Is it possible to copy data to device constant memory?

[patrickcsullivan]

When using C CUDA it's possible to allocate constant memory on the GPU by using cudaMemcpyToSymbol rather than cudaMemcpy. Is there a similar mechanism in the cust library for copying an array with a constant size from the host to the device constant memory?

[RDambrosio016]

I believe module::Symbol's impl of CopyDestination should work for this, i don't think the driver API actually has memcpyToSymbol, at least from what i can see, we just use HtoD memcpys which will probably work the same.

[patrickcsullivan]

Thanks for the response!

Using module::Symbol makes sense to me. However, do you know how I can declare a variable in my GPU module so that it is compiled as a global symbol which I can then access by calling the Module::get_global function?

Here’s an example of what I’m trying to do:

// GPU Code

use cuda_std::prelude::*;

// I assume this is the wrong way to declare a global symbol.
// If this were C CUDA, then I'd annotate MY_NUM with __const__
pub static MY_NUM: i64 = 0;

#[kernel]
#[allow(improper_ctypes_definitions, clippy::missing_safety_doc)]
pub unsafe fn simple_kernel(out: *mut i64, n: usize) {
    let mut tid = (thread::thread_idx_x() + thread::block_idx_x() * thread::block_dim_x()) as usize;
    while tid < n {
        *out.add(tid) = MY_NUM;
        tid += (thread::block_dim_x() * thread::grid_dim_x()) as usize;
    }
}

// CPU Code

let _ctx = cust::quick_init()?;
let module = Module::from_ptx(PTX, &[])?;
let kernel = module.get_function("simple_kernel")?;

let symbol_name = CString::new("MY_NUM")?;
let mut symbol = module.get_global::<i64>(symbol_name.as_c_str())?;

let my_num: i64 = 1234;
symbol.copy_from(&my_num)?;

When I run this I get the following error:

Error: NotFound

I assume that MY_NUM is not getting compiled as a global symbol, so module.get_global can’t find it.

If I were writing C CUDA then I assume I’d need to annotate MY_NUM with __const__. Do you know what I need to do in my Rust GPU code to ensure that MY_NUM gets compiled as a global symbol?

[Stock84-dev]

#[no_mangle]
static DATA: [f32; 1024] = [0.; 1024];