From a quick look at the code it seems that zpl_isize is used to represent indices, counts, etc and in those cases -1 is often used as an "invalid" value with s

From a quick look at the code it seems that zpl_isize is used to represent indices, counts, etc and in those cases -1 is often used as an "invalid" value with special meaning (e.g. in the hashtable -1 is used as a "not found" index).