utils: add lxc_find_next_power2()

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/utils.c

@@ -2518,3 +2518,21 @@ int parse_byte_size_string(const char *s, int64_t *converted)
 	*converted = overflow;
 	return 0;
 }
+
+uint64_t lxc_find_next_power2(uint64_t n)
+{
+	/* 0 is not valid input. We return 0 to the caller since 0 is not a
+	 * valid power of two.
+	 */
+	if (n == 0)
+		return 0;
+
+	if (!(n & (n - 1)))
+		return n;
+
+	while (n & (n - 1))
+		n = n & (n - 1);
+
+	n = n << 1;
+	return n;
+}

src/lxc/utils.h

@@ -472,4 +472,14 @@ extern bool lxc_nic_exists(char *nic);
 extern int lxc_make_tmpfile(char *template, bool rm);
 extern uint64_t lxc_getpagesize(void);
 
+/* If n is not a power of 2 this function will return the next power of 2
+ * greater than that number. Note that this function always returns the *next*
+ * power of 2 *greater* that number not the *nearest*. For example, passing 1025
+ * as argument this function will return 2048 although the closest power of 2
+ * would be 1024.
+ * If the caller passes in 0 they will receive 0 in return since this is invalid
+ * input and 0 is not a power of 2.
+ */
+extern uint64_t lxc_find_next_power2(uint64_t n);
+
 #endif /* __LXC_UTILS_H */