lua/ldebug.c

/*
** $Id: ldebug.c,v 2.155 2018/02/17 19:29:29 roberto Exp roberto $
** Debug Interface
** See Copyright Notice in lua.h
*/

#define ldebug_c
#define LUA_CORE

#include "lprefix.h"


#include <stdarg.h>
#include <stddef.h>
#include <string.h>

#include "lua.h"

#include "lapi.h"
#include "lcode.h"
#include "ldebug.h"
#include "ldo.h"
#include "lfunc.h"
#include "lobject.h"
#include "lopcodes.h"
#include "lstate.h"
#include "lstring.h"
#include "ltable.h"
#include "ltm.h"
#include "lvm.h"



#define noLuaClosure(f)		((f) == NULL || (f)->c.tt == LUA_TCCL)


/* Active Lua function (given call info) */
#define ci_func(ci)		(clLvalue(s2v((ci)->func)))


static const char *funcnamefromcode (lua_State *L, CallInfo *ci,
                                    const char **name);


static int currentpc (CallInfo *ci) {
  lua_assert(isLua(ci));
  return pcRel(ci->u.l.savedpc, ci_func(ci)->p);
}


/*
** Get a "base line" to find the line corresponding to an instruction.
** For that, search the array of absolute line info for the largest saved
** instruction smaller or equal to the wanted instruction. A special
** case is when there is no absolute info or the instruction is before
** the first absolute one.
*/
static int getbaseline (Proto *f, int pc, int *basepc) {
  if (f->sizeabslineinfo == 0 || pc < f->abslineinfo[0].pc) {
    *basepc = -1;  /* start from the beginning */
    return f->linedefined;
  }
  else {
    unsigned int i;
    if (pc >= f->abslineinfo[f->sizeabslineinfo - 1].pc)
      i = f->sizeabslineinfo - 1;  /* instruction is after last saved one */
    else {  /* binary search */
      unsigned int j = f->sizeabslineinfo - 1;  /* pc < anchorlines[j] */
      i = 0;  /* abslineinfo[i] <= pc */
      while (i < j - 1) {
        unsigned int m = (j + i) / 2;
        if (pc >= f->abslineinfo[m].pc)
          i = m;
        else
          j = m;
      }
    }
    *basepc = f->abslineinfo[i].pc;
    return f->abslineinfo[i].line;
  }
}


/*
** Get the line corresponding to instruction 'pc' in function 'f';
** first gets a base line and from there does the increments until
** the desired instruction.
*/
int luaG_getfuncline (Proto *f, int pc) {
  if (f->lineinfo == NULL)  /* no debug information? */
    return -1;
  else {
    int basepc;
    int baseline = getbaseline(f, pc, &basepc);
    while (basepc++ < pc) {  /* walk until given instruction */
      lua_assert(f->lineinfo[basepc] != ABSLINEINFO);
      baseline += f->lineinfo[basepc];  /* correct line */
    }
    return baseline;
  }
}


static int currentline (CallInfo *ci) {
  return luaG_getfuncline(ci_func(ci)->p, currentpc(ci));
}


/*
** This function can be called asynchronously (e.g. during a signal),
** under "reasonable" assumptions. A new 'ci' is completely linked
** in the list before it becomes part of the "active" list, and
** we assume that pointers are atomic (see comment in next function).
** (If we traverse one more item, there is no problem. If we traverse
** one less item, the worst that can happen is that the signal will
** not interrupt the script.)
*/
static void settraps (CallInfo *ci) {
  for (; ci != NULL; ci = ci->previous)
    if (isLua(ci))
      ci->u.l.trap = 1;
}


