SpinLock: Add acquire/release() and spinPause()

We're very careful to ensure that we add a cpu_relax hint.

include/spinLock.h

@@ -2,6 +2,17 @@
 #define SPIN_LOCK_H
 
 #include <atomic>
+#ifdef __x86_64__
+#include <immintrin.h>
+#elif defined(__i386__)
+#include <xmmintrin.h>
+#elif defined(__arm__)
+#include <arm_neon.h>
+#elif defined(__aarch64__)
+#include <arm_neon.h>
+#elif defined(__aarch32__)
+#include <arm_neon.h>
+#endif
 
 namespace smo {
 
@@ -21,6 +32,41 @@ public:
 		return locked.compare_exchange_strong(expected, true);
 	}
 
+	inline void spinPause()
+	{
+#ifdef __x86_64__
+		_mm_pause();
+#elif defined(__i386__)
+		_mm_pause();
+#elif defined(__arm__)
+		__asm__ volatile("yield");
+#elif defined(__aarch64__)
+		__asm__ volatile("yield");
+#elif defined(__aarch32__)
+		__asm__ volatile("yield");
+#else
+# error "Unsupported architecture"
+#endif
+	}
+
+	void acquire()
+	{
+		while (!tryAcquire())
+		{
+			/**	EXPLANATION:
+			 * Busy-wait: keep trying to acquire the lock
+			 * The CPU will spin here until the lock becomes available
+			 *
+			 * The spinPause() function is architecture-specific and is
+			 * essential because I once fried an older Intel M-class laptop CPU
+			 * when I forgot to include a PAUSE instruction in a for (;;){}
+			 * loop. I'm not interested in frying my RPi or my other testbed
+			 * robot boards.
+			 */
+			spinPause();
+		}
+	}
+
 	void release()
 	{
 		locked.store(false);