Improve traceability for constant memory placement for static variables

[PMQ9]

For this issue: #218

[LegNeato]

Thank you and welcome to the project! Looks pretty good, just some small things

[PMQ9]

One more change:

• The flag --use-constant-memory-space seems to be missing, I just added that.

Test example to trigger the error:

• /examples/cuda/const_memory_overflow/
• kernels/src/lib.rs

#![no_std]
#![crate_type = "staticlib"]
use cuda_std::*;

// 35KB per array, 3 arrays = 105KB total (well above 64KB limit)
const ARRAY_SIZE: usize = 35 * 1024 / 4; 

static BIG_ARRAY_1: [u32; ARRAY_SIZE] = [111u32; ARRAY_SIZE];
static BIG_ARRAY_2: [u32; ARRAY_SIZE] = [222u32; ARRAY_SIZE];
static BIG_ARRAY_3: [u32; ARRAY_SIZE] = [333u32; ARRAY_SIZE];

pub unsafe fn test_kernel(out: *mut u32) {
    *out = BIG_ARRAY_1[0] + BIG_ARRAY_2[0] + BIG_ARRAY_3[0];
}

• build.rs

use std::path::PathBuf;

fn main() {
    let manifest_dir = PathBuf::from(std::env::var("CARGO_MANIFEST_DIR").unwrap());
    let out_path = PathBuf::from(std::env::var("OUT_DIR").unwrap());

    println!("cargo::warning=Building GPU kernels with constant memory ENABLED");
    println!("cargo::warning=This should trigger constant memory overspill errors");
    
    cuda_builder::CudaBuilder::new(manifest_dir.join("kernels"))
        .copy_to(out_path.join("kernels.ptx"))
        // Enable constant memory placement to trigger the overspill error
        .use_constant_memory_space(true)
        .build()
        .unwrap();
}

• Cargo.toml

[workspace]

[package]
name = "const_memory_overflow"
version = "0.1.0"
edition = "2021"

[dependencies]
cust = { path = "../../../crates/cust" }

[build-dependencies]
cuda_builder = { path = "../../../crates/cuda_builder", features = ["rustc_codegen_nvvm"] }

• kernels/Cargo.toml

[workspace]

[package]
name = "const_memory_overflow_kernels"
version = "0.1.0"
edition = "2021"

[dependencies]
cuda_std = { path = "../../../../crates/cuda_std" }

[lib]
crate-type = ["cdylib", "rlib"]

• src/main,rs

use cust::prelude::*;

fn main() {  
    println!("This example is meant to fail at compile time!");
    println!("It demonstrates the improved error messages for constant memory overflow.");
}

Expected Failure

error: cannot place static `BIG_ARRAY_2` (35840 bytes) in constant memory:
cumulative constant memory usage would be 71680 bytes, exceeding the 65536 byte limit
= note: current constant memory usage: 35840 bytes
= note: static size: 35840 bytes
= note: would result in: 71680 bytes total
= help: move this or other statics to global memory...

[PMQ9]

Option 1: Move BIG_ARRAY_2 to global

static BIG_ARRAY_1: [u32; ARRAY_SIZE] = [111u32; ARRAY_SIZE];
#[cuda_std::address_space(global)]
static BIG_ARRAY_2: [u32; ARRAY_SIZE] = [222u32; ARRAY_SIZE];

Option 2: Disable feature competely

.use_constant_memory_space(false)