Jump to content
  • Advertisement
Sign in to follow this  
Gage64

Unity Parameter value optimization?

This topic is 3631 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

This thread got me thinking. Suppose I do this:
struct Obj {
    int a;
    char b;

    Obj(int a, char b):a(a), b(b) {}
};

std::vector<Obj> vec;
Obj obj(5, 'b');
vec.push_back(obj);
In this code, first Obj's constructor is called, then it's copy constructor is called to copy it into the vector, then it's destructor is called. But suppose that I have no need for obj. I only want to create an object "inside" the vector, so to speak. If it was possible to create the object directly into the vector's memory, then the copy constructor and destructor would not need to be called. This situation kind of reminds me of the return value optimization, so my question is: is there something like this with parameters? That is, if I change the above code to this: vec.push_back(Obj(5, 'b')); would the compiler be able to optimize the temporary away? Also, I've heard that there's also the named return optimization, where the compiler can optimize away the temporary even if it's created as an object with a name (i.e., not just in the return statement). Can the compiler do something like this with the first code snippet? Thanks in advance.

Share this post


Link to post
Share on other sites
Advertisement
In theory, yes. You could write code such as:

void foo(Bar a);
int main() { foo(Bar()); }


And no temporary value would be created, skipping the copy construction and instantiating the argument directly in the function.

In practice, what happens is that the push-back function usually looks like:

void push_back(const value_type &t)
{
if (size() == capacity()) { reserve(capacity() * 2); }
new (this->_data + size()) value_type(t);
++ this->_size;
}


Meaning the calling function cannot know where the object should be instantiated, and thus no optimization can be applied. Of course, some compilers may apply some additional optimizations to their associated SC++L implementations, but I don't think this one happens.

Share this post


Link to post
Share on other sites
It's tricky. Very tricky. Mostly because for your very simple example, it may behave differently than a *real* example.

Compilers are actually surprisingly stupid when you really confront them with something that is semantically obvious, but syntactically complicated, like what you are putting forward here. It is clear what your intent is. However, with all programming languages, there isn't always a direct mapping between what a programmer means, and what a programmer says [thus, logically different code that is *meant* to be the same]. For example, what you have here.

Consider this:
Obj c;
c = Obj(a,b);
vs:
Obj c(a,b)
or even:
Obj c = Obj(a,b)
What you MEAN, is clear. You mean for these all to be the same. What you SAY, isn't the same. And the compiler can only mess with what you say. The first case resolves into a fully completing empty constructor followed by an assingment. The second is the (int,char) constructor, and it turns out that the third case is also the (int,char) constructor [though in a rather strange form].

So.... The std::vector<T>::pushback(T&) function will most certainly be inlined. In all likelihood, so will your constructors and your assignment in this simple case. And once it is all inlined, the compiler will see these redundant assignments of variables, and hack them up. It will optimize the code, but it will likely only actually end up being expressed as you are describing in the most simplistic of cases. Cases where everything gets inlined, and everything gets chewed up, and everything means the same thing [which turns out very frequently to be semantically equivalent, but not structurally equivalent]. It turns out that this isn't going to be a specific optimization with its own fancy name [at least not a name I'm familiar with], but instead an effect of function inlining and an a few expression simplification tricks.

But again, as always, it depends heavily on the individual compiler.

And yes, a compiler can optimize away a return value pretty easily in many cases.

*EDIT* Toohrvyk brings up a good point about the complications brought up by using the vector class. On a side note though, it doesn't have to know exactly where the data will go to grind on the assignments and simplify the equations if everything has been inlined successfully.

Share this post


Link to post
Share on other sites
I think this can be solved if you could do something like this:

void push_back(arguments)
{
if (size() == capacity()) { reserve(capacity() * 2); }
new (this->_data + size()) value_type(arguments);
++ this->_size;
}


Then this:

vec.push_back(Obj(5, 'b'));

would become this:

vec.push_back(5, 'b');

That is, if push_back could accept a variable number of arguments, then it could construct the object directly in memory, and then there would be no need to create a temporary object.

I think this is what's being requested in the other thread that I linked. I guess doing this in C++ is very complicated, if it's even possible.

Share this post


Link to post
Share on other sites
Isn't C++0x going to solve this issue with move semantics? Hooray, another meaning for the double-ampersand :)

Share this post


Link to post
Share on other sites
Quote:
Original post by Gage64
I think this can be solved if you could do something like this:


There are hundreds of ways this could be solved, with small patches to the language. But ultimately, it's the entire language philosophy and semantics that cause the issue: the C++ timidly supports the distinction between initialized and uninitialized memory, meaning it's good enough to cause problems but not good enough to solve them elegantly.

