Add unstable book entry

Author: jonhoo

src/doc/unstable-book/src/SUMMARY.md

@@ -37,6 +37,7 @@
 - [collections](collections.md)
 - [collections_range](collections-range.md)
 - [command_envs](command-envs.md)
+- [compiler_barriers](compiler-barriers.md)
 - [compiler_builtins](compiler-builtins.md)
 - [compiler_builtins_lib](compiler-builtins-lib.md)
 - [concat_idents](concat-idents.md)

src/doc/unstable-book/src/compiler-barriers.md

@@ -0,0 +1,98 @@
+# `compiler_barriers`
+
+The tracking issue for this feature is: [#41092]
+
+[#41092]: https://github.com/rust-lang/rust/issues/41092
+
+------------------------
+
+The `compiler_barriers` feature exposes the `compiler_barrier` function
+in `std::sync::atomic`. This function is conceptually similar to C++'s
+`atomic_signal_fence`, which can currently only be accessed in nightly
+Rust using the `atomic_singlethreadfence_*` instrinsic functions in
+`core`, or through the mostly equivalent literal assembly:
+
+```rust
+#![feature(asm)]
+unsafe { asm!("" ::: "memory" : "volatile") };
+```
+
+A `compiler_barrier` 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_barrier`.
+
+## Examples
+
+The need to prevent re-ordering of reads and writes often arises when
+working with low-level devices. Consider a piece of code that interacts
+with an ethernet card with a set of internal registers that are accessed
+through an address port register (`a: &mut usize`) and a data port
+register (`d: &usize`). To read internal register 5, the following code
+might then be used:
+
+```rust
+fn read_fifth(a: &mut usize, d: &usize) -> usize {
+    *a = 5;
+    *d
+}
+```
+
+In this case, the compiler is free to re-order these two statements if
+it thinks doing so might result in better performance, register use, or
+anything else compilers care about. However, in doing so, it would break
+the code, as `x` would be set to the value of some other device
+register!
+
+By inserting a compiler barrier, we can force the compiler to not
+re-arrange these two statements, making the code function correctly
+again:
+
+```rust
+#![feature(compiler_barriers)]
+use std::sync::atomic;
+
+fn read_fifth(a: &mut usize, d: &usize) -> usize {
+    *a = 5;
+    atomic::compiler_barrier(atomic::Ordering::SeqCst);
+    *d
+}
+```
+
+Compiler barriers are also useful in code that implements low-level
+synchronization primitives. Consider a structure with two different
+atomic variables, with a dependency chain between them:
+
+```rust
+use std::sync::atomic;
+
+fn thread1(x: &atomic::AtomicUsize, y: &atomic::AtomicUsize) {
+    x.store(1, atomic::Ordering::Release);
+    let v1 = y.load(atomic::Ordering::Acquire);
+}
+fn thread2(x: &atomic::AtomicUsize, y: &atomic::AtomicUsize) {
+    y.store(1, atomic::Ordering::Release);
+    let v2 = x.load(atomic::Ordering::Acquire);
+}
+```
+
+This code will guarantee that `thread1` sees any writes to `y` made by
+`thread2`, and that `thread2` sees any writes to `x`. Intuitively, one
+might also expect that if `thread2` sees `v2 == 0`, `thread1` must see
+`v1 == 1` (since `thread2`'s store happened before its `load`, and its
+load did not see `thread1`'s store). However, the code as written does
+*not* guarantee this, because the compiler is allowed to re-order the
+store and load within each thread. To enforce this particular behavior,
+a call to `compiler_barrier(Ordering::SeqCst)` would need to be inserted
+between the `store` and `load` in both functions.
+
+Compiler barriers with weaker re-ordering semantics (such as
+`Ordering::Acquire`) can also be useful, but are beyond the scope of
+this text. Curious readers are encouraged to read the Linux kernel's
+discussion of [memory barriers][1], as well as C++ references on
+[`std::memory_order`][2] and [`atomic_signal_fence`][3].
+
+[1]: https://www.kernel.org/doc/Documentation/memory-barriers.txt
+[2]: http://en.cppreference.com/w/cpp/atomic/memory_order
+[3]: http://www.cplusplus.com/reference/atomic/atomic_signal_fence/

src/libcore/sync/atomic.rs

@@ -1598,7 +1598,7 @@ pub fn fence(order: Ordering) {
 /// [`AcqRel`]: enum.Ordering.html#variant.AcqRel
 /// [`Relaxed`]: enum.Ordering.html#variant.Relaxed
 #[inline]
-#[unstable(feature = "std_compiler_fences", issue = "41091")]
+#[unstable(feature = "compiler_barriers", issue = "41091")]
 pub fn compiler_barrier(order: Ordering) {
     unsafe {
         match order {