C++ OOP Performance Tips–Part 1


We often encounter inheritance and composition implementations that are too flexible and too generic for the problem domain. They may perform computations that are rarely or never required. In practice, it is not surprising to discover performance overhead associated with inheritance and composition. This is a tradeoff between code reuse and performance. Oftentimes, reusable code will
compute things you don’t really need in a specific scenario. Any time you call functions that do more than you really need, you will take a performance hit. This article will highlight some of these scenarios.

This is not to say that inheritance is fundamentally a performance obstacle. We must make a distinction between the overall computational cost, required cost, and computational penalty. The overall computational cost is the set of all instructions executed in a computation. The required cost is that subset of instructions whose results are necessary. This part of the computation is mandatory; computational penalty = overall computational costrequired cost. This is the part of the computation that could have been eliminated by an alternative design or implementation. (e.g inheritance hierarchy, composition layout, etc…)

So we cannot make a blanket statement that complex inheritance designs are necessarily bad, nor do they always carry a performance penalty. All we can say is that overall cost grows with the size of the derivation tree. If all those computations are valuable then it is all required cost. In practice, inheritance hierarchies are not likely to be perfect. In that case, they are likely to impose a computational penalty.

Constructors and Destructors

Following is a snippet from Visual Studio 2008 Disassembly Window for a simple Foo() function creating Point object locally on the stack and destroying it on returning.

class Point
{
public:
    Point() : X(0), Y(0) { cout << "Hello!\n"; }
    ~Point() { cout << "Bye bye\n"; }
    int X;
    int Y;
};

void Foo()
{
010820B0  push        ebp  
010820B1  mov         ebp,esp 
010820B3  sub         esp,8 
    Point obj;
010820B6  lea         ecx,[obj] // Pass obj address to the constructor 
010820B9  call        Point::Point (10813ACh) // Call Constructor 
}
010820BE  lea         ecx,[obj] // Pass obj address to the destructor
010820C1  call        Point::~Point (10813B1h) // Call Destructor
010820C6  xor         eax,eax 
010820C8  mov         esp,ebp 
010820CA  pop         ebp  
010820CB  ret             

The translated assembly instructions of each C++ statement (in black) appears directly below it. As you can see, constructors and destructors (in Red) are normal functions like any C++ function. The compiler knows exactly where and when to call them. A constructor or destructor call costs 2 assembly instructions.

The following is the code generated for an empty Point constructor, total of 8 assembly instructions.

class Point
{
public:
    Point() 
00E12960  push        ebp  
00E12961  mov         ebp,esp 
00E12963  push        ecx  
00E12964  mov         dword ptr [ebp-4],ecx 
    {
        
    }
00E12967  mov         eax,dword ptr [this] 
00E1296A  mov         esp,ebp 
00E1296C  pop         ebp  
00E1296D  ret   

The following is the code generated for an empty Point destructor, total of 7 assembly instructions.

    ~Point()
    {
00E12580  push        ebp  
00E12581  mov         ebp,esp 
00E12583  push        ecx  
00E12584  mov         dword ptr [ebp-4],ecx   
    }
00E12587  mov         esp,ebp 
00E12589  pop         ebp  
00E1258A  ret        

Let’s calculate a base-line for the number of assembly instructions required to construct/destruct a Point object:

  1. 2 instructions to call Point constructor Point::Point().
  2. 8 instructions to implement Point empty constructor body.
  3. 2 instructions to call Point destructor Point:~Point()
  4. 7 instructions to implement Point empty destructor body.
  5. Total of 2 + 8 + 2 + 7 = 19 instruction to construct/destruct a Point object!
    Enough theory, let’s get our hands dirty and write some code and figure out the effect of these extra 19 instruction.
    Consider the code snippet below (Version 0):
// Version 0
Point dummy
; for (int i = 0; i < 100000000; ++i) { Point p; p.X = p.Y = i; dummy.X = p.X; dummy.Y = p.Y; }

The above code doesn’t make sense in a real-life program, and it is meaningless, but it will help illustrate our theory.  We will focus only on the Point object construction/destruction. As you can see, each iteration costs 19 instruction (mentioned up) per Point construction/destruction, and the overall cost for construction/destruction in the previous for-loop is 20 multiplied by the number f iterations which is 100 million ~= 1900000000 instruction only for Point construction/destruction.  Note that we didn’t calculate the instructions required for p and dummy objects members initialization because they are irrelevant to our case-study. We care only for Point construction/destruction.

