You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mod.rs
    path of file: ./repos/tock/arch/riscv/src/csr
    
    the code of the file until where you have to start completion: // Licensed under the Apache License, Version 2.0 or the MIT License.
// SPDX-License-Identifier: Apache-2.0 OR MIT
// Copyright Tock Contributors 2022.

//! Tock Register in
    
    pub fn read_cycle_counter(&self) -> u64 {
        let (mut top, mut bot): (usize, usize);

        // Need to handle the potential for rollover between reading the lower
        // and upper bits. We do this by reading twice, and seeing if the upper
        // bits change between reads. This should only ever loop at most twice.
        loop {
            top = CSR.mcycleh.read(mcycle::mcycleh::mcycleh);
            bot = CSR.mcycle.read(mcycle::mcycle::mcycle);
            if top == CSR.mcycleh.read(mcycle::mcycleh::mcycleh) {
                break;
            }
        }

        (top as u64).checked_shl(32).unwrap() + bot as u64
    }