-#define IS_ADJACENT(io, nio) \
- ((io)->io_offset + (io)->io_size == (nio)->io_offset)
+/*
+ * Compute the range spanned by two i/os, which is the endpoint of the last
+ * (lio->io_offset + lio->io_size) minus start of the first (fio->io_offset).
+ * Conveniently, the gap between fio and lio is given by -IO_SPAN(lio, fio);
+ * thus fio and lio are adjacent if and only if IO_SPAN(lio, fio) == 0.
+ */
+#define IO_SPAN(fio, lio) ((lio)->io_offset + (lio)->io_size - (fio)->io_offset)
+#define IO_GAP(fio, lio) (-IO_SPAN(lio, fio))