base-native-int63 is just a virtual OPAM package that determineswhetherInt63.t in the Base library is exposed as aprivate int on64 bit sytems or as an abstract type whose implementation is chosen atruntime.

If you install this package, operations onBase.Int63.t values willbe faster as the compiler will be able to generate optimizedcode. However, you won't be able to use the generated byte-code withjs_of_ocaml.