From 1acf3219e49d5194740cc600a76e68962fbdce6f Mon Sep 17 00:00:00 2001
From: Brian Norris <banorris@uci.edu>
Date: Tue, 9 Oct 2012 11:15:36 -0700
Subject: [PATCH] barrier: add hand-written barrier implementation

Grabbed from:
http://stackoverflow.com/questions/8115267/writing-a-spinning-thread-barrier-using-c11-atomics
---
 barrier/barrier.h | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 barrier/barrier.h

diff --git a/barrier/barrier.h b/barrier/barrier.h
new file mode 100644
index 0000000..871e10f
--- /dev/null
+++ b/barrier/barrier.h
@@ -0,0 +1,33 @@
+#include <atomic>
+
+class spinning_barrier {
+ public:
+	spinning_barrier (unsigned int n) : n_ (n), nwait_ (0), step_(0) {}
+
+	bool wait() {
+		unsigned int step = step_.load ();
+
+		if (nwait_.fetch_add (1) == n_ - 1) {
+			/* OK, last thread to come.  */
+			nwait_.store (0); // XXX: maybe can use relaxed ordering here ??
+			step_.fetch_add (1);
+			return true;
+		} else {
+			/* Run in circles and scream like a little girl.  */
+			while (step_.load () == step)
+				;
+			return false;
+		}
+	}
+
+ protected:
+	/* Number of synchronized threads. */
+	const unsigned int n_;
+
+	/* Number of threads currently spinning.  */
+	std::atomic<unsigned int> nwait_;
+
+	/* Number of barrier syncronizations completed so far, 
+	 *      * it's OK to wrap.  */
+	std::atomic<unsigned int> step_;
+};
-- 
2.34.1