/*
** This function can be called asynchronously (e.g. during a signal),
** under "reasonable" assumptions.
** Fields 'oldpc', 'basehookcount', and 'hookcount' (set by
** 'resethookcount') are for debug only, and it is no problem if they
** get arbitrary values (causes at most one wrong hook call). 'hookmask'
** is an atomic value. We assume that pointers are atomic too (e.g., gcc
** ensures that for all platforms where it runs). Moreover, 'hook' is
** always checked before being called (see 'luaD_hook').
*/
LUA_API void lua_sethook (lua_State *L, lua_Hook func, int mask, int count) {
  if (func == NULL || mask == 0) {  /* turn off hooks? */
    mask = 0;
    func = NULL;
  }
  if (isLua(L->ci))
    L->oldpc = L->ci->u.l.savedpc;
  L->hook = func;
  L->basehookcount = count;
  resethookcount(L);
  L->hookmask = cast_byte(mask);
  if (mask)
    settraps(L->ci);  /* to trace inside 'luaV_execute' */
}


LUA_API lua_Hook lua_gethook (lua_State *L) {
  return L->hook;
}


LUA_API int lua_gethookmask (lua_State *L) {
  return L->hookmask;
}


LUA_API int lua_gethookcount (lua_State *L) {
  return L->basehookcount;
}


LUA_API int lua_getstack (lua_State *L, int level, lua_Debug *ar) {
  int status;
  CallInfo *ci;
  if (level < 0) return 0;  /* invalid (negative) level */
  lua_lock(L);
  for (ci = L->ci; level > 0 && ci != &L->base_ci; ci = ci->previous)
    level--;
  if (level == 0 && ci != &L->base_ci) {  /* level found? */
    status = 1;
    ar->i_ci = ci;
  }
  else status = 0;  /* no such level */
  lua_unlock(L);
  return status;
}


static const char *upvalname (Proto *p, int uv) {
  TString *s = check_exp(uv < p->sizeupvalues, p->upvalues[uv].name);
  if (s == NULL) return "?";
  else return getstr(s);
}


static const char *findvararg (CallInfo *ci, int n, StkId *pos) {
  if (clLvalue(s2v(ci->func))->p->is_vararg) {
    int nextra = ci->u.l.nextraargs;
    if (n <= nextra) {
      *pos = ci->func - nextra + (n - 1);
      return "(*vararg)";  /* generic name for any vararg */
    }
  }
  return NULL;  /* no such vararg */
}


static const char *findlocal (lua_State *L, CallInfo *ci, int n,
                              StkId *pos) {
  StkId base = ci->func + 1;
  const char *name = NULL;
  if (isLua(ci)) {
    if (n < 0)  /* access to vararg values? */
      return findvararg(ci, -n, pos);
    else
      name = luaF_getlocalname(ci_func(ci)->p, n, currentpc(ci));
  }
  if (name == NULL) {  /* no 'standard' name? */
    StkId limit = (ci == L->ci) ? L->top : ci->next->func;
    if (limit - base >= n && n > 0)  /* is 'n' inside 'ci' stack? */
      name = "(*temporary)";  /* generic name for any valid slot */
    else
      return NULL;  /* no name */
  }
  *pos = base + (n - 1);
  return name;
}


LUA_API const char *lua_getlocal (lua_State *L, const lua_Debug *ar, int n) {
  const char *name;
  lua_lock(L);
  if (ar == NULL) {  /* information about non-active function? */
    if (!isLfunction(s2v(L->top - 1)))  /* not a Lua function? */
      name = NULL;
    else  /* consider live variables at function start (parameters) */
      name = luaF_getlocalname(clLvalue(s2v(L->top - 1))->p, n, 0);
  }
  else {  /* active function; get information through 'ar' */
    StkId pos = NULL;  /* to avoid warnings */
    name = findlocal(L, ar->i_ci, n, &pos);
    if (name) {
      setobjs2s(L, L->top, pos);
      api_incr_top(L);
    }
  }
  lua_unlock(L);
  return name;
}


