luaO_arith
setnvalue(o1, luaO_arith(op, nvalue(o1), nvalue(o2)));
r = luaO_arith(op - OP_ADD + LUA_OPADD, e1->u.nval, e2->u.nval);
LUAI_FUNC lua_Number luaO_arith (int op, lua_Number v1, lua_Number v2);
res = luaO_arith(op - TM_ADD + LUA_OPADD, nvalue(b), nvalue(c));