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    //! Initialization code ("crt0") written in Rust
    //!
    //! This is for bare metal systems where there is no ELF loader or OS to take
    //! care of initializing RAM for the program.
    //!
    //! # Initializing RAM
    //!
    //! On the linker script side, we must assign names (symbols) to the boundaries
    //! of the `.bss` and `.data` sections.
    //!
    //! ``` text
    //! .bss : ALIGN(4)
    //! {
    //!     _sbss = .;
    //!     *(.bss.*);
    //!     _ebss = ALIGN(4);
    //! } > RAM
    //!
    //! .data : ALIGN(4)
    //! {
    //!     _sdata = .;
    //!     *(.data.*);
    //!     _edata = ALIGN(4);
    //! } > RAM AT > FLASH
    //!
    //! _sidata = LOADADDR(.data);
    //! ```
    //!
    //! On the Rust side, we must bind to those symbols using an `extern` block.
    //!
    //! ```
    //! unsafe fn before_main() {
    //!     // The type, `u32`, indicates that the memory is 4-byte aligned
    //!     extern "C" {
    //!         static mut _sbss: u32;
    //!         static mut _ebss: u32;
    //!
    //!         static mut _sdata: u32;
    //!         static mut _edata: u32;
    //!
    //!         static _sidata: u32;
    //!     }
    //!
    //!     zero_bss(&mut _sbss, &mut _ebss);
    //!     init_data(&mut _sdata, &mut _edata, &_sidata);
    //! }
    //! ```
    //!
    //! # `.init_array` & `.pre_init_array`
    //!
    //! This crate also provides an API to add "life before main" functionality to
    //! bare metal systems.
    //!
    //! On the linker script side, instruct the linker to keep the `.init_array`
    //! sections from input object files. Store the start and end address of the
    //! merged `.init_array` section.
    //!
    //! ``` text
    //! .text :
    //! {
    //!   /* .. */
    //!   _init_array_start = ALIGN(4);
    //!   KEEP(*(.init_array));
    //!   _init_array_end = ALIGN(4);
    //!   /* .. */
    //! }
    //! ```
    //!
    //! On the startup code, invoke the `run_init_array` function *before* you call
    //! the user `main`.
    //!
    //! ```
    //! unsafe fn start() {
    //!     extern "C" {
    //!         static _init_array_start: extern "C" fn();
    //!         static _init_array_end: extern "C" fn();
    //!     }
    //!
    //!     ::r0::run_init_array(&_init_array_start, &_init_array_end);
    //!
    //!     extern "C" {
    //!         fn main(argc: isize, argv: *const *const u8) -> isize;
    //!     }
    //!
    //!     main();
    //! }
    //! ```
    //!
    //! Then the user application can use this crate `init_array!` macro to run code
    //! before `main`.
    //!
    //! ```
    //! init_array!(before_main, {
    //!     println!("Hello");
    //! });
    //!
    //! fn main() {
    //!     println!("World");
    //! }
    //! ```

    #![deny(warnings)]
    #![no_std]

    use core::{mem, ptr, slice};

    /// Initializes the `.data` section
    ///
    /// # Arguments
    ///
    /// - `sdata`. Pointer to the start of the `.data` section.
    /// - `edata`. Pointer to the open/non-inclusive end of the `.data` section.
    ///   (The value behind this pointer will not be modified)
    /// - `sidata`. `.data` section Load Memory Address (LMA)
    /// - Use `T` to indicate the alignment of the `.data` section and its LMA.
    ///
    /// # Safety
    ///
    /// - Must be called exactly once
    /// - `mem::size_of::<T>()` must be non-zero
    /// - `edata >= sdata`
    /// - The `sdata -> edata` region must not overlap with the `sidata -> ...`
    ///   region
    /// - `sdata`, `edata` and `sidata` must be `T` aligned.
    pub unsafe fn init_data<T>(
        mut sdata: *mut T,
        edata: *mut T,
        mut sidata: *const T,
    ) where
        T: Copy,
    {
        while sdata < edata {
            ptr::write(sdata, ptr::read(sidata));
            sdata = sdata.offset(1);
            sidata = sidata.offset(1);
        }
    }

    pub unsafe fn run_init_array(
        init_array_start: &extern "C" fn(),
        init_array_end: &extern "C" fn(),
    ) {
        let n = (init_array_end as *const _ as usize -
                     init_array_start as *const _ as usize) /
            mem::size_of::<extern "C" fn()>();

        for f in slice::from_raw_parts(init_array_start, n) {
            f();
        }
    }

    /// Zeroes the `.bss` section
    ///
    /// # Arguments
    ///
    /// - `sbss`. Pointer to the start of the `.bss` section.
    /// - `ebss`. Pointer to the open/non-inclusive end of the `.bss` section.
    ///   (The value behind this pointer will not be modified)
    /// - Use `T` to indicate the alignment of the `.bss` section.
    ///
    /// # Safety
    ///
    /// - Must be called exactly once
    /// - `mem::size_of::<T>()` must be non-zero
    /// - `ebss >= sbss`
    /// - `sbss` and `ebss` must be `T` aligned.
    pub unsafe fn zero_bss<T>(mut sbss: *mut T, ebss: *mut T)
    where
        T: Copy,
    {
        while sbss < ebss {
            // NOTE(volatile) to prevent this from being transformed into `memclr`
            ptr::write_volatile(sbss, mem::zeroed());
            sbss = sbss.offset(1);
        }
    }

    #[macro_export]
    macro_rules! pre_init_array {
        ($name:ident, $body:expr) => {
            #[allow(dead_code)]
            unsafe extern "C" fn $name() {
                #[link_section = ".pre_init_array"]
                #[used]
                static PRE_INIT_ARRAY_ELEMENT: unsafe extern "C" fn() = $name;

                #[inline(always)]
                fn inner() {
                    $body
                }

                inner()
            }
        }
    }

    #[macro_export]
    macro_rules! init_array {
        ($name:ident, $body:expr) => {
            #[allow(dead_code)]
            unsafe extern "C" fn $name() {
                #[link_section = ".init_array"]
                #[used]
                static INIT_ARRAY_ELEMENT: unsafe extern "C" fn() = $name;

                #[inline(always)]
                fn inner() {
                    $body
                }

                inner()
            }
        }
    }

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