LUA_API const char *lua_setlocal (lua_State *L, const lua_Debug *ar, int n) {
  StkId pos = NULL;  /* to avoid warnings */
  const char *name;
  lua_lock(L);
  name = findlocal(L, ar->i_ci, n, &pos);
  if (name) {
    setobjs2s(L, pos, L->top - 1);
    L->top--;  /* pop value */
  }
  lua_unlock(L);
  return name;
}


static void funcinfo (lua_Debug *ar, Closure *cl) {
  if (noLuaClosure(cl)) {
    ar->source = "=[C]";
    ar->linedefined = -1;
    ar->lastlinedefined = -1;
    ar->what = "C";
  }
  else {
    Proto *p = cl->l.p;
    ar->source = p->source ? getstr(p->source) : "=?";
    ar->linedefined = p->linedefined;
    ar->lastlinedefined = p->lastlinedefined;
    ar->what = (ar->linedefined == 0) ? "main" : "Lua";
  }
  luaO_chunkid(ar->short_src, ar->source, LUA_IDSIZE);
}


static int nextline (Proto *p, int currentline, int pc) {
  if (p->lineinfo[pc] != ABSLINEINFO)
    return currentline + p->lineinfo[pc];
  else
    return luaG_getfuncline(p, pc);
}


static void collectvalidlines (lua_State *L, Closure *f) {
  if (noLuaClosure(f)) {
    setnilvalue(s2v(L->top));
    api_incr_top(L);
  }
  else {
    int i;
    TValue v;
    Proto *p = f->l.p;
    int currentline = p->linedefined;
    Table *t = luaH_new(L);  /* new table to store active lines */
    sethvalue2s(L, L->top, t);  /* push it on stack */
    api_incr_top(L);
    setbvalue(&v, 1);  /* boolean 'true' to be the value of all indices */
    for (i = 0; i < p->sizelineinfo; i++) {  /* for all lines with code */
      currentline = nextline(p, currentline, i);
      luaH_setint(L, t, currentline, &v);  /* table[line] = true */
    }
  }
}


static const char *getfuncname (lua_State *L, CallInfo *ci, const char **name) {
  if (ci == NULL)  /* no 'ci'? */
    return NULL;  /* no info */
  else if (ci->callstatus & CIST_FIN) {  /* is this a finalizer? */
    *name = "__gc";
    return "metamethod";  /* report it as such */
  }
  /* calling function is a known Lua function? */
  else if (!(ci->callstatus & CIST_TAIL) && isLua(ci->previous))
    return funcnamefromcode(L, ci->previous, name);
  else return NULL;  /* no way to find a name */
}


static int auxgetinfo (lua_State *L, const char *what, lua_Debug *ar,
                       Closure *f, CallInfo *ci) {
  int status = 1;
  for (; *what; what++) {
    switch (*what) {
      case 'S': {
        funcinfo(ar, f);
        break;
      }
      case 'l': {
        ar->currentline = (ci && isLua(ci)) ? currentline(ci) : -1;
        break;
      }
      case 'u': {
        ar->nups = (f == NULL) ? 0 : f->c.nupvalues;
        if (noLuaClosure(f)) {
          ar->isvararg = 1;
          ar->nparams = 0;
        }
        else {
          ar->isvararg = f->l.p->is_vararg;
          ar->nparams = f->l.p->numparams;
        }
        break;
      }
      case 't': {
        ar->istailcall = (ci) ? ci->callstatus & CIST_TAIL : 0;
        break;
      }
      case 'n': {
        ar->namewhat = getfuncname(L, ci, &ar->name);
        if (ar->namewhat == NULL) {
          ar->namewhat = "";  /* not found */
          ar->name = NULL;
        }
        break;
      }
      case 'r': {
        if (ci == NULL || !(ci->callstatus & CIST_TRAN))
          ar->ftransfer = ar->ntransfer = 0;
        else {
          ar->ftransfer = ci->u2.transferinfo.ftransfer;
          ar->ntransfer = ci->u2.transferinfo.ntransfer;
        }
      }
      case 'L':
      case 'f':  /* handled by lua_getinfo */
        break;
      default: status = 0;  /* invalid option */
    }
  }
  return status;
}


