 2024 1

Stacked borrows Initiative

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Comments (9)

Konard commented on July 19, 2024 1

If Rust forces us to be not UB here, we should try to make code better.

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uselessgoddess commented on July 19, 2024

To implement it, we need:

Fix Szb trait from trees-rs to remove self parameter from all methods. Add explicit data argument (currently &mut [T])
Remove all data fields from Szb trait implementors from doublets
Rework all Store's functional to use new trees.

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Konard commented on July 19, 2024

At the moment our tree implementations do not support thread safety. Can you make a benchmark of this proposal? Does it give any improvement in speed by itself?

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uselessgoddess commented on July 19, 2024

Any code in Rust is always thread-safe, otherwise it's UB.
Also, intentional violation of Stacked Borrows is also UB.

This code is works, but actually it's UB (later)

let mut data = 10;
let ref1 = &mut data;
let ref2 = ref1 as *mut _;
unsafe {
    *ref1 += 1;
    *ref2 += 2;
}

error: Undefined Behavior: attempting a read access using <untagged> at alloc2284[0x0], but that tag does not exist in the borrow stack for this location
 --> src\main.rs:8:9
  |
8 |         *ref2 += 2;
  |         ^^^^^^^^^^
  |         |
  |
4 |     let ref2 = ref1 as *mut _;
  |                ^^^^
help: tag was later invalidated at offsets [0x0..0x4]
 --> src\main.rs:7:9
  |
7 |         *ref1 += 1;
  |         ^^^^^^^^^^
  = note: inside `main` at src\main.rs:8:9

Since the compiler uses two rules for optimization:

cannot borrow X as mutable more than once at a time
cannot borrow X as mutable because it is also borrowed as immutable

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uselessgoddess commented on July 19, 2024

Does it give any improvement in speed by itself?

I can only promise compiler optimizations and no UB

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uselessgoddess commented on July 19, 2024

It is my initiative, but it will help us to make the code better and without possible UB

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uselessgoddess commented on July 19, 2024

Why is bad?
Try to write this code
It's actually the valid code, but the compiler isn't that smart:

let mut data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let monitor = &data[..5];
data[..5].copy_from_slice(&[10, 9, 8, 7, 6]);
println!("hey, i immutable: {monitor:?}");
println!("{data:?}");

error[E0502]: cannot borrow `data` as mutable because it is also borrowed as immutable
 --> src\main.rs:5:10
  |
4 |     let monitor = &data[..5];
  |                    ---- immutable borrow occurs here
5 |     &mut data[..5].copy_from_slice(&[10, 9, 8, 7, 6]);
  |          ^^^^ mutable borrow occurs here
6 |
7 |     println!("hey, i immutable: {monitor:?}");
  |                                  ------- immutable borrow later used here

Ok, unsafe time:

let mut data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let monitor = unsafe { from_raw_parts(data.as_ptr(), data.len()).get_unchecked(..5) };
data[..5].copy_from_slice(&[10, 9, 8, 7, 6]);
println!("hey, i immutable: {monitor:?}");
println!("{data:?}");

It works:

hey, i immutable: [10, 9, 8, 7, 6]
[10, 9, 8, 7, 6, 6, 7, 8, 9, 10]

But miri...

     |
2371 | fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     | |
     | trying to reborrow from <4390> for SharedReadOnly permission at alloc2110[0x0], but that tag does not exist in the borrow stack for this location
     | this error occurs as part of a reborrow at alloc2110[0x0..0x14]
     |

Hm. println! hinders us. Remove it:

let mut data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let monitor = unsafe { from_raw_parts(data.as_ptr(), data.len()).get_unchecked(..5) };
data[..5].copy_from_slice(&[10, 9, 8, 7, 6]);
let _ = monitor;

error: Undefined Behavior: trying to reborrow from <4390> for SharedReadOnly permission at alloc2102[0x0], but that tag does not exist in the borrow stack for this location
 --> src\main.rs:9:19
  |
9 |     let monitor = monitor;
  |                   ^^^^^^^
  |                   |
  |                   trying to reborrow from <4390> for SharedReadOnly permission at alloc2102[0x0], but that tag does not exist in the borrow stack for this location
  |                   this error occurs as part of a reborrow at alloc2102[0x0..0x14]
  |

Why are you doing that?
from_raw_parts(data.as_ptr(), data.len()) borrow whole data. Miri think that we borrow data[0..10]. Let's rewrite the code:

let mut data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let monitor = unsafe { from_raw_parts(data.as_ptr().add(5), data.len() - 5) };
data[..5].copy_from_slice(&[10, 9, 8, 7, 6]);

Nice. Miri silent.
I also recommend using a safe rust:

let mut data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let (monitor, hot) = data.split_at_mut(5);
hot.copy_from_slice(&[10, 9, 8, 7, 6]);

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uselessgoddess commented on July 19, 2024

It's not exactly dangerous, but possible has potential bugs

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uselessgoddess commented on July 19, 2024

Develompent in stacked-borrows-initiative branch

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Stacked borrows Initiative about doublets-rs HOT 9 OPEN

Comments (9)

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