+/**
+ * Mark a variable as used and prevent the compiler from optimizing it
+ * away. This is useful for variables accessed only from inline
+ * assembler without the compiler being aware.
+ */
+#ifndef av_used
+#if AV_GCC_VERSION_AT_LEAST(3,1)
+# define av_used __attribute__((used))
+#else
+# define av_used
+#endif
+#endif
+