LUA_API int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar) {
  int status;
  Closure *cl;
  CallInfo *ci;
  TValue *func;
  lua_lock(L);
  if (*what == '>') {
    ci = NULL;
    func = s2v(L->top - 1);
    api_check(L, ttisfunction(func), "function expected");
    what++;  /* skip the '>' */
    L->top--;  /* pop function */
  }
  else {
    ci = ar->i_ci;
    func = s2v(ci->func);
    lua_assert(ttisfunction(func));
  }
  cl = ttisclosure(func) ? clvalue(func) : NULL;
  status = auxgetinfo(L, what, ar, cl, ci);
  if (strchr(what, 'f')) {
    setobj2s(L, L->top, func);
    api_incr_top(L);
  }
  if (strchr(what, 'L'))
    collectvalidlines(L, cl);
  lua_unlock(L);
  return status;
}


/*
** {======================================================
** Symbolic Execution
** =======================================================
*/

static const char *getobjname (Proto *p, int lastpc, int reg,
                               const char **name);


/*
** Find a "name" for the constant 'c'.
*/
static void kname (Proto *p, int c, const char **name) {
  TValue *kvalue = &p->k[c];
  *name = (ttisstring(kvalue)) ? svalue(kvalue) : "?";
}


/*
** Find a "name" for the register 'c'.
*/
static void rname (Proto *p, int pc, int c, const char **name) {
  const char *what = getobjname(p, pc, c, name); /* search for 'c' */
  if (!(what && *what == 'c'))  /* did not find a constant name? */
    *name = "?";
}


/*
** Find a "name" for a 'C' value in an RK instruction.
*/
static void rkname (Proto *p, int pc, Instruction i, const char **name) {
  int c = GETARG_C(i);  /* key index */
  if (GETARG_k(i))  /* is 'c' a constant? */
    kname(p, c, name);
  else  /* 'c' is a register */
    rname(p, pc, c, name);
}


static int filterpc (int pc, int jmptarget) {
  if (pc < jmptarget)  /* is code conditional (inside a jump)? */
    return -1;  /* cannot know who sets that register */
  else return pc;  /* current position sets that register */
}


/*
** try to find last instruction before 'lastpc' that modified register 'reg'
*/
static int findsetreg (Proto *p, int lastpc, int reg) {
  int pc;
  int setreg = -1;  /* keep last instruction that changed 'reg' */
  int jmptarget = 0;  /* any code before this address is conditional */
  for (pc = 0; pc < lastpc; pc++) {
    Instruction i = p->code[pc];
    OpCode op = GET_OPCODE(i);
    int a = GETARG_A(i);
    int change;  /* true if current instruction changed 'reg' */
    switch (op) {
      case OP_LOADNIL: {  /* set registers from 'a' to 'a+b' */
        int b = GETARG_B(i);
        change = (a <= reg && reg <= a + b);
        break;
      }
      case OP_TFORCALL: {  /* affect all regs above its base */
        change = (reg >= a + 2);
        break;
      }
      case OP_CALL:
      case OP_TAILCALL: {  /* affect all registers above base */
        change = (reg >= a);
        break;
      }
      case OP_JMP: {  /* doesn't change registers, but changes 'jmptarget' */
        int b = GETARG_sJ(i);
        int dest = pc + 1 + b;
        /* jump does not skip 'lastpc' and is larger than current one? */
        if (dest <= lastpc && dest > jmptarget)
          jmptarget = dest;  /* update 'jmptarget' */
        change = 0;
        break;
      }
      default:  /* any instruction that sets A */
        change = (testAMode(op) && reg == a);
        break;
    }
    if (change)
      setreg = filterpc(pc, jmptarget);
  }
  return setreg;
}


