虚函数调用始终比正常函数调用快。为什么？

Question

I wrote the following program to test how much virtual functions cost on my machine:

#include <iostream>
#include <ctime>
#define NUM_ITER 10000000000
//   5 seconds = 1000000000

static volatile int global_a;

void spin()
{
    int a = global_a;
    int b = a*a;
    int c = a+5;
    int d = a^b^c;
    global_a = b*d;
}

struct A {
    virtual void a() = 0;
};

struct B : A {
    virtual void a() { spin(); }
};

struct C : A {
    virtual void a() { spin(); }
};

void run_A1(A* a)
{
    a->a();
}

void run_A(A* a)
{
    for (long long i = 0; i < NUM_ITER; i++) {
        run_A1(a);
    }
}

void run()
{
    for (long long i = 0; i < NUM_ITER; i++) {
        spin();
    }
}

int main()
{
    global_a = 2;

    A* a1 = new B;
    A* a2 = new C;

    std::clock_t c_begin, c_end;

    c_begin = std::clock();
    run_A(a1);
    c_end = std::clock();

    std::cout << "Virtual | CPU time used: "
              << 1000.0 * (c_end-c_begin) / CLOCKS_PER_SEC
              << " ms\n";

    c_begin = std::clock();
    run_A(a2);
    c_end = std::clock();

    std::cout << "Virtual | CPU time used: "
              << 1000.0 * (c_end-c_begin) / CLOCKS_PER_SEC
              << " ms\n";

    c_begin = std::clock();
    run();
    c_end = std::clock();

    std::cout << "Normal  | CPU time used: "
              << 1000.0 * (c_end-c_begin) / CLOCKS_PER_SEC
              << " ms\n";

    delete a1;
    delete a2;
}

The results were opposite than I expected: the virtual functions were consistently faster. For example, this is one of the outputs I got with NUM_ITER = 10000000000 :

Virtual | CPU time used: 49600 ms
Virtual | CPU time used: 50270 ms
Normal  | CPU time used: 52890 ms

From the analysis of the resulting assembler file I can confirm that the compiler hasn't optimized out anything important. I've used GCC-4.7 with the following options:

g++ -O3 -std=c++11 -save-temps -masm=intel -g0 -fno-exceptions -fno-inline test.cc -o test

Why are the virtual function calls faster? Or why are the non-virtual function calls slower? Have the branch predictors become so good? Or maybe it's just my machine. Maybe someone could also test and post his timings?

Answer 1

Try reseting global_a before each call to run() :

void run()
{
    global_a = 2;

    ...
}

void run_A(A *a)
{    
    global_a = 2;

    ...
}

Not sure if this is having any impact, but not all mathematical operations take the same amount of time!

Answer 2

The compiler might be smart enough to see that the virtual functions call a global function spin() and devirtualize them. The calls probably get inlined too.

Check this .