Share this post


Link to post
Share on other sites
I've been looking around a bit and it looks like boost::object_pool allows something like this using construct(). Maybe it's possible to modify std::vector to allow this as well? Would it be worth doing?

Share this post


Link to post
Share on other sites
btw, sometimes, to avoid the temporary you can just grow your vector first and take a reference on the last element. The resize will internally call the default constructor of T though.


std::vector<T> myVector;
...
size_t size = myVector.size();
myVector.resize( size + 1 ); // default constructor called
T& myElement = myVector[size]; // initialize the element afterwards

Share this post


Link to post
Share on other sites
Quote:
Original post by fboivin
The resize will internally call the default constructor of T though.
Which is exactly why it doesn't help the situation any. You are also breaking encapsulation in doing this, and requiring that your objects are designed with entirely public fields for this to have any performance impact at all. In short, you're trading one performance problem that is likely rather mild, for a design problem that can be more significant.
Quote:
Original post by ToohrVyk
There are hundreds of ways this could be solved, with small patches to the language. But ultimately, it's the entire language philosophy and semantics that cause the issue: the C++ timidly supports the distinction between initialized and uninitialized memory, meaning it's good enough to cause problems but not good enough to solve them elegantly
I'm not going to go into it too much for fear of starting some sort of language war, but you bring up a good point about C++ in particular. This issue really is only the tip of the iceberg of reasons that compiler writers hate C and C++. There are so many little details of the C and C++ specifications that make certain compile time optimization intensely difficult or even completely impossible that are rather simple in other languages. Couple this with adoption of common practices that are not actually part of the standard resulting in code that compiles fine if optimizations that rely on standards compliance are all turned off, and breaks horribly if optimizations are turned on, and you have a monster of a mess on your hands.

[Edited by - Drigovas on July 8, 2008 6:29:29 AM]

Share this post


Link to post
Share on other sites
Quote:
Original post by fboivin
btw, sometimes, to avoid the temporary you can just grow your vector first and take a reference on the last element. The resize will internally call the default constructor of T though.

*** Source Snippet Removed ***


No, in that case also unnecessary assignment operator call will be made, if the compiler is not "smart enough" to optimize it. In perfect case we should be able to construct the element *directly* in the vector.

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Tags

  • Similar Content

    • By D34DPOOL
      Edit Your Profile D34DPOOL 0 Threads 0 Updates 0 Messages Network Mod DB GameFront Sign Out Add jobEdit jobDeleteC# Programmer for a Unity FPS at Anywhere   Programmers located Anywhere.
      Posted by D34DPOOL on May 20th, 2018
      Hello, my name is Mason, and I've been working on a Quake style arena shooter about destroying boxes on and off for about a year now. I have a proof of concept with all of the basic features, but as an artist with little programming skill I've reached the end of my abilities as a programmer haha. I need someone to help fix bugs, optomize code, and to implent new features into the game. As a programmer you will have creative freedom to suggest new features and modes to add into the game if you choose to, I'm usually very open to suggestions :).
      What is required:
      Skill using C#
      Experience with Unity
      Experience using UNET (since it is a multiplayer game), or the effort and ability to learn it
      Compensation:
      Since the game currently has no funding, we can split whatever revenue the game makes in the future. However if you would perfer I can create 2D and/or 3D assets for whatever you need in return for your time and work.
      It's a very open and chill enviornment, where you'll have relative creative freedom. I hope you are interested in joining the team, and have a good day!
       
      To apply email me at mangemason@yahoo.com
    • By davejones
      Is there a way to automatically change the start position of an animation? I have a bunch of animations set up on 3D models in unity. The issue is that I need to move the 3D models, however when I do so the animation start positions are not updated and I have to do it manually.

      Changing the transform of key frames is time consuming with the amount of animations I have, so I was wondering if there was a way to do it automatically?
    • By MoreLion
      hey all! We are looking for members for our Unity horror game! 
      Here’s the story:
      After a deadly virus plunges the world into chaos killing 85% of the human population there are now what they call “zones” these zones are watched very closely by the surviving government, people are checked every day for the virus, even if you touch the spit or any human waste or fluids of the victim who is infected, you will die. But one day, people in the west zone start to go missing, 1 woman goes outside the walls to uncover the mystery, is there more to the virus than meets the eye?, That is where your story starts.
      This game is not a long development game, I have loads other game ideas,
      I will also allow you to have a bit of creative freedom if you wish to add or share a idea!
      And no, it’s not a zombie game lol I feel like zombie games are too generic, in this game you will encounter terrifying beasts!
      There is some concept art one of our concept artists have made
      If interested email liondude12@gmail.com
    • By Canadian Map Makers
      GOVERNOR is a modernized version of the highly popular series of “Caesar” games. Our small team has already developed maps, written specifications, acquired music and performed the historical research needed to create a good base for the programming part of the project.

