`sxpinfo'标头定义为下面的64位C结构：
struct sxpinfo_struct { /* Type and namedcnt in first byte */ unsigned int nmcnt : 3; /* count of "names" referring to object */ unsigned int type : 5; /* discussed above */ /* Garbage collector stuff - keep in one byte to maybe speed up access */ unsigned int gccls : 3; /* node class for garbage collector */ unsigned int gcgen : 1; /* old generation number - may be best first */ unsigned int mark : 1; /* marks object as in use in garbage collector */ /* Object flag */ unsigned int obj : 1; /* set if this is an S3 or S4 object */ /* Flags to synchronize with helper threads */ unsigned int in_use: 1; /* whether contents may be in use by a helper */ unsigned int being_computed : 1; /* whether helper may be computing this */ /* "general purpose" field, used for miscellaneous purposes */ unsigned int gp : 16; /* The "general purpose" field */ union { struct { /* field below is for vectors only */ R_len_t truelength; /* for old stuff - may someday be defunct... */ } vec; struct { /* fields below are for non-vectors only */ /* Debugging */ unsigned int debug : 1; /* function/environment is being debugged */ unsigned int rstep : 1; /* function is to be debugged, but only once */ unsigned int trace : 1; /* function is being traced */ /* Symbol binding */ unsigned int basec : 1; /* sym has base binding in global cache */ unsigned int spec_sym : 1; /* this is a "special" symbol */ unsigned int no_spec_sym : 1; /* environment has no special symbols */ unsigned int unused : 2; /* not yet used */ /* Primitive operations */ unsigned char var1, var2;/* variants for evals of fast primitive args */ unsigned char pending_ok;/* whether args can have computation pending */ } nonvec; /* - only one bit, but maybe faster as byte */ } u; };
这给我留下了两个问题，关于nmcnt和type字段（第一个字节）：
SEXPTYPEs用 5 位代表2 when = 64？25？提前致谢。