# Function atomic_polyfill::compiler_fence

1.21.0 ·

    pub fn compiler_fence(order: Ordering)

Expand description

A compiler memory fence.

`compiler_fence` does not emit any machine code, but restricts the kinds of memory re-ordering the compiler is allowed to do. Specifically, depending on the given `Ordering` semantics, the compiler may be disallowed from moving reads or writes from before or after the call to the other side of the call to `compiler_fence`. Note that it does not prevent the hardware from doing such re-ordering. This is not a problem in a single-threaded, execution context, but when other threads may modify memory at the same time, stronger synchronization primitives such as `fence` are required.

The re-ordering prevented by the different ordering semantics are:

- with `SeqCst`, no re-ordering of reads and writes across this point is allowed.
- with `Release`, preceding reads and writes cannot be moved past subsequent writes.
- with `Acquire`, subsequent reads and writes cannot be moved ahead of preceding reads.
- with `AcqRel`, both of the above rules are enforced.

`compiler_fence` is generally only useful for preventing a thread from racing with itself. That is, if a given thread is executing one piece of code, and is then interrupted, and starts executing code elsewhere (while still in the same thread, and conceptually still on the same core). In traditional programs, this can only occur when a signal handler is registered. In more low-level code, such situations can also arise when handling interrupts, when implementing green threads with pre-emption, etc. Curious readers are encouraged to read the Linux kernel’s discussion of memory barriers.

## Panics

Panics if `order` is `Relaxed`.

## Examples

Without `compiler_fence`, the `assert_eq!` in following code is not guaranteed to succeed, despite everything happening in a single thread. To see why, remember that the compiler is free to swap the stores to `IMPORTANT_VARIABLE` and `IS_READY` since they are both `Ordering::Relaxed`. If it does, and the signal handler is invoked right after `IS_READY` is updated, then the signal handler will see `IS_READY=1`, but `IMPORTANT_VARIABLE=0`. Using a `compiler_fence` remedies this situation.

    use std::sync::atomic::{AtomicBool, AtomicUsize};
    use std::sync::atomic::Ordering;
    use std::sync::atomic::compiler_fence;

    static IMPORTANT_VARIABLE: AtomicUsize = AtomicUsize::new(0);
    static IS_READY: AtomicBool = AtomicBool::new(false);

    fn main() {
        IMPORTANT_VARIABLE.store(42, Ordering::Relaxed);
        // prevent earlier writes from being moved beyond this point
        compiler_fence(Ordering::Release);
        IS_READY.store(true, Ordering::Relaxed);
    }

    fn signal_handler() {
        if IS_READY.load(Ordering::Relaxed) {
            assert_eq!(IMPORTANT_VARIABLE.load(Ordering::Relaxed), 42);
        }
    }