// Tagged value 的 struct 定義

typedef LJ_ALIGN(8) union TValue {
  uint64_t u64;		/* 64 bit pattern overlaps number. */
  lua_Number n;		/* Number object overlaps split tag/value object. */
  struct {
    LJ_ENDIAN_LOHI(
      union {
	GCRef gcr;	/* GCobj reference (if any). */
	int32_t i;	/* Integer value. */
      };
    , uint32_t it;	/* Internal object tag. Must overlap MSW of number. */
    )
  };
  struct {
    LJ_ENDIAN_LOHI(
      GCRef func;	/* Function for next frame (or dummy L). */
    , FrameLink tp;	/* Link to previous frame. */
    )
  } fr;
  struct {
    LJ_ENDIAN_LOHI(
      uint32_t lo;	/* Lower 32 bits of number. */
    , uint32_t hi;	/* Upper 32 bits of number. */
    )
  } u32;
} TValue;

// Tag 的型別規格
/* Internal object tags.
**
** Internal tags overlap the MSW of a number object (must be a double).
** Interpreted as a double these are special NaNs. The FPU only generates
** one type of NaN (0xfff8_0000_0000_0000). So MSWs > 0xfff80000 are available
** for use as internal tags. Small negative numbers are used to shorten the
** encoding of type comparisons (reg/mem against sign-ext. 8 bit immediate).
**
**                  ---MSW---.---LSW---
** primitive types |  itype  |         |
** lightuserdata   |  itype  |  void * |  (32 bit platforms)
** lightuserdata   |ffff|    void *    |  (64 bit platforms, 47 bit pointers)
** GC objects      |  itype  |  GCRef  |
** int (LJ_DUALNUM)|  itype  |   int   |
** number           -------double------
**
** ORDER LJ_T
** Primitive types nil/false/true must be first, lightuserdata next.
** GC objects are at the end, table/userdata must be lowest.
** Also check lj_ir.h for similar ordering constraints.
*/
#define LJ_TNIL			(~0u)
#define LJ_TFALSE		(~1u)
#define LJ_TTRUE		(~2u)
#define LJ_TLIGHTUD		(~3u)
#define LJ_TSTR			(~4u)
#define LJ_TUPVAL		(~5u)
#define LJ_TTHREAD		(~6u)
#define LJ_TPROTO		(~7u)
#define LJ_TFUNC		(~8u)
#define LJ_TTRACE		(~9u)
#define LJ_TCDATA		(~10u)
#define LJ_TTAB			(~11u)
#define LJ_TUDATA		(~12u)
/* This is just the canonical number type used in some places. */
#define LJ_TNUMX		(~13u)

// 判斷型別的方式
/* Macros to test types. */
#define itype(o)	((o)->it)
#define tvisnil(o)	(itype(o) == LJ_TNIL)
#define tvisfalse(o)	(itype(o) == LJ_TFALSE)
#define tvistrue(o)	(itype(o) == LJ_TTRUE)
#define tvisbool(o)	(tvisfalse(o) || tvistrue(o))
#if LJ_64
#define tvislightud(o)	(((int32_t)itype(o) >> 15) == -2)
#else
#define tvislightud(o)	(itype(o) == LJ_TLIGHTUD)
#endif
#define tvisstr(o)	(itype(o) == LJ_TSTR)
#define tvisfunc(o)	(itype(o) == LJ_TFUNC)
#define tvisthread(o)	(itype(o) == LJ_TTHREAD)
#define tvisproto(o)	(itype(o) == LJ_TPROTO)
#define tviscdata(o)	(itype(o) == LJ_TCDATA)
#define tvistab(o)	(itype(o) == LJ_TTAB)
#define tvisudata(o)	(itype(o) == LJ_TUDATA)
#define tvisnumber(o)	(itype(o) <= LJ_TISNUM)
#define tvisint(o)	(LJ_DUALNUM && itype(o) == LJ_TISNUM)
#define tvisnum(o)	(itype(o) < LJ_TISNUM)

#define tvistruecond(o)	(itype(o) < LJ_TISTRUECOND)
#define tvispri(o)	(itype(o) >= LJ_TISPRI)
#define tvistabud(o)	(itype(o) <= LJ_TISTABUD)  /* && !tvisnum() */
#define tvisgcv(o)	((itype(o) - LJ_TISGCV) > (LJ_TNUMX - LJ_TISGCV))

/* Special macros to test numbers for NaN, +0, -0, +1 and raw equality. */
#define tvisnan(o)	((o)->n != (o)->n)
#if LJ_64
#define tviszero(o)	(((o)->u64 << 1) == 0)
#else
#define tviszero(o)	(((o)->u32.lo | ((o)->u32.hi << 1)) == 0)
#endif
#define tvispzero(o)	((o)->u64 == 0)
#define tvismzero(o)	((o)->u64 == U64x(80000000,00000000))
#define tvispone(o)	((o)->u64 == U64x(3ff00000,00000000))
#define rawnumequal(o1, o2)	((o1)->u64 == (o2)->u64)