Cast between pointer types

[izissise]

Hello
I'm trying to use a storage buffer of u8 to store a i32

#[spirv(compute(threads(256)))]
pub fn main_cs(
    #[spirv(global_invocation_id)] global_invocation_id: UVec3,
    #[spirv(local_invocation_id)] local_invocation_id: UVec3,
    #[spirv(workgroup_id)] workgroup_id: UVec3,
    #[spirv(storage_buffer, descriptor_set = 0, binding = 0)] inoutbuf: &mut [u8]
) {
...
    let gid = global_invocation_id.x as usize;
    let lid = local_invocation_id.x as usize; // thread id
    
    inoutbuf[gid] = 0; // zero out to use it as an output
    let out: &mut u32 = unsafe {
        let ptr = inoutbuf.as_mut_ptr() as *mut u32;
        &mut *ptr
    };
    *out = 42;
...
}

But I keep getting this error

error: cannot cast between pointer types
       from `*u8`
         to `*u32`

It works in rust playground https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=2949039e961bbadd0c8920c5926d62c2

How can I do it?

Thanks,
Hugues

[LegNeato]

I think you can use something like:

buffer[0..4].copy_from_slice(&value.to_le_bytes());

Or use bytemuck.

[izissise]

I tried with bytemuck but got a similar error, using to_ne_bytes , I get

    let mut res = 0;
    let mut res_b = [0_u8; 4];
    reduce_sum(lid, wgshared, &mut res);
    if gid == 0 {
        res_b[0..4].copy_from_slice(&res.to_le_bytes());
        inoutbuf[0] = res_b[0];
        inoutbuf[1] = res_b[1];
        inoutbuf[2] = res_b[2];
        inoutbuf[3] = res_b[3];
    }

I still get an error about casting

error: cannot cast between pointer types
    from `*[u8; 4]`
      to `*[u8]`
--> src/lib.rs:246:9
 |
246 |         res_b[0..4].copy_from_slice(&res.to_le_bytes());
 |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Also I'd like to avoid the extra copy

[LegNeato]

Hm, this might be #46

[izissise]

A minimal non compiling example

#[spirv(compute(threads(256)))]
pub fn main_cs(
    #[spirv(storage_buffer, descriptor_set = 0, binding = 0)] inoutbuf: &mut [u8],
) {
    let answer: i32 = 42;
    inoutbuf[0..4].copy_from_slice(&answer.to_le_bytes());
}

[LegNeato]

Filed as #164 to track.

[izissise]

As a workaround I'm doing it the other way, in the compute shader I take the storage buffer as a &[u32]

and I access individual byte like so

    let iob_idx  = gid / 4;
    let byte_idx = gid % 4;
    let b = ((inoutbuf[iob_idx] >> (32 - (byte_idx * 8))) & 0xff) as u8;

I can simply give the first element to the reduce function

reduce_sum(lid, wgshared, &mut inoutbuf[0] );

This works correctly but I'd rather cast the pointer

[izissise]

Today I had a similar error when trying to iterate over an array of u8

157 |        let chs = [0_u8, 2, 4];
158 | /     for c in chs {
159 | |         sum += match c {
...   |
166 | |         };
167 | |     }
    | |_____^


error: cannot cast between pointer types
       from `*[u8; 3]`
         to `*[u8]`

[LegNeato]

Ok, it looks like this is an existing issue. Our slice support is incomplete unless the slice is an input or output buffer.