Cannot use 0 as an address in wasm in with opt-level >= 1 #57897
Description
I want to use Rust to work on the raw chunk of memory that WebAssembly creates for me. Consider the following rust code:
#![no_std]
#![no_main]
use core::panic::PanicInfo;
use core::slice::from_raw_parts_mut;
#[no_mangle]
fn test() {
let in_b: &mut [u32];
unsafe {
in_b = from_raw_parts_mut::<u32>(0 as *mut u32, 3);
}
in_b[0] = 123;
}
#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
loop {}
}Compile with
rustc --target=wasm32-unknown-unknown -o rust.wasm rotate.rs
And load with this HTML file:
<!DOCTYPE html>
<script>
async function init() {
const { instance } = await WebAssembly.instantiate(
await fetch("/rust.wasm").then(r => r.arrayBuffer())
);
instance.exports.test();
console.log(new Uint32Array(instance.exports.memory.buffer, 0).slice(0, 3));
}
init();
</script>This will work fine (i.e. no errors).
If you add -C opt-level=1 (or 2 or 3) to the rustc invocation, the wasm module will fail with unreachable.
If you change the rust code as following
- in_b = from_raw_parts_mut::<u32>(0 as *mut u32, 3);
+ in_b = from_raw_parts_mut::<u32>(4 as *mut u32, 3);the code will work again.
I think if I can use 0 as an address in a debug build I should also be able to do it in release mode 😄