Implementation of sync_val_compare_and_swap in ESP8266 (it is very general for embedded actually).

sync_val_compare_and_swap.rs

// See https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.3/com.ibm.xlc1313.aix.doc/compiler_ref/bif_gcc_atomic_val_comp_swap.html
#[cfg(not(feature = "disable-fake-atomics"))]
#[no_mangle]
fn __sync_val_compare_and_swap_1(ptr_exch: *mut u8, cmp: u8, exch: u8) -> u8 {
    let oldval = unsafe { *ptr_exch };
    if cmp == oldval {
        unsafe { core::ptr::write(ptr_exch, cmp) };
    }
    oldval
}

#[cfg(not(feature = "disable-fake-atomics"))]
#[no_mangle]
fn __sync_val_compare_and_swap_2(ptr_exch: *mut u16, cmp: u16, exch: u16) -> u16 {
    let oldval = unsafe { *ptr_exch };
    if cmp == oldval {
        unsafe { core::ptr::write(ptr_exch, cmp) };
    }
    oldval
}

#[cfg(not(feature = "disable-fake-atomics"))]
#[no_mangle]
fn __sync_val_compare_and_swap_4(ptr_exch: *mut u32, cmp: u32, exch: u32) -> u32 {
    let oldval = unsafe { *ptr_exch };
    if cmp == oldval {
        unsafe { core::ptr::write(ptr_exch, cmp) };
    }
    oldval
}