lua 备忘

lparser.c

//http://www.cnblogs.com/zxh1210603696/category/682573.html

/*
** check whether, in an assignment to an upvalue/local variable, the
** upvalue/local variable is begin used in a previous assignment to a
** table. If so, save original upvalue/local value in a safe place and
** use this safe copy in the previous assignment.
*/
static void check_conflict (LexState *ls, struct LHS_assign *lh, expdesc *v) {
  FuncState *fs = ls->fs;
  int extra = fs->freereg;  /* eventual position to save local variable */
  int conflict = 0;
  for (; lh; lh = lh->prev) {  /* check all previous assignments */
    if (lh->v.k == VINDEXED) {  /* assigning to a table? */
      /* table is the upvalue/local being assigned now? */
      if (lh->v.u.ind.vt == v->k && lh->v.u.ind.t == v->u.info) {
        conflict = 1;
        lh->v.u.ind.vt = VLOCAL;
        lh->v.u.ind.t = extra;  /* previous assignment will use safe copy */
      }
      /* index is the local being assigned? (index cannot be upvalue) */
      if (v->k == VLOCAL && lh->v.u.ind.idx == v->u.info) {
        conflict = 1;
        lh->v.u.ind.idx = extra;  /* previous assignment will use safe copy */
      }
    }
  }
  if (conflict) {
    /* copy upvalue/local value to a temporary (in position 'extra') */
    OpCode op = (v->k == VLOCAL) ? OP_MOVE : OP_GETUPVAL;
    luaK_codeABC(fs, op, extra, v->u.info, 0);
    luaK_reserveregs(fs, 1);
  }
}


/*
local a
a,a = 1,2
print(a) 


main <test.lua:0,0> (8 instructions at 0x7f846f403490)
0+ params, 3 slots, 1 upvalue, 1 local, 3 constants, 0 functions
  1  [1]  LOADNIL    0 0
  2  [2]  LOADK      1 -1  ; 1 // 会多用一个临时变量
  3  [2]  LOADK      0 -2  ; 2
  4  [2]  MOVE       0 1   
  5  [3]  GETTABUP   1 0 -3  ; _ENV "print"
  6  [3]  MOVE       2 0
  7  [3]  CALL       1 2 1
  8  [3]  RETURN     0 1
*/