Virtual functions and performance - C++

Question

In my class design, I use abstract classes and virtual functions extensively. I had a feeling that virtual functions affects the performance. Is this true? But I think this performance difference is not noticeable and looks like I am doing premature optimization. Right?

Answer 1

From page 44 of Agner Fog's "Optimizing Software in C++" manual:

The time it takes to call a virtual member function is a few clock cycles more than it takes to call a non-virtual member function, provided that the function call statement always calls the same version of the virtual function. If the version changes then you will get a misprediction penalty of 10 - 30 clock cycles. The rules for prediction and misprediction of virtual function calls is the same as for switch statements...

Answer 2

When Objective-C (where all methods are virtual) is the primary language for the iPhone and freakin' Java is the main language for Android, I think it's pretty safe to use C++ virtual functions on our 3 GHz dual-core towers.

Answer 3

Yes, you're right and if you curious about the cost of virtual function call you might find this post interesting.

Answer 4

Your question made me curious, so I went ahead and ran some timings on the 3GHz in-order PowerPC CPU we work with. The test I ran was to make a simple 4d vector class with get/set functions

class TestVec 
{
    float x,y,z,w; 
public:
    float GetX() { return x; }
    float SetX(float to) { return x=to; }  // and so on for the other three 
}

Then I set up three arrays each containing 1024 of these vectors (small enough to fit in L1) and ran a loop that added them to one another (A.x = B.x + C.x) 1000 times. I ran this with the functions defined as inline, virtual, and regular function calls. Here are the results:

• inline: 8ms (0.65ns per call)
• direct: 68ms (5.53ns per call)
• virtual: 160ms (13ns per call)

So, in this case (where everything fits in cache) the virtual function calls were about 20x slower than the inline calls. But what does this really mean? Each trip through the loop caused exactly 3 * 4 * 1024 = 12,288 function calls (1024 vectors times four components times three calls per add), so these times represent 1000 * 12,288 = 12,288,000 function calls. The virtual loop took 92ms longer than the direct loop, so the additional overhead per call was 7 nanoseconds per function.

From this I conclude: yes, virtual functions are much slower than direct functions, and no, unless you're planning on calling them ten million times per second, it doesn't matter.

See also: comparison of the generated assembly.

Answer 5

In very performance critical applications (like video games) a virtual function call can be too slow. With modern hardware, the biggest performance concern is the cache miss. If data isn't in the cache, it may be hundreds of cycles before it's available.

A normal function call can generate an instruction cache miss when the CPU fetches the first instruction of the new function and it's not in the cache.

A virtual function call first needs to load the vtable pointer from the object. This can result in a data cache miss. Then it loads the function pointer from the vtable which can result in another data cache miss. Then it calls the function which can result in an instruction cache miss like a non-virtual function.

In many cases, two extra cache misses are not a concern, but in a tight loop on performance critical code it can dramatically reduce performance.

Answer 6

absolutely. It was a problem way back when computers ran at 100Mhz, as every method call required a lookup on the vtable before it was called. But today.. on a 3Ghz CPU that has 1st level cache with more memory than my first computer had? Not at all. Allocating memory from main RAM will cost you more time than if all your functions were virtual.

Its like the old, old days where people said structured programming was slow because all the code was split into functions, each function required stack allocations and a function call!

The only time I would even think of bothering to consider the performance impact of a virtual function, is if it was very heavily used and instantiated in templated code that ended up throughout everything. Even then, I wouldn't spend too much effort on it!

PS think of other 'easy to use' languages - all their methods are virtual under the covers and they don't crawl nowadays.