      Our ultimate goal is to create a world class multi-level strategic city building game, but to start with we would like to create some of the simpler modules to demonstrate proof of concept and graphical elegance.

       

      We would like programmers and graphical artists to come onboard to (initially) create:

      A module where Province wide infrastructure can be built on an interactive 3D map of one of the ancient Roman Provinces.
      A module where city infrastructure can be built on a real 3D interactive landscape.
      For both parts, geographically and historically accurate base maps will be prepared by our team cartographer. Graphics development will be using Blender. The game engine will be Unity.

       

      More information, and examples of the work carried out so far can be found at http://playgovernor.com/ (most of the interesting content is under the Encyclopedia tab).

       

      This project represents a good opportunity for upcoming programmers and 3D modeling artists to develop something for their portfolios in a relatively short time span, working closely with one of Canada’s leading cartographers. There is also the possibility of being involved in this project to the point of a finished game and commercial success! Above all, this is a fun project to work on.

       

      Best regards,

      Steve Chapman (Canadian Map Makers)

       
    • By Scouting Ninja
      So I have hundreds of moving objects that need to check there speed. One of the reasons they need to check there speed is so they don't accelerate into oblivion, as more and more force is added to each object.
      At first I was just using the Unity vector3.magnitude. However this is actually very slow; when used hundreds of times.
      Next I tried the dot-product check:  vector3.dot(this.transform.foward, ShipBody.velocity) The performance boost was fantastic. However this only measures speed in the forward direction. Resulting in bouncing objects accelerating way past the allowed limit.
       
      I am hoping someone else knows a good way for me to check the speed with accuracy, that is fast on the CPU. Or just any magnitude calculations that I can test when I get home later.
       
      What if I used  vector3.dot(ShipBody.velocity.normalized, ShipBody.velocity)?
      How slow is it to normalize a vector, compared to asking it's magnitude?
    • By Ds ds
      Hi, my name is Andres, I'm a programmer with a technician degree and a Diploma in C#, looking for a project in Unity to start my career in game development. I don't do it for a paid but a recognition and start a portfolio, preferably a 2D game. Thanks for read, have a nice day. 
       
    • By Victor Rodriguez
      Hi there! Is the first time that I'm posting here so I'm sorry if I'm doing it wrong ha. 
      So here it comes, my doubt is, I'm doing a game with different levels, each of these levels in one different scene. Each scene contains to cameras that you can change pressing a button. Everything works fine. 
      The only problem is that I would like it to look a bit more professional, and I would like that if you finish the level with camera2, the next level start the same way. I've been thinking about using dontdestroyonloadon both cameras, but obviously this cameras need to be attached to the player to make the movement work, what do you recommend? Sorry If I've explained it in a messy way, and feel free to dm me for anything. Thanks in advance! 
    • By Ike aka Dk
      Hello everyone 
      I am a programmer from Baku.
      I need a 3D Modeller for my shooter project in unity.I have 2 years Unity exp.
      Project will paid when we finish the work 
      If you interested write me on email:
      mr.danilo911@gmail.com
    • By markoal
      Hi,
      I'm Unity developer from Croatia and I'm looking to work on the paid project in my spare time.
      I have 5+ years of experience in Unity and I'm familiar with almost anything, including all platforms (also Switch, PS4 and Xbox).
      Feel free to contact me.
    • By bartekm777
      Hello
      About me
      Lvl 28   Programmer (day job: non-gamedev-programmer, making games as a hobby for about 2 years) Some vector art experience - tried to make some assets on my own using vector software and scripts   Some design experience (designing my own games ) About game
      Turn-based fantasy rpg inspired by games like Heroes 3 (also WoG mod), NEO Scavenger, Battle Brothers I would like to create easy to use editor for creating custom scenarios (similar to the one from Heroes 3) World and story are clean slate, I did some drafts but I'm not good at it so it's possibly subject to change I decided to create graphics using vector software + scripts to make it faster (rpg's tend to have lots of assets), also it's more precise and easier to create tileable graphics (for example: rivers, paths) No sound/music work has been done yet Who do I look for?
      Definitely someone with 2d art skills  I would like to focus more on programming 2D animator (skletal animations are preferred) Additional programmer could make development faster Someone for creating sounds/music/both It's a hobby project, I work on it in my free time. In case the project make it to the finish line and get shipped  - I can offer rev-share  
      Below should be few screens of what I already did (about 2 months of work) - some graphics, editor prototype screenshot and game prototype screenshot




  • Advertisement
  • Popular Now

  • Forum Statistics

    • Total Topics
      631354
    • Total Posts
      2999492
×

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

Participate in the game development conversation and more when you create an account on GameDev.net!

Sign me up!