/*
** Check whether table being indexed by instruction 'i' is the
** environment '_ENV'
*/
static const char *gxf (Proto *p, int pc, Instruction i, int isup) {
  int t = GETARG_B(i);  /* table index */
  const char *name;  /* name of indexed variable */
  if (isup)  /* is an upvalue? */
    name = upvalname(p, t);
  else
    getobjname(p, pc, t, &name);
  return (name && strcmp(name, LUA_ENV) == 0) ? "global" : "field";
}


 const char *getobjname (Proto *p, int lastpc, int reg, const char **name) {
  int pc;
  *name = luaF_getlocalname(p, reg + 1, lastpc);
  if (*name)  /* is a local? */
    return "local";
  /* else try symbolic execution */
  pc = findsetreg(p, lastpc, reg);
  if (pc != -1) {  /* could find instruction? */
    Instruction i = p->code[pc];
    OpCode op = GET_OPCODE(i);
    switch (op) {
      case OP_MOVE: {
        int b = GETARG_B(i);  /* move from 'b' to 'a' */
        if (b < GETARG_A(i))
          return getobjname(p, pc, b, name);  /* get name for 'b' */
        break;
      }
      case OP_GETTABUP: {
        int k = GETARG_C(i);  /* key index */
        kname(p, k, name);
        return gxf(p, pc, i, 1);
      }
      case OP_GETTABLE: {
        int k = GETARG_C(i);  /* key index */
        rname(p, pc, k, name);
        return gxf(p, pc, i, 0);
      }
      case OP_GETI: {
        *name = "integer index";
        return "field";
      }
      case OP_GETFIELD: {
        int k = GETARG_C(i);  /* key index */
        kname(p, k, name);
        return gxf(p, pc, i, 0);
      }
      case OP_GETUPVAL: {
        *name = upvalname(p, GETARG_B(i));
        return "upvalue";
      }
      case OP_LOADK:
      case OP_LOADKX: {
        int b = (op == OP_LOADK) ? GETARG_Bx(i)
                                 : GETARG_Ax(p->code[pc + 1]);
        if (ttisstring(&p->k[b])) {
          *name = svalue(&p->k[b]);
          return "constant";
        }
        break;
      }
      case OP_SELF: {
        rkname(p, pc, i, name);
        return "method";
      }
      default: break;  /* go through to return NULL */
    }
  }
  return NULL;  /* could not find reasonable name */
}


/*
** Try to find a name for a function based on the code that called it.
** (Only works when function was called by a Lua function.)
** Returns what the name is (e.g., "for iterator", "method",
** "metamethod") and sets '*name' to point to the name.
*/
static const char *funcnamefromcode (lua_State *L, CallInfo *ci,
                                     const char **name) {
  TMS tm = (TMS)0;  /* (initial value avoids warnings) */
  Proto *p = ci_func(ci)->p;  /* calling function */
  int pc = currentpc(ci);  /* calling instruction index */
  Instruction i = p->code[pc];  /* calling instruction */
  if (ci->callstatus & CIST_HOOKED) {  /* was it called inside a hook? */
    *name = "?";
    return "hook";
  }
  switch (GET_OPCODE(i)) {
    case OP_CALL:
    case OP_TAILCALL:
      return getobjname(p, pc, GETARG_A(i), name);  /* get function name */
    case OP_TFORCALL: {  /* for iterator */
      *name = "for iterator";
       return "for iterator";
    }
    /* other instructions can do calls through metamethods */
    case OP_SELF: case OP_GETTABUP: case OP_GETTABLE:
    case OP_GETI: case OP_GETFIELD:
      tm = TM_INDEX;
      break;
    case OP_SETTABUP: case OP_SETTABLE: case OP_SETI: case OP_SETFIELD:
      tm = TM_NEWINDEX;
      break;
    case OP_ADDI: case OP_SUBI: case OP_MULI: case OP_MODI:
    case OP_POWI: case OP_DIVI: case OP_IDIVI:
    case OP_BANDK: case OP_BORK: case OP_BXORK: {
      int offset = GET_OPCODE(i) - OP_ADDI;  /* ORDER OP */
      tm = cast(TMS, offset + TM_ADD);  /* ORDER TM */
      break;
    }
    case OP_ADD: case OP_SUB: case OP_MUL: case OP_MOD:
    case OP_POW: case OP_DIV: case OP_IDIV: case OP_BAND:
    case OP_BOR: case OP_BXOR: case OP_SHL: case OP_SHR: {
      int offset = GET_OPCODE(i) - OP_ADD;  /* ORDER OP */
      tm = cast(TMS, offset + TM_ADD);  /* ORDER TM */
      break;
    }
    case OP_UNM: tm = TM_UNM; break;
    case OP_BNOT: tm = TM_BNOT; break;
    case OP_LEN: tm = TM_LEN; break;
    case OP_CONCAT: tm = TM_CONCAT; break;
    case OP_EQ: tm = TM_EQ; break;
    case OP_LT: case OP_LE: case OP_LTI: case OP_LEI:
      *name = "order";  /* '<=' can call '__lt', etc. */
      return "metamethod";
    case OP_SHRI: case OP_SHLI:
      *name = "shift";
      return "metamethod";
    default:
      return NULL;  /* cannot find a reasonable name */
  }
  *name = getstr(G(L)->tmname[tm]) + 2;
  return "metamethod";
}

