lvm_loops.cpp

/*
** $Id: lvm_loops.c $
** For-loop operations for Lua VM
** See Copyright Notice in lua.h
*/

#define lvm_c
#define LUA_CORE

#include "lprefix.h"

#include "lua.h"

#include "ldebug.h"
#include "ldo.h"
#include "lobject.h"
#include "lstate.h"
#include "lvm.h"


/*
** Try to convert a 'for' limit to an integer, preserving the semantics
** of the loop. Return true if the loop must not run; otherwise, '*p'
** gets the integer limit.
** (The following explanation assumes a positive step; it is valid for
** negative steps mutatis mutandis.)
** If the limit is an integer or can be converted to an integer,
** rounding down, that is the limit.
** Otherwise, check whether the limit can be converted to a float. If
** the float is too large, clip it to LUA_MAXINTEGER.  If the float
** is too negative, the loop should not run, because any initial
** integer value is greater than such limit; so, the function returns
** true to signal that. (For this latter case, no integer limit would be
** correct; even a limit of LUA_MININTEGER would run the loop once for
** an initial value equal to LUA_MININTEGER.)
*/
int lua_State::forLimit(lua_Integer init, const TValue *lim,
                        lua_Integer *p, lua_Integer step) {
  if (!luaV_tointeger(lim, p, (step < 0 ? F2Imod::F2Iceil : F2Imod::F2Ifloor))) {
    /* not coercible to in integer */
    lua_Number flim;  /* try to convert to float */
    if (!tonumber(lim, &flim)) /* cannot convert to float? */
      luaG_forerror(this, lim, "limit");
    /* else 'flim' is a float out of integer bounds */
    if (luai_numlt(0, flim)) {  /* if it is positive, it is too large */
      if (step < 0) return 1;  /* initial value must be less than it */
      *p = LUA_MAXINTEGER;  /* truncate */
    }
    else {  /* it is less than min integer */
      if (step > 0) return 1;  /* initial value must be greater than it */
      *p = LUA_MININTEGER;  /* truncate */
    }
  }
  return (step > 0 ? init > *p : init < *p);  /* not to run? */
}


/*
** Prepare a numerical for loop (opcode OP_FORPREP).
** Before execution, stack is as follows:
**   ra     : initial value
**   ra + 1 : limit
**   ra + 2 : step
** Return true to skip the loop. Otherwise,
** after preparation, stack will be as follows:
**   ra     : loop counter (integer loops) or limit (float loops)
**   ra + 1 : step
**   ra + 2 : control variable
*/
int lua_State::forPrep(StkId ra) {
  TValue *pinit = s2v(ra);
  TValue *plimit = s2v(ra + 1);
  TValue *pstep = s2v(ra + 2);
  if (ttisinteger(pinit) && ttisinteger(pstep)) { /* integer loop? */
    lua_Integer init = ivalue(pinit);
    lua_Integer step = ivalue(pstep);
    lua_Integer limit;
    if (step == 0)
      luaG_runerror(this, "'for' step is zero");
    if (this->forLimit(init, plimit, &limit, step))
      return 1;  /* skip the loop */
    else {  /* prepare loop counter */
      lua_Unsigned count;
      if (step > 0) {  /* ascending loop? */
        count = l_castS2U(limit) - l_castS2U(init);
        if (step != 1)  /* avoid division in the too common case */
          count /= l_castS2U(step);
      }
      else {  /* step < 0; descending loop */
        count = l_castS2U(init) - l_castS2U(limit);
        /* Handle LUA_MININTEGER edge case explicitly */
        if (l_unlikely(step == LUA_MININTEGER)) {
          /* For step == LUA_MININTEGER, count should be divided by max value */
          count /= l_castS2U(LUA_MAXINTEGER) + 1u;
        }
        else {
          /* 'step+1' avoids negating 'mininteger' in normal case */
          count /= l_castS2U(-(step + 1)) + 1u;
        }
      }
      /* use 'chgivalue' for places that for sure had integers */
      chgivalue(s2v(ra), l_castU2S(count));  /* change init to count */
      setivalue(s2v(ra + 1), step);  /* change limit to step */
      chgivalue(s2v(ra + 2), init);  /* change step to init */
    }
  }
  else {  /* try making all values floats */
    lua_Number init; lua_Number limit; lua_Number step;
    if (l_unlikely(!tonumber(plimit, &limit)))
      luaG_forerror(this, plimit, "limit");
    if (l_unlikely(!tonumber(pstep, &step)))
      luaG_forerror(this, pstep, "step");
    if (l_unlikely(!tonumber(pinit, &init)))
      luaG_forerror(this, pinit, "initial value");
    if (step == 0)
      luaG_runerror(this, "'for' step is zero");
    if (luai_numlt(0, step) ? luai_numlt(limit, init)
                            : luai_numlt(init, limit))
      return 1;  /* skip the loop */
    else {
      /* make sure all values are floats */
      setfltvalue(s2v(ra), limit);
      setfltvalue(s2v(ra + 1), step);
      setfltvalue(s2v(ra + 2), init);  /* control variable */
    }
  }
  return 0;
}


/*
** Execute a step of a float numerical for loop, returning
** true iff the loop must continue. (The integer case is
** written online with opcode OP_FORLOOP, for performance.)
*/
int lua_State::floatForLoop(StkId ra) {
  lua_Number step = fltvalue(s2v(ra + 1));
  lua_Number limit = fltvalue(s2v(ra));
  lua_Number idx = fltvalue(s2v(ra + 2));  /* control variable */
  idx = luai_numadd(this, idx, step);  /* increment index */
  if (luai_numlt(0, step) ? luai_numle(idx, limit)
                          : luai_numle(limit, idx)) {
    chgfltvalue(s2v(ra + 2), idx);  /* update control variable */
    return 1;  /* jump back */
  }
  else
    return 0;  /* finish the loop */
}