If it's a power of two you can use a longer than required index. An empty buffer will have all index bits equal. A full buffer will have the higher bits differe

If it's a power of two you can use a longer than required index. An empty buffer will have all index bits equal. A full buffer will have the higher bits different but the lower bits equal. This requires extracting only the lower bits from the index before using it, but depending on the instruction set and bit position this is either very cheap or completely free.