Auto merge of #41207 - sstewartgallus:spinpause, r=alexcrichton

Spin loop pause function redux

GitHub's interface is screwy.

This is the same PR as #40537

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

@@ -145,6 +145,7 @@
     - [future_atomic_orderings](library-features/future-atomic-orderings.md)
     - [get_type_id](library-features/get-type-id.md)
     - [heap_api](library-features/heap-api.md)
+    - [hint_core_should_pause](library-features/hint-core-should-pause.md)
     - [i128](library-features/i128.md)
     - [inclusive_range](library-features/inclusive-range.md)
     - [integer_atomics](library-features/integer-atomics.md)

src/doc/unstable-book/src/library-features/hint-core-should-pause.md

@@ -0,0 +1,41 @@
+# `hint_core_should_pause`
+
+The tracking issue for this feature is: [#41196]
+
+[#41196]: https://github.com/rust-lang/rust/issues/41196
+
+------------------------
+
+Many programs have spin loops like the following:
+
+```rust,no_run
+use std::sync::atomic::{AtomicBool,Ordering};
+
+fn spin_loop(value: &AtomicBool) {
+    loop {
+        if value.load(Ordering::Acquire) {
+             break;
+        }
+    }
+}
+```
+
+These programs can be improved in performance like so:
+
+```rust,no_run
+#![feature(hint_core_should_pause)]
+use std::sync::atomic;
+use std::sync::atomic::{AtomicBool,Ordering};
+
+fn spin_loop(value: &AtomicBool) {
+    loop {
+        if value.load(Ordering::Acquire) {
+             break;
+        }
+        atomic::hint_core_should_pause();
+    }
+}
+```
+
+Further improvements could combine `hint_core_should_pause` with
+exponential backoff or `std::thread::yield_now`.

src/libcore/sync/atomic.rs

@@ -94,6 +94,29 @@ use intrinsics;
 use cell::UnsafeCell;
 use fmt;
 
+/// Save power or switch hyperthreads in a busy-wait spin-loop.
+///
+/// This function is deliberately more primitive than
+/// `std::thread::yield_now` and does not directly yield to the
+/// system's scheduler.  In some cases it might be useful to use a
+/// combination of both functions.  Careful benchmarking is advised.
+///
+/// On some platforms this function may not do anything at all.
+#[inline]
+#[unstable(feature = "hint_core_should_pause", issue = "41196")]
+pub fn hint_core_should_pause()
+{
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    unsafe {
+        asm!("pause" ::: "memory" : "volatile");
+    }
+
+    #[cfg(target_arch = "aarch64")]
+    unsafe {
+        asm!("yield" ::: "memory" : "volatile");
+    }
+}
+
 /// A boolean type which can be safely shared between threads.
 ///
 /// This type has the same in-memory representation as a `bool`.