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Tracking issue for uninitialized constructors for Box, Rc, Arc #63291
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Just from looking at the current source code: pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
let ptr = unsafe {
Global.alloc(layout)
.unwrap_or_else(|_| alloc::handle_alloc_error(layout))
};
Box(ptr.cast().into())
} When
Did I have overseen something or is this a bug? |
Good point! I copied this pattern from
|
I have noticed this when implementing This is my implementation for the sliced version. |
Requesting a zero-size allocation is not allowed, return a dangling pointer instead. CC rust-lang#63291 (comment)
Fix zero-size uninitialized boxes Requesting a zero-size allocation is not allowed, return a dangling pointer instead. CC rust-lang#63291 (comment)
Fix zero-size uninitialized boxes Requesting a zero-size allocation is not allowed, return a dangling pointer instead. CC rust-lang#63291 (comment)
Fix zero-size uninitialized boxes Requesting a zero-size allocation is not allowed, return a dangling pointer instead. CC rust-lang#63291 (comment)
Fix zero-size uninitialized boxes Requesting a zero-size allocation is not allowed, return a dangling pointer instead. CC rust-lang#63291 (comment)
alloc: Add new_zeroed() versions like new_uninit(). MaybeUninit has both uninit() and zeroed(), it seems reasonable to have the same surface on Box/Rc/Arc. Needs tests. cc rust-lang#63291
What are the requirements for stabilizing the I don't think using |
Yes, that’s exactly the idea with having |
Great! I still think it'd be better to provide safe abstraction for it, but at least it'd be possible to do in a library. |
Similar to |
It is possible to create a zeroed array or slice directly on the heap on Stable Rust and without using const N: usize = 4096*4096;
pub fn make_array() -> Box<[u8; N]> {
vec![0; N].into_boxed_slice().try_into().unwrap()
}
pub fn make_slice(n: usize) -> Box<[u8]> {
vec![0; n].into_boxed_slice()
} This doesn't blow the stack even with |
That’s an optimization (based on specialization of a private trait) in current versions of the standard library, but it only works for some types and is not a documented guarantee. |
Regarding the |
This is correct, but it's also true that we'd probably need a very good reason to regress it. If some code's correctness (somehow) relies on this, they should find another method, but for performance reasons I think it's safe enough to rely on. That said, as you point out it does not work for all types. |
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. See [1] [2] [3] for background information. [1] Rust-for-Linux/linux#879 [2] Rust-for-Linux/linux#2 [3] rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. See [1] [2] [3] for background information. [1] Rust-for-Linux/linux#879 [2] Rust-for-Linux/linux#2 [3] rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. See [1] [2] [3] for background information. [1] Rust-for-Linux#879 [2] Rust-for-Linux#2 [3] rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: #879 Link: #2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [Reworded to use `Link` tags] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: #879 Link: #2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux#879 Link: Rust-for-Linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
API looks very strange. So, please make signature so: impl<T> Box<MaybeUninit<T>, Global> {
fn new_uninit() -> Box<MaybeUninit<T>, Global> { ... }
} The same applies to other functions, for example, |
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux#879 Link: Rust-for-Linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux/linux#879 Link: Rust-for-Linux/linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux/linux#879 Link: Rust-for-Linux/linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux/linux#879 Link: Rust-for-Linux/linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux/linux#879 Link: Rust-for-Linux/linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux/linux#879 Link: Rust-for-Linux/linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux/linux#879 Link: Rust-for-Linux/linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
The unstable new_uninit feature enables various library APIs to create uninitialized containers, such as `Box::assume_init()`. This is necessary to build abstractions that directly initialize memory at the target location, instead of doing copies through the stack. Will be used by the DRM scheduler abstraction in the kernel crate, and by field-wise initialization (e.g. using `place!()` or a future replacement macro which may itself live in `kernel`) in driver crates. Link: Rust-for-Linux/linux#879 Link: Rust-for-Linux/linux#2 Link: rust-lang/rust#63291 Signed-off-by: Asahi Lina <lina@asahilina.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Link: https://lore.kernel.org/r/20230224-rust-new_uninit-v1-1-c951443d9e26@asahilina.net [ Reworded to use `Link` tags. ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This could be resolved by not making |
Assigning
MaybeUninit::<Foo>::uninit()
to a local variable is usually free, even whensize_of::<Foo>()
is large. However, passing it for example toArc::new
causes at least one copy (from the stack to the newly allocated heap memory) even though there is no meaningful data. It is theoretically possible that a Sufficiently Advanced Compiler could optimize this copy away, but this is reportedly unlikely to happen soon in LLVM.This issue tracks constructors for containers (
Box
,Rc
,Arc
) ofMaybeUninit<T>
or[MaybeUninit<T>]
that do not initialized the data, and unsafe conversions to the known-initialized types (withoutMaybeUninit
). The constructors are guaranteed not to make unnecessary copies.PR #62451 adds:
PR #66128 adds:
Unresolved question:
Box<MaybeUninit<T>>
might “belong” more as an associated function of that same type, rather thanBox<T>
. (And similarly for other constructors.) However this would make a call likeBox::<u32>::new_uninit()
becomesBox::<MaybeUnint<u32>>::new_uninit()
which feels unnecessarily verbose. I suspect that this turbofish will be needed in a lot of cases to appease type inference.The text was updated successfully, but these errors were encountered: