Re: C string pointers to Lua

[Peter Cawley <lua@...>]

On Tue, Oct 12, 2010 at 11:33 AM, Robert Raschke
<rtrlists@googlemail.com> wrote:
>
> On Tue, Oct 12, 2010 at 11:09 AM, Rebel Neurofog <rebelneurofog@gmail.com>
> wrote:
>>
>> I've been thinking about Lua string uniqueness and though out of the
>> optimization could be made:
>>
>> ===============================
>> static const char *strconst_rgba = "rgba";
>> static const char *strconst_rgba = "rgb";
>> static const char *strconst_rgba = "abgr";
>> static const char *strconst_rgba = "bgr";
>>
>> static int some_c_function (lua_State *L)
>> {
>>    if (lua_gettop (L) != 1) return 0;
>>    const char *arg = lua_tostring (L, 1);
>>     /* no 'strcmp ()' here: */
>>    if (arg == strconst_rgba) {
>>        lua_pushnumber (L, 1);
>>        return 1;
>>    } else if (arg == strconst_rgb) {
>>        lua_pushnumber (L, 1);
>>        return 1;
>>    } else if (arg == strconst_abgr) {
>>        lua_pushnumber (L, 1);
>>        return 1;
>>    } else if (arg == strconst_bgr) {
>>        lua_pushnumber (L, 1);
>>        return 1;
>>    }
>>    return 0;
>> }
>>
>> int main ()
>> {
>>    lua_State *L = luaL_newstate ();
>>    assert (lua_loadstrconst (L, strconst_rgba)); /* Check for uniqueness
>> */
>>    assert (lua_loadstrconst (L, strconst_rgb)); /* Check for uniqueness */
>>    assert (lua_loadstrconst (L, strconst_abgr)); /* Check for uniqueness
>> */
>>    assert (lua_loadstrconst (L, strconst_bgr)); /* Check for uniqueness */
>>
>>    /* bind 'some_c_function ()' here */
>>    /* execute some code */
>>
>>    return 0;
>> }
>> =======================
>>
>> The primary idea is to avoid 'strcmp ()'.
>> Instead we compare pointers (just the way things work inside Lua VM).
>>
>> What do you think? Is it too mad?
>>
>> P. S. luaL_checkoption () uses strcmp ()
>>
>
> I'd assume that strcmp() is already pretty heavily optimised. Compilers will
> inline it when at least one of the two args is a literal or a local const
> (at least that's what was happening 15 years ago, been a while since I
> worked on a compiler). And I'd be slightly shocked if it didn't already do
> the pointer comparison before jumping into byte comparisons.
>
> Robby

I would lean toward an approach like the following:

static int some_c_function(lua_State* L)
{
  if(lua_gettop(L) != 0) return 0;
  lua_rawget(L, lua_upvalueindex(1));
  switch(lua_tointeger(L, 1))
  {
    case 1: // RGBA
    case 2: // RGB
    case 3: // ABGR
    case 4: //  BGR
      lua_pushnumber(L, 1);
      return 1;
    default:
      return 0;
  }
}

int main()
{
  // etc.

  lua_newtable(L);
  #define keyval(k, v) \
    lua_pushliteral(L, k); \
    lua_pushinteger(L, v); \
    lua_rawset(L, -3)
  keyval("rgba", 1);
  keyval("rgb" , 2);
  keyval("abgr", 3);
  keyval( "bgr", 4);
  #undef keyval
  lua_pushcclosure(L, some_c_function, 1);

  // etc.
}