+/* Many space usage primitives use all 1 bits to denote a value that is
+ not applicable or unknown. Propagate this information by returning
+ a uintmax_t value that is all 1 bits if the argument is all 1 bits,
+ even if the argument is unsigned and smaller than uintmax_t. */
+#define PROPAGATE_ALL_ONES(x) ((x) == -1 ? (uintmax_t) -1 : (uintmax_t) (x))
+
+/* Extract the top bit of X as an uintmax_t value. */
+#define EXTRACT_TOP_BIT(x) ((x) \
+ & ((uintmax_t) 1 << (sizeof (x) * CHAR_BIT - 1)))
+
+/* If a value is negative, many space usage primitives store it into an
+ integer variable by assignment, even if the variable's type is unsigned.
+ So, if a space usage variable X's top bit is set, convert X to the
+ uintmax_t value V such that (- (uintmax_t) V) is the negative of
+ the original value. If X's top bit is clear, just yield X.
+ Use PROPAGATE_TOP_BIT if the original value might be negative;
+ otherwise, use PROPAGATE_ALL_ONES. */
+#define PROPAGATE_TOP_BIT(x) ((x) | ~ (EXTRACT_TOP_BIT (x) - 1))