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Add an unsafe method Uint8Array::view_mut_raw(ptr: *mut u8, length: usize)
so that JS can efficiently initialize a wasm buffer
#1643
Comments
Seems reasonable to me to add! I'm not 100% convinced we need this for mutability reasons, it seems a long stretch for anything bad to happen. Having this for unininitialized memory though is more compelling to me and seems pretty reasonable! |
In the same vein, I'd like to see a version of |
Oops, I think I meant |
|
Seems reasonable to me to add as well! |
@That3Percent Do you mean you want to be able to pass the uninitialized |
is there any way to check if |
Here's the workflow as it exists today and how it could be improved by the issue. Task: Given some Today:
Step 2 is what we want to get rid of. Why write 0 when it's going to be overridden immediately? But, it's not possible without invoking Undefined Behavior, because it's not valid to even have a Proposed:
|
I just took a look at #1855 . I think that what you built there is a decent step toward a more performant version of the existing |
@That3Percent does this match your use case?
It is not exactly what you asked for, the signature of the function is:
|
@That3Percent I think you can use the existing
Do I miss anything? Here is the entire test case:
|
Can anyone review the |
This touches more APIs, for example: pub fn read_pixels_with_opt_u8_array(
this: &WebGlRenderingContext,
(...)
pixels: Option<&mut [u8]>,
) -> Result<(), JsValue>; Currently there's no (legal and sound from Rust's POV) way of avoiding zero-initialization without changing this API. |
Motivation
I am using the following pattern to efficiently initialize a large buffer inside wasm memory from JavaScript without copies (see #1079 for full motivation):
There are two problems with this:
Uint8Array::view
takes an immutable slice ref, but then I go ahead and mutate the contents. Lying to the compiler is dangerous.If I wanted to eliminate the zero initialization, I could do that using
reserve
+set_len
on the vec. However, rust forbids wrapping uninitialized values in a slice - you have to use raw pointers when initializing uninitialized memory. But the current signature ofUint8Array::view
requires me to make a slice.Proposed Solution
I propose adding a new method to
Uint8Array
(and maybe to the other typed array variants):pub unsafe fn view_mut_raw(ptr: *mut u8, length: usize)
This would let me change my rust code to the following:
It solves the problems: We've eliminated the zero initialization, we're not constructing a slice around uninitialized memory, and we're not lying to the compiler about the mutation anymore.
Alternatives
I'm not aware of any obvious alternatives.
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