Is there any performance difference between ++i and i++ in C#?

Question

Is there any performance difference between using something like

for(int i = 0; i < 10; i++) { ... } 

and

for(int i = 0; i < 10; ++i) { ... } 

or is the compiler able to optimize in such a way that they are equally fast in the case where they are functionally equivalent?

Edit: This was asked because I had a discussion with a co-worker about it, not because I think its a useful optimization in any practical sense. It is largely academic.

Answer 1

There is no difference in the generated intermediate code for ++i and i++ in this case. Given this program:

class Program {     const int counter = 1024 * 1024;     static void Main(string[] args)     {         for (int i = 0; i < counter; ++i)         {             Console.WriteLine(i);         }          for (int i = 0; i < counter; i++)         {             Console.WriteLine(i);         }     } } 

The generated IL code is the same for both loops:

  IL_0000:  ldc.i4.0   IL_0001:  stloc.0   // Start of first loop   IL_0002:  ldc.i4.0   IL_0003:  stloc.0   IL_0004:  br.s       IL_0010   IL_0006:  ldloc.0   IL_0007:  call       void [mscorlib]System.Console::WriteLine(int32)   IL_000c:  ldloc.0   IL_000d:  ldc.i4.1   IL_000e:  add   IL_000f:  stloc.0   IL_0010:  ldloc.0   IL_0011:  ldc.i4     0x100000   IL_0016:  blt.s      IL_0006   // Start of second loop   IL_0018:  ldc.i4.0   IL_0019:  stloc.0   IL_001a:  br.s       IL_0026   IL_001c:  ldloc.0   IL_001d:  call       void [mscorlib]System.Console::WriteLine(int32)   IL_0022:  ldloc.0   IL_0023:  ldc.i4.1   IL_0024:  add   IL_0025:  stloc.0   IL_0026:  ldloc.0   IL_0027:  ldc.i4     0x100000   IL_002c:  blt.s      IL_001c   IL_002e:  ret 

That said, it's possible (although highly unlikely) that the JIT compiler can do some optimizations in certain contexts that will favor one version over the other. If there is such an optimization, though, it would likely only affect the final (or perhaps the first) iteration of a loop.

In short, there will be no difference in the runtime of simple pre-increment or post-increment of the control variable in the looping construct that you've described.

Answer 2

If you're asking this question, you're trying to solve the wrong problem.

The first question to ask is "how to I improve customer satisfaction with my software by making it run faster?" and the answer is almost never "use ++i instead of i++" or vice versa.

From Coding Horror's post "Hardware is Cheap, Programmers are Expensive":

Rules of Optimization:
Rule 1: Don't do it.
Rule 2 (for experts only): Don't do it yet.
-- M.A. Jackson

I read rule 2 to mean "first write clean, clear code that meets your customer's needs, then speed it up where it's too slow". It's highly unlikely that ++i vs. i++ is going to be the solution.