Rollup merge of rust-lang#56996 - clarcharr:spin_loop_hint, r=KodrAus

Move spin_loop_hint to core::hint module

As mentioned in rust-lang#55002. The new name is kept unstable to decide whether the function should have `_hint` in its name.

src/libcore/hint.rs

@@ -49,3 +49,26 @@ use intrinsics;
 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");
+    }
+}

src/libcore/sync/atomic.rs

@@ -84,6 +84,8 @@ use intrinsics;
 use cell::UnsafeCell;
 use fmt;
 
+use hint::spin_loop;
+
 /// Save power or switch hyperthreads in a busy-wait spin-loop.
 ///
 /// This function is deliberately more primitive than
@@ -96,15 +98,7 @@ use fmt;
 #[inline]
 #[stable(feature = "spin_loop_hint", since = "1.24.0")]
 pub fn spin_loop_hint() {
-    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
-    unsafe {
-        asm!("pause" ::: "memory" : "volatile");
-    }
-
-    #[cfg(target_arch = "aarch64")]
-    unsafe {
-        asm!("yield" ::: "memory" : "volatile");
-    }
+    spin_loop()
 }
 
 /// A boolean type which can be safely shared between threads.

src/libstd/lib.rs

@@ -286,6 +286,7 @@
 #![feature(staged_api)]
 #![feature(stmt_expr_attributes)]
 #![feature(str_internals)]
+#![feature(renamed_spin_loop)]
 #![feature(rustc_private)]
 #![feature(thread_local)]
 #![feature(toowned_clone_into)]