no_mangle on DEVICE_PERIPHERALS prevents optimizing write away

[bauen1]

For fun (and profit) I'm trying to reduce the .bss usage of a rust embedded program down to zero, relying only on the stack for memory.

Actually the only users of .bss apart from the stack are the singleton guards of the stm32l4r5::CorePeripherals and stm32l4r5::Peripherals structs.

And by replacing take with steal, since I'm sure that main will only be called "once", these should have been eliminated, however for some weird reason DEVICE_PERIPHERALS persisted.

The reason for that is outlined in the short snippet below:

// Type your code here, or load an example.
#![no_std]
#![no_main]

use core::panic::PanicInfo;

#[panic_handler]
fn panic(_panic: &PanicInfo<'_>) -> ! {
    loop {}
}

// As of Rust 1.75, small functions are automatically
// marked as `#[inline]` so they will not show up in
// the output when compiling with optimisations. Use
// `#[no_mangle]` or `#[inline(never)]` to work around
// this issue.
// See https://github.com/compiler-explorer/compiler-explorer/issues/5939

// If you use `main()`, declare it as `pub` to see it in the output:
// pub fn main() { ... }

#[no_mangle]
static mut USED1: bool = false;

static mut USED2: bool = false;

#[inline(never)]
pub fn main() -> () {

  unsafe { USED1 = true };

  unsafe { USED2 = true };

  loop {

  }
}

godbolt

Only USED1 appears in the output, USED2 is optimized away.
Maybe you could also use no_mangle in the same way cortex-m does ?
https://docs.rs/cortex-m/latest/src/cortex_m/peripheral/mod.rs.html#156-162

When using the steal method, LLVM is then actually smart enough to completely eliminate the variable that's written only once, like #151 (comment) thought.