Re: lua in high performance apps

["Joshua Jensen" <jjensen@...>]

From: "Steve Dekorte" <steve@dekorte.com>
Sent: Monday, April 16, 2001 10:13 PM
> On Monday, April 16, 2001, at 08:15 PM, Joshua Jensen wrote:
>
> > 2) for loops are quite a bit faster than while loops, since they have
> > specialized virtual machine instructions.
>
> By "for loop" do you mean lua 4.0's for loop or foreach() and foreachi()?
> Anyone now how foreach and foreachi compare in speed to while or for?

I mean the built-in for loop:

for i = 1, 1000, 1 do
   -- Stuff
end

As far as foreach and foreachi, a quick check of the source code reveals:

static int luaB_foreachi (lua_State *L) {
  int n, i;
  luaL_checktype(L, 1, LUA_TTABLE);
  luaL_checktype(L, 2, LUA_TFUNCTION);
  n = lua_getn(L, 1);
  for (i=1; i<=n; i++) {
    lua_pushvalue(L, 2);  /* function */
    lua_pushnumber(L, i);  /* 1st argument */
    lua_rawgeti(L, 1, i);  /* 2nd argument */
    lua_rawcall(L, 2, 1);
    if (!lua_isnil(L, -1))
      return 1;
    lua_pop(L, 1);  /* remove nil result */
  }
  return 0;
}

static int luaB_foreach (lua_State *L) {
  luaL_checktype(L, 1, LUA_TTABLE);
  luaL_checktype(L, 2, LUA_TFUNCTION);
  lua_pushnil(L);  /* first index */
  for (;;) {
    if (lua_next(L, 1) == 0)
      return 0;
    lua_pushvalue(L, 2);  /* function */
    lua_pushvalue(L, -3);  /* key */
    lua_pushvalue(L, -3);  /* value */
    lua_rawcall(L, 2, 1);
    if (!lua_isnil(L, -1))
      return 1;
    lua_pop(L, 2);  /* remove value and result */
  }
}

Based on what I see above, I'd wager that a straight for loop in Lua is a
lot faster than foreachi, if only for the reason that foreachi calls
lua_rawcall.  A 'for key, value in table' in Lua is probably also faster
than foreach for the same reason.  But I haven't benchmarked either to be
sure.

Thanks,
Josh
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