Re: [PATCH v5 14/15] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit`

[Alice Ryhl]

On 4/3/23 18:05, Benno Lossin wrote:
> `UniqueArc::try_new_uninit` calls `Arc::try_new(MaybeUninit::uninit())`.
> This results in the uninitialized memory being placed on the stack,
> which may be arbitrarily large due to the generic `T` and thus could
> cause a stack overflow for large types.
>
> Change the implementation to use the pin-init API which enables in-place
> initialization. In particular it avoids having to first construct and
> then move the uninitialized memory from the stack into the final location.
>
> Signed-off-by: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
> Cc: Gary Guo <gary@xxxxxxxxxxx>
> Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Cc: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>

Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

>       /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
>       pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
> -        Ok(UniqueArc::<MaybeUninit<T>> {
> +        // INVARIANT: The refcount is initialised to a non-zero value.
> +        let inner = Box::try_init::<AllocError>(try_init!(ArcInner {
> +            // SAFETY: There are no safety requirements for this FFI call.
> +            refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> +            data <- init::uninit::<T, AllocError>(),
> +        }? AllocError))?;
> +        Ok(UniqueArc {
>               // INVARIANT: The newly-created object has a ref-count of 1.
> -            inner: Arc::try_new(MaybeUninit::uninit())?,
> +            // SAFETY: The pointer from the `Box` is valid.
> +            inner: unsafe { Arc::from_inner(Box::leak(inner).into()) },
>           })
>       }
>   }

I'm curious - do you know whether this compiles to the same machine code 
as this?

pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
    let inner: Box<MaybeUninit<ArcInner<T>>> = Box::try_new_uninit()?;
    let ptr = Box::into_raw(inner) as *mut ArcInner<T>;
    addr_of_mut!((*ptr).refcount).write(bindings::REFCOUNT_INIT(1));
    Ok(UniqueArc {
        inner: Arc {
            ptr: unsafe { NonNull::new_unchecked(ptr) },
            _p: PhantomData,
        }
    })
}