/* }====================================================== */



/*
** The subtraction of two potentially unrelated pointers is
** not ISO C, but it should not crash a program; the subsequent
** checks are ISO C and ensure a correct result.
*/
static int isinstack (CallInfo *ci, const TValue *o) {
  StkId base = ci->func + 1;
  ptrdiff_t i = cast(StkId, o) - base;
  return (0 <= i && i < (ci->top - base) && s2v(base + i) == o);
}


/*
** Checks whether value 'o' came from an upvalue. (That can only happen
** with instructions OP_GETTABUP/OP_SETTABUP, which operate directly on
** upvalues.)
*/
static const char *getupvalname (CallInfo *ci, const TValue *o,
                                 const char **name) {
  LClosure *c = ci_func(ci);
  int i;
  for (i = 0; i < c->nupvalues; i++) {
    if (c->upvals[i]->v == o) {
      *name = upvalname(c->p, i);
      return "upvalue";
    }
  }
  return NULL;
}


static const char *varinfo (lua_State *L, const TValue *o) {
  const char *name = NULL;  /* to avoid warnings */
  CallInfo *ci = L->ci;
  const char *kind = NULL;
  if (isLua(ci)) {
    kind = getupvalname(ci, o, &name);  /* check whether 'o' is an upvalue */
    if (!kind && isinstack(ci, o))  /* no? try a register */
      kind = getobjname(ci_func(ci)->p, currentpc(ci),
                        cast_int(cast(StkId, o) - (ci->func + 1)), &name);
  }
  return (kind) ? luaO_pushfstring(L, " (%s '%s')", kind, name) : "";
}


l_noret luaG_typeerror (lua_State *L, const TValue *o, const char *op) {
  const char *t = luaT_objtypename(L, o);
  luaG_runerror(L, "attempt to %s a %s value%s", op, t, varinfo(L, o));
}


l_noret luaG_concaterror (lua_State *L, const TValue *p1, const TValue *p2) {
  if (ttisstring(p1) || cvt2str(p1)) p1 = p2;
  luaG_typeerror(L, p1, "concatenate");
}


l_noret luaG_opinterror (lua_State *L, const TValue *p1,
                         const TValue *p2, const char *msg) {
  if (!ttisnumber(p1))  /* first operand is wrong? */
    p2 = p1;  /* now second is wrong */
  luaG_typeerror(L, p2, msg);
}


/*
** Error when both values are convertible to numbers, but not to integers
*/
l_noret luaG_tointerror (lua_State *L, const TValue *p1, const TValue *p2) {
  lua_Integer temp;
  if (!tointegerns(p1, &temp))
    p2 = p1;
  luaG_runerror(L, "number%s has no integer representation", varinfo(L, p2));
}


l_noret luaG_ordererror (lua_State *L, const TValue *p1, const TValue *p2) {
  const char *t1 = luaT_objtypename(L, p1);
  const char *t2 = luaT_objtypename(L, p2);
  if (strcmp(t1, t2) == 0)
    luaG_runerror(L, "attempt to compare two %s values", t1);
  else
    luaG_runerror(L, "attempt to compare %s with %s", t1, t2);
}


