Atomic compare exchange for ARM platforms

Makes similar kernel-specific implementations unnecessary,
although i don't delete them with this commit.

base/include/arm/cpu/atomic.h

@@ -0,0 +1,54 @@
+/*
+ * \brief  Atomic operations for ARM
+ * \author Norman Feske
+ * \author Stefan Kalkowski
+ * \date   2007-04-28
+ */
+
+/*
+ * Copyright (C) 2007-2012 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU General Public License version 2.
+ */
+
+#ifndef _INCLUDE__ARM__CPU__ATOMIC_H_
+#define _INCLUDE__ARM__CPU__ATOMIC_H_
+
+namespace Genode {
+
+	/**
+	 * Atomic compare and exchange
+	 *
+	 * This function compares the value at dest with cmp_val.
+	 * If both values are equal, dest is set to new_val. If
+	 * both values are different, the value at dest remains
+	 * unchanged.
+	 *
+	 * \return  1 if the value was successfully changed to new_val,
+	 *          0 if cmp_val and the value at dest differ.
+	 */
+	inline int cmpxchg(volatile int *dest, int cmp_val, int new_val)
+	{
+		unsigned long equal, not_exclusive;
+
+		__asm__ __volatile__(
+			"@ cmpxchg\n"
+			"  1:                  \n"
+			"  ldrex   %0, [%2]    \n"
+			"  cmp     %0, %3      \n"
+			"  bne     2f          \n"
+			"  strexeq %0, %4, [%2]\n"
+			"  teqeq   %0, #0      \n"
+			"  bne     1b          \n"
+			"  moveq   %1, #1      \n"
+			"  2:                  \n"
+			"  movne   %1, #0      \n"
+			: "=&r" (not_exclusive), "=&r" (equal)
+			: "r" (dest), "r" (cmp_val), "r" (new_val)
+			: "cc");
+		return equal && !not_exclusive;
+	}
+}
+
+#endif /* _INCLUDE__ARM__CPU__ATOMIC_H_ */