Can I force Rust to not optimize a single function?

I have a function where Rust's/LLVM's optimization fails and leads to a panic (in the release version), while the unoptimized code (debug version) just works fine. If I compare the generated assembly code, I can not even grasp an idea what the optimizer tries to accomplish. (A reason might be that this very function uses inline assembler.)

Is there any way to tell Rust to leave certain functions alone during the optimisation, or do I have to switch off all optimizations?

Here is the specific function:

#[naked]
pub extern "C" fn dispatch_svc(){
    Cpu::save_context();
    let mut nr: u32 = 0;
    unsafe {
        asm!("ldr r0, [lr, #-4]
              bic $0, r0, #0xff000000":"=r"(nr)::"r0":"volatile")
    };
    swi_service_routine(nr);
    Cpu::restore_context_and_return();
}