GLib: add bounds-checked unsigned int arithmetic

Add some helpers for builds-checked unsigned integer arithmetic to GLib.
These will be based on compiler intrinsics where they are available,
falling back to standard manual checks otherwise.

The fallback case needs to be implemented as a function (which we do
inline) because we cannot rely on statement expressions.  We also
implement the intrinsics case as an inline in order to avoid people
accidentally writing non-portable code which depends on static
evaluation of the builtin.

For now there is only support for addition and multiplication for guint,
guint64 and gsize.  It may make sense to add support for subtraction or
for the signed equivalents of those types in the future if we find a use
for that.

https://bugzilla.gnome.org/show_bug.cgi?id=503096

glib/docs.c

@@ -1423,6 +1423,130 @@
  * Returns: @val converted to the opposite byte order
  */
  
+/* Bounds-checked integer arithmetic {{{1 */
+/**
+ * SECTION:checkedmath
+ * @title: Bounds-checking integer arithmetic
+ * @short_description: a set of helpers for performing checked integer arithmetic
+ *
+ * GLib offers a set of macros for doing additions and multiplications
+ * of unsigned integers, with checks for overflows.
+ *
+ * The helpers all have three arguments.  A pointer to the destination
+ * is always the first argument and the operands to the operation are
+ * the other two.
+ *
+ * Following standard GLib convention, the helpers return %TRUE in case
+ * of success (ie: no overflow).
+ *
+ * The helpers may be macros, normal functions or inlines.  They may be
+ * implemented with inline assembly or compiler intrinsics where
+ * available.
+ *
+ * Since: 2.48
+ */
+
+/**
+ * g_uint_checked_add
+ * @dest: a pointer to the #guint destination
+ * @a: the #guint left operand
+ * @b: the #guint right operand
+ *
+ * Performs a checked addition of @a and @b, storing the result in
+ * @dest.
+ *
+ * If the operation is successful, %TRUE is returned.  If the operation
+ * overflows then the state of @dest is undefined and %FALSE is
+ * returned.
+ *
+ * Returns: %TRUE if there was no overflow
+ * Since: 2.48
+ */
+
+/**
+ * g_uint_checked_mul
+ * @dest: a pointer to the #guint destination
+ * @a: the #guint left operand
+ * @b: the #guint right operand
+ *
+ * Performs a checked multiplication of @a and @b, storing the result in
+ * @dest.
+ *
+ * If the operation is successful, %TRUE is returned.  If the operation
+ * overflows then the state of @dest is undefined and %FALSE is
+ * returned.
+ *
+ * Returns: %TRUE if there was no overflow
+ * Since: 2.48
+ */
+
+/**
+ * g_uint64_checked_add
+ * @dest: a pointer to the #guint64 destination
+ * @a: the #guint64 left operand
+ * @b: the #guint64 right operand
+ *
+ * Performs a checked addition of @a and @b, storing the result in
+ * @dest.
+ *
+ * If the operation is successful, %TRUE is returned.  If the operation
+ * overflows then the state of @dest is undefined and %FALSE is
+ * returned.
+ *
+ * Returns: %TRUE if there was no overflow
+ * Since: 2.48
+ */
+
+/**
+ * g_uint64_checked_mul
+ * @dest: a pointer to the #guint64 destination
+ * @a: the #guint64 left operand
+ * @b: the #guint64 right operand
+ *
+ * Performs a checked multiplication of @a and @b, storing the result in
+ * @dest.
+ *
+ * If the operation is successful, %TRUE is returned.  If the operation
+ * overflows then the state of @dest is undefined and %FALSE is
+ * returned.
+ *
+ * Returns: %TRUE if there was no overflow
+ * Since: 2.48
+ */
+
+/**
+ * g_size_checked_add
+ * @dest: a pointer to the #gsize destination
+ * @a: the #gsize left operand
+ * @b: the #gsize right operand
+ *
+ * Performs a checked addition of @a and @b, storing the result in
+ * @dest.
+ *
+ * If the operation is successful, %TRUE is returned.  If the operation
+ * overflows then the state of @dest is undefined and %FALSE is
+ * returned.
+ *
+ * Returns: %TRUE if there was no overflow
+ * Since: 2.48
+ */
+
+/**
+ * g_size_checked_mul
+ * @dest: a pointer to the #gsize destination
+ * @a: the #gsize left operand
+ * @b: the #gsize right operand
+ *
+ * Performs a checked multiplication of @a and @b, storing the result in
+ * @dest.
+ *
+ * If the operation is successful, %TRUE is returned.  If the operation
+ * overflows then the state of @dest is undefined and %FALSE is
+ * returned.
+ *
+ * Returns: %TRUE if there was no overflow
+ * Since: 2.48
+ */
 /* Numerical Definitions {{{1 */
 
 /**