Wasmer 3.0.0 and fast data transferral

[Icedude907]

Hi there,

I've been experimenting with Wasmer as a hobby, and one of the things I've been doing is moving a large amount of data back and forth. Previously, I had a function like this which dumped ~100mb of data into the guest.

// Wasmer 2.3.0
#[derive(Copy, Clone)] pub struct Arr<T, const N: usize>(pub [T; N]);
unsafe impl<T: Copy, const N: usize> ValueType for Arr<T, N>{}

fn send_big_buffer(env: &EnvWithMemory, outbuf: WasmPtr<Arr<u8, 104857600>>)
    let mem = unsafe{outbuf.deref(env.memory_ref_unchecked())}.expect("Pointer provided was invalid");
    let mem = unsafe{ mem.get_mut() };

    mem.0.copy_from_slice(&data[..]);
}

Given large api changes (and my utilisation of a depreciated function) this is no longer possible. As per the comment on #1249, it seems that the correct way to do this is with WasmSlice, so my function becomes:

// Wasmer 3.0.0-rc.2
pub fn send_big_buffer(fnenv: FunctionEnvMut<EnvMem>, outbuf: WasmPtr<[u8;104857600]>){
    let mem = fnenv.data().memory.view(&fnenv);

    let outbuf: WasmPtr<u8> = unsafe{ std::mem::transmute(outbuf) };
    outbuf.slice(&mem, 104857600).unwrap().write_slice(&data[..]).expect("pointer flows out of bounds");
}

I understand these two methods aren't comparable in function (memory safety) - but I can't do the old way anymore and its a non-insugnificant slowdown. Wasm 3.0 on my machine took (dbg)0.38s/(rel)0.021s, whilst 2.3 just took a flat 0.01s.

As it stands, I am unsure if there's a better way to go about this. Documentation is still in its infancy reguarding 3.0 and lots of it is out of date, so I've had trouble finding info.

Guest code is this for both versions:

fn receive_big_buffer() -> Box::<[u8;104857600]>{
    unsafe{
        let mut buf = Box::<[u8;104857600]>::new([0;104857600]); // Should use new_uninit (nightly)
        super::externs::receive_big_buffer(buf.as_mut());
        return buf;
    }
}
mod externs { extern "C" {
    #[cfg_attr(target_arch = "wasm32", link_name = "receive_big_buffer(ptr)")]
    pub fn receive_big_buffer(outbuf: &mut [u8;104857600]);
}}

Thanks for listening.

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