Normative: add Atomics.pause

spec.html

@@ -47210,6 +47210,24 @@ <h1>Atomics.notify ( _ta_, _index_, _count_ )</h1>
       </emu-alg>
     </emu-clause>
 
+    <emu-clause id="Atomics.pause">
+      <h1>Atomics.pause ( )</h1>
+      <p>This function provides a hint to the CPU that the program is spin looping while waiting on a value.</p>
+      <p>It performs the following steps when called:</p>
+      <emu-alg>
+        1. If the execution environment of the ECMAScript implementation supports signaling to the operating system or CPU that the current executing code is in a spin-wait loop, send that signal.
+        1. Return *undefined*.
+      </emu-alg>
+      <emu-note>
+        <p>This method is designed for programs implementing spin-wait loops, such as spinlock fast paths inside of mutexes, to provide a hint to the CPU that it is spinning while waiting on a value. It has no observable behaviour other than timing.</p>
+        <p>Implementations are expected to implement a pause or yield instruction if the best practices of the underlying architecture recommends such instructions in spin loops. For example, the <a href="https://www.intel.com/content/www/us/en/content-details/671488/intel-64-and-ia-32-architectures-optimization-reference-manual-volume-1.html">Intel Optimization Manual</a> recommends the <code>pause</code> instruction.</p>
+        <p>Implementations are encouraged to have an internal upper bound on the maximum amount of time paused on the order of tens to hundreds of nanoseconds.</p>
+      </emu-note>
+      <emu-note>
+        <p>Due to the overhead of function calls, it is reasonable that an inlined call to this method in an optimizing compiler waits a different amount of time than a non-inlined call.</p>
+      </emu-note>
+    </emu-clause>
+
     <emu-clause id="sec-atomics.xor" type="built-in function">
       <h1>Atomics.xor ( _ta_, _index_, _value_ )</h1>
       <p>This function performs the following steps when called:</p>