[Migrated] Language feature: barriers

Bắt đầu bằng việc xem xét GroupSharedArray, GroupSharedWriter, GroupSharedReader và ví dụ barrier trong issue. Điều tra hạn chế DerefMut/IndexMut đã nêu và các trường hợp cho phép ghi hoặc đọc lặp lại; công việc chỉ được xem là hoàn tất khi có một thiết kế rõ ràng, hợp lý để thực thi các barrier, thay vì prototype chưa hoàn chỉnh hiện tại.

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/8
Old labels: t: design
Originally creatd by Jasper-Bekkers on 2020-08-18T17:33:10Z

We want to have the same types of safety on the GPU as we do on the CPU. One of those areas that make it easy to introduce race conditions is with memory barriers. HLSL and GLSL require the programmer to explicitly put them in the right locations without any formal verifiction.

This is an attempt at solving that within the Rust type system (attempt still has racy bugs and is largely incomplete).

use core::ops::Deref;
use core::ops::DerefMut;
use core::ops::Index;
use core::ops::IndexMut;

struct MyFoo {
    test: GroupSharedArray<u32>,
}

struct GroupSharedArray<T>
where
    T: Default + Copy,
{
    data: Box<[T]>,
    size: usize,
}

impl<T: Default + Copy> GroupSharedArray<T> {
    fn new() -> Self {
        Self {
            size: 100,
            data: Box::new([T::default(); 100]),
        }
    }
}

struct GroupSharedWriter<T> {
    data: T,
}

impl<T> Deref for GroupSharedWriter<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.data
    }
}

impl<T> DerefMut for GroupSharedWriter<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.data
    }
}

impl<T> GroupSharedWriter<T> {
    fn new(data: T) -> Self {
        Self { data }
    }
    fn barrier(self) -> GroupSharedReader<T> {
        GroupSharedReader { data: self.data }
    }
}

struct GroupSharedReader<T> {
    data: T,
}

impl<T> Deref for GroupSharedReader<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.data
    }
}

impl<T> GroupSharedReader<T> {
    fn barrier(self) -> GroupSharedWriter<T> {
        GroupSharedWriter { data: self.data }
    }
}

impl<T: Default + Copy> Index<Uniform<u32>> for GroupSharedArray<T> {
    type Output = T;

    fn index(&self, index: Uniform<u32>) -> &Self::Output {
        &self.data[index.data as usize]
    }
}

impl<T: Default + Copy> IndexMut<Uniform<u32>> for GroupSharedArray<T> {
    fn index_mut(&mut self, index: Uniform<u32>) -> &mut Self::Output {
        &mut self.data[index.data as usize]
    }
}

struct Uniform<T> {
    data: T,
}

impl<T> Uniform<T> {
    fn new(data: T) -> Self {
        Self { data }
    }
}

fn thread_idx() -> Uniform<u32> {
    Uniform::new(0)
}

fn main() {
    let mut data = GroupSharedWriter::new(MyFoo {
        test: GroupSharedArray::new(),
    }); // new does barrier

    data.test[thread_idx()] = thread_idx().data;
    data.test[thread_idx()] = 666; // should be illegal

    let value = data.test[thread_idx()]; // should be illegal

    //data.test[0] = thread_idx().data; // what happens in this case? it's unsafe/unsound. race cnd over contents of `test`

    let other_data = data.barrier(); // `barrier` return GroupSharedReader
    
    // some race-y cases handled correctly:
    // data.test[thread_idx()] = 666; // is correctly marked illegal
    // other_data.test[thread_idx()] = 666; // is correctly marked illegal

    let test = other_data.data.test[thread_idx()];
}

// tl;dr
// - use move semantics to enforce barriers betwen reading and writing
// - broken right now because DerefMut and IndexMut require Deref and Index to be implemented
// - broken right now because we can write to the same location multiple times (data race)