Generic vs not-generic performance in C#

Question

I\'ve written two equivalent methods:

static bool F(T a, T b) where T : class
{
    return a == b;
}

static bool F2(A a, A b)
{
    return a == b;
         


        

Answer 1

I did make some changes to your code to measure perf correctly.

1. Use Stopwatch
2. Execute Release Mode
3. Prevent Inlining.
4. Use GetHashCode() to do some real work
5. Look at the generated Assembly code

Here is the code:

class A
{
}

[MethodImpl(MethodImplOptions.NoInlining)]
static bool F(T a, T b) where T : class
{
    return a.GetHashCode() == b.GetHashCode();
}

[MethodImpl(MethodImplOptions.NoInlining)]
static bool F2(A a, A b)
{
    return a.GetHashCode() == b.GetHashCode();
}

static int Main(string[] args)
{
    const int Runs = 100 * 1000 * 1000;
    var a = new A();
    bool lret = F(a, a);
    var sw = Stopwatch.StartNew();
    for (int i = 0; i < Runs; i++)
    {
        F(a, a);
    }
    sw.Stop();
    Console.WriteLine("Generic: {0:F2}s", sw.Elapsed.TotalSeconds);

    lret = F2(a, a);
    sw = Stopwatch.StartNew();
    for (int i = 0; i < Runs; i++)
    {
        F2(a, a);
    }
    sw.Stop();
    Console.WriteLine("Non Generic: {0:F2}s", sw.Elapsed.TotalSeconds);

    return lret ? 1 : 0;
}

During my tests the non generic version was slightly faster (.NET 4.5 x32 Windows 7). But there is practically no measurable difference in speed. I would say the are both equal. For completeness here is the assembly code of the generic version: I got the assembly code via the debugger in Release mode with JIT optimizations enabled.The default is to disable JIT optimizations during debugging to make setting breakpoints and variables inspection easier.

Generic

static bool F(T a, T b) where T : class
{
        return a.GetHashCode() == b.GetHashCode();
}

push        ebp 
mov         ebp,esp 
push        ebx 
sub         esp,8 // reserve stack for two locals 
mov         dword ptr [ebp-8],ecx // store first arg on stack
mov         dword ptr [ebp-0Ch],edx // store second arg on stack
mov         ecx,dword ptr [ebp-8] // get first arg from stack --> stupid!
mov         eax,dword ptr [ecx]   // load MT pointer from a instance
mov         eax,dword ptr [eax+28h] // Locate method table start
call        dword ptr [eax+8] //GetHashCode // call GetHashCode function pointer which is the second method starting from the method table
mov         ebx,eax           // store result in ebx
mov         ecx,dword ptr [ebp-0Ch] // get second arg
mov         eax,dword ptr [ecx]     // call method as usual ...
mov         eax,dword ptr [eax+28h] 
call        dword ptr [eax+8] //GetHashCode
cmp         ebx,eax 
sete        al 
movzx       eax,al 
lea         esp,[ebp-4] 
pop         ebx 
pop         ebp 
ret         4 

Non Generic

static bool F2(A a, A b)
{
  return a.GetHashCode() == b.GetHashCode();
}

push        ebp 
mov         ebp,esp 
push        esi 
push        ebx 
mov         esi,edx 
mov         eax,dword ptr [ecx] 
mov         eax,dword ptr [eax+28h] 
call        dword ptr [eax+8] //GetHashCode
mov         ebx,eax 
mov         ecx,esi 
mov         eax,dword ptr [ecx] 
mov         eax,dword ptr [eax+28h] 
call        dword ptr [eax+8] //GetHashCode
cmp         ebx,eax 
sete        al 
movzx       eax,al 
pop         ebx 
pop         esi 
pop         ebp 
ret 

As you can see the generic version looks slightly more inefficient due to more stack memoy operations which are not perfect but in reality the difference is not measurable since all is fitting into the L1 cache of the processor which makes the memory operations less costly compared to the pure register operations of the non generic version. I would suspect that the non generic version should perform a little better in real world if you need to pay for real memory access not coming from any CPU cache.

For all practical purposes these both methods are identical. You should look at some other place for real world performance gains. I would first look at the data access patterns and used data structures. Algorithmic changes tend to bring much more perf gain than such low level stuff.

Edit1: If you want to use == then you will find

00000000  push        ebp 
00000001  mov         ebp,esp 
00000003  cmp         ecx,edx // Check for reference equality 
00000005  sete        al 
00000008  movzx       eax,al 
0000000b  pop         ebp 
0000000c  ret         4 

both methods produce exactly the same machine code. Any difference you did measure are your measurement errors.