A make-sense optimization for the previous for loop is to bring the Point p; line outside the for loop so that the Point object is constructed/destructed only once. In theory, the overall construction/destruction should drop from 1900000000 to 20 ! We can call this overhead: “Silent Overhead”.

The optimized for loop will look as follows (Version 1):

// Version 1
Point p
; Point dummy; for (int i = 0; i < 100000000; ++i) { p.X = p.Y = i; dummy.X = p.X; dummy.Y = p.Y; }

Results

We chose number of iterations to be a rather very high 100000000 iteration to highlight the performance drop. Our experimental computer is equipped with a very fast Intel Core i7 processor with 8 logical processors. And 1 million iteration is too small for it, the i7 was able to execute 1 million iteration in 2 milliseconds in average !! That’s why we chose a very high number of iterations to scale this difference.

image

For Version 0 and Version 1, We ran the for-loop for 100 million iteration and take the average running time for 100 sample. The optimized Version 1 is approximately 3 times faster than Version 0 .

From the previous experiment, It is obvious that object construction/destruction can lead to a big drop in performance if they are called unnecessarily as in code snippet Version 0.

Key Points

  1. Watch out for the combinatorial explosion of created objects in complex hierarchies. The construction (destruction) of an object triggers recursive construction (destruction) of parent and member objects.
  2. Defer object construction (i.e local variable declaration, dynamic object allocation) to the scope in which it is manipulated. The object life cycle is not free of cost. At the very least, construction and destruction of an object may consume CPU cycles. Don’t create an object unless you are going to use it.
  3. Use the object constructor initialization list to complete the member object creation. Because compilers must initialize contained member objects prior to entering the constructor body. This will save the overhead of calling the assignment operator later in the constructor body. In some cases, it will also avoid the generation of temporary objects.

Do/ Don’t Do

Do this:

void Foo(int n)
{
    if (n == 0)
        return;
    else
    {
        HeavyClass heavyObj;
        heavyObj.DoWork();
    }
}

Don’t do this:

void Foo(int n)
{
    HeavyClass heavyObj;
    
    if (n == 0)
        return;
    else
    {
        heavyObj.DoWork();
    }
}

Do this:

class Person
{
public:
    Person(const char* name) : Name(name) {}
    string Name;
};

Don’t do this:

class Person
{
public:
    Person(const char* name) /* Name constructor called */
    {
        Name = name;
    }
    string Name;
};

Reference

  • Efficient C++ Performance Programming Techniques book By Dov Bulka, David Mayhew
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10 Responses to “C++ OOP Performance Tips–Part 1”

  1. Anas Says:

    Great One!! I liked this way of comparison very much …
    Simply .. ana metkayef 😀

  2. abuMuslim Amr Says:

    It is just perfect, useful and interesting … I liked it so much, I liked its scientific value as well as the way you present it. keep it up dude!

    • MHesham Says:

      Thanks dude, If you like this dirty stuff, I recommend for you the book referenced in this post, I am reading it right now, and it inspired me to write this post.

  3. mohammedsharaf Says:

    Wow, and now Filipino is reading C++ Efficiency, i can remember your face when you are talking to Zikas Telling him “waaaaaaaaaaaaaa` When can i read in that Zikas waaaaaaaaaaa` 😥 :'(”
    علي رأي العلامة الكبير محمد منير، لو بطلنا نحلم نموت
    😀 😀

    • MHesham Says:

      YES ! I remember this 😀
      إمتي بقي أتخرج و أقري في ال
      Optimization

      isA part 2 will be about Virtual Functions …

      • mohammedsharaf Says:

        For me i want to read about another things 😀

        Good Luck dude (Y)

      • MHesham Says:

        سبحان من خلق الناس مختلفين
        لو كلنا كنا بنحب نفس الحاجة كانت الدنيا خربت

      • Anas Says:

        The same dream here …!!
        امتى اتخرج وأقرأ في الحاجات دي … أنا بتمزج منها الصراحة ..

      • MHesham Says:

        إنت شكلك من نفس فصيلتي .. بتحب قلة الأدب اللي من النوع ده
        طول ما إنت بتحب قلة الأدب يبقي هتوصل إن شاء الله :D:D


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