/* add src:line information to 'msg' */
const char *luaG_addinfo (lua_State *L, const char *msg, TString *src,
                                        int line) {
  char buff[LUA_IDSIZE];
  if (src)
    luaO_chunkid(buff, getstr(src), LUA_IDSIZE);
  else {  /* no source available; use "?" instead */
    buff[0] = '?'; buff[1] = '\0';
  }
  return luaO_pushfstring(L, "%s:%d: %s", buff, line, msg);
}


l_noret luaG_errormsg (lua_State *L) {
  if (L->errfunc != 0) {  /* is there an error handling function? */
    StkId errfunc = restorestack(L, L->errfunc);
    lua_assert(ttisfunction(s2v(errfunc)));
    setobjs2s(L, L->top, L->top - 1);  /* move argument */
    setobjs2s(L, L->top - 1, errfunc);  /* push function */
    L->top++;  /* assume EXTRA_STACK */
    luaD_callnoyield(L, L->top - 2, 1);  /* call it */
  }
  luaD_throw(L, LUA_ERRRUN);
}


l_noret luaG_runerror (lua_State *L, const char *fmt, ...) {
  CallInfo *ci = L->ci;
  const char *msg;
  va_list argp;
  luaC_checkGC(L);  /* error message uses memory */
  va_start(argp, fmt);
  msg = luaO_pushvfstring(L, fmt, argp);  /* format message */
  va_end(argp);
  if (isLua(ci))  /* if Lua function, add source:line information */
    luaG_addinfo(L, msg, ci_func(ci)->p->source, currentline(ci));
  luaG_errormsg(L);
}


/*
** Check whether new instruction 'newpc' is in a different line from
** previous instruction 'oldpc'.
*/
static int changedline (Proto *p, int oldpc, int newpc) {
  while (oldpc++ < newpc) {
    if (p->lineinfo[oldpc] != 0)
      return (luaG_getfuncline(p, oldpc - 1) != luaG_getfuncline(p, newpc));
  }
  return 0;  /* no line changes in the way */
}


void luaG_traceexec (lua_State *L) {
  CallInfo *ci = L->ci;
  lu_byte mask = L->hookmask;
  int counthook = (--L->hookcount == 0 && (mask & LUA_MASKCOUNT));
  if (counthook)
    resethookcount(L);  /* reset count */
  else if (!(mask & LUA_MASKLINE))
    return;  /* no line hook and count != 0; nothing to be done */
  if (ci->callstatus & CIST_HOOKYIELD) {  /* called hook last time? */
    ci->callstatus &= ~CIST_HOOKYIELD;  /* erase mark */
    return;  /* do not call hook again (VM yielded, so it did not move) */
  }
  if (!isIT(*(ci->u.l.savedpc - 1)))
    L->top = ci->top;  /* prepare top */
  if (counthook)
    luaD_hook(L, LUA_HOOKCOUNT, -1, 0, 0);  /* call count hook */
  if (mask & LUA_MASKLINE) {
    Proto *p = ci_func(ci)->p;
    const Instruction *npc = ci->u.l.savedpc;
    int npci = pcRel(npc, p);
    if (npci == 0 ||  /* call linehook when enter a new function, */
        npc <= L->oldpc ||  /* when jump back (loop), or when */
        changedline(p, pcRel(L->oldpc, p), npci)) {  /* enter new line */
      int newline = luaG_getfuncline(p, npci);  /* new line */
      luaD_hook(L, LUA_HOOKLINE, newline, 0, 0);  /* call line hook */
    }
    L->oldpc = npc;
  }
  if (L->status == LUA_YIELD) {  /* did hook yield? */
    if (counthook)
      L->hookcount = 1;  /* undo decrement to zero */
    ci->u.l.savedpc--;  /* undo increment (resume will increment it again) */
    ci->callstatus |= CIST_HOOKYIELD;  /* mark that it yielded */
    luaD_throw(L, LUA_YIELD);
  }
}