Auto merge of #57714 - matthewjasper:wellformed-unreachable, r=pnkfelix

[rust.git] / src / libcore / hint.rs

diff --git a/src/libcore/hint.rs b/src/libcore/hint.rs
index 0bfdd937abd630c1431100f9aa5139b7b6be8b0c..ad5a2071a7381c033bd5243441753b775f154a39 100644 (file)
--- a/src/libcore/hint.rs
+++ b/src/libcore/hint.rs
@@ -1,13 +1,3 @@
-// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
 #![stable(feature = "core_hint", since = "1.27.0")]
 
 //! Hints to compiler that affects how code should be emitted or optimized.
@@ -59,3 +49,26 @@
 pub unsafe fn unreachable_unchecked() -> ! {
     intrinsics::unreachable()
 }
+
+/// Save power or switch hyperthreads in a busy-wait spin-loop.
+///
+/// This function is deliberately more primitive than
+/// [`std::thread::yield_now`](../../std/thread/fn.yield_now.html) 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 = "renamed_spin_loop", issue = "55002")]
+pub fn spin_loop() {
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    unsafe {
+        asm!("pause" ::: "memory" : "volatile");
+    }
+
+    #[cfg(target_arch = "aarch64")]
+    unsafe {
+        asm!("yield" ::: "memory" : "volatile");
+    }
+}