gallop_left() and gallop_right() functions explicitly rely on overflowing behavior of Py_ssize_t (https://github.com/python/cpython/blob/6015cc50bc38b9e920ce4986ee10658eaa14f561/Objects/listobject.c#L1361):

    ofs = (ofs << 1) + 1;
    if (ofs <= 0)                   /* int overflow */
        ofs = maxofs;

Signed integer overflow is undefined in C, and the above is guaranteed to work only if compiler-specific workarounds are applied, such as GCC's -fwrapv (that is what CPython does). Without such workarounds the compiler would be free to remove the if statement.