Is There a Performance Difference Between I++ and ++I in C++

Is there a performance difference between i++ and ++i in C?

Executive summary: No.

i++ could potentially be slower than ++i, since the old value of i
might need to be saved for later use, but in practice all modern
compilers will optimize this away.

We can demonstrate this by looking at the code for this function,
both with ++i and i++.

$ cat i++.c
extern void g(int i);
void f()
{
    int i;

    for (i = 0; i < 100; i++)
        g(i);

}

The files are the same, except for ++i and i++:

$ diff i++.c ++i.c
6c6
<     for (i = 0; i < 100; i++)
---
>     for (i = 0; i < 100; ++i)

We'll compile them, and also get the generated assembler:

$ gcc -c i++.c ++i.c
$ gcc -S i++.c ++i.c

And we can see that both the generated object and assembler files are the same.

$ md5 i++.s ++i.s
MD5 (i++.s) = 90f620dda862cd0205cd5db1f2c8c06e
MD5 (++i.s) = 90f620dda862cd0205cd5db1f2c8c06e

$ md5 *.o
MD5 (++i.o) = dd3ef1408d3a9e4287facccec53f7d22
MD5 (i++.o) = dd3ef1408d3a9e4287facccec53f7d22

Is there a performance difference between i++ and ++i in C++?

[Executive Summary: Use ++i if you don't have a specific reason to use i++.]

For C++, the answer is a bit more complicated.

If i is a simple type (not an instance of a C++ class), then the answer given for C ("No there is no performance difference") holds, since the compiler is generating the code.

However, if i is an instance of a C++ class, then i++ and ++i are making calls to one of the operator++ functions. Here's a standard pair of these functions:

Foo& Foo::operator++()   // called for ++i
{
    this->data += 1;
    return *this;
}

Foo Foo::operator++(int ignored_dummy_value)   // called for i++
{
    Foo tmp(*this);   // variable "tmp" cannot be optimized away by the compiler
    ++(*this);
    return tmp;
}

Since the compiler isn't generating code, but just calling an operator++ function, there is no way to optimize away the tmp variable and its associated copy constructor. If the copy constructor is expensive, then this can have a significant performance impact.

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

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.

What is the difference between ++i and i++?

++i will increment the value of i, and then return the incremented value.
```
 i = 1;
 j = ++i;
 (i is 2, j is 2)
```
i++ will increment the value of i, but return the original value that i held before being incremented.
```
 i = 1;
 j = i++;
 (i is 2, j is 1)
```

For a for loop, either works. ++i seems more common, perhaps because that is what is used in K&R.

In any case, follow the guideline "prefer ++i over i++" and you won't go wrong.

There's a couple of comments regarding the efficiency of ++i and i++. In any non-student-project compiler, there will be no performance difference. You can verify this by looking at the generated code, which will be identical.

The efficiency question is interesting... here's my attempt at an answer:
Is there a performance difference between i++ and ++i in C?

As @OnFreund notes, it's different for a C++ object, since operator++() is a function and the compiler can't know to optimize away the creation of a temporary object to hold the intermediate value.

Is there any performance difference with ++i vs i += 1 in C#?

It is easy to answer: the C# compiler translates C# source code to IL opcodes. There is no dedicated IL opcode that performs the equivalent of the ++ operator. Which is easy to see if you look at the generated IL with the ildasm.exe tool. This sample C# snippet:

        int ix = 0;
        ix++;
        ix = ix + 1;

Generates:

  IL_0000:  ldc.i4.0               // load 0
  IL_0001:  stloc.0                // ix = 0

  IL_0002:  ldloc.0                // load ix
  IL_0003:  ldc.i4.1               // load 1
  IL_0004:  add                    // ix + 1
  IL_0005:  stloc.0                // ix = ix + 1

  IL_0006:  ldloc.0                // load ix
  IL_0007:  ldc.i4.1               // load 1
  IL_0008:  add                    // ix + 1
  IL_0009:  stloc.0                // ix = ix + 1

It generates the exact same code. Nothing the jitter can do but generate machine code that is equally fast.

The pre/post increment operator is syntax sugar in C#, use it wherever it makes your code more legible. Or perhaps more relevant: avoid it where it makes it less legible. They do have a knack for letting you create expressions that have too many side-effects.

Speed comparison between i = i + 1 and ++i [duplicate]

Apart from compiler you are using, it also depends on the optimization setting you used. If you want to know, disassemble it to see the result of compilation for specific compiler + optimization setting.

However at least there are something that are mostly true:

i++ will not be slower than i = i+1
++i will not be slower than i++ if you are only doing incrementation.
It is always preferred to write ++i if possible even for simple type. Not because of performance, but to build up a good practice so you do not need to double-think when you are doing ++ on an object.

Why is ++i more efficient than i++? [duplicate]

i++ increments i and returns the initial value of i. Which means:

int i = 1;
i++; // == 1 but i == 2

But ++i returns the actual incremented value:

int i = 1;
++i; // == 2 and i == 2 too, so no need for a temporary variable

In the first case, the compiler has to create a temporary variable (when used) for returning 1 instead of 2 (in the case where it's not a constant of course but a dynamic value, a return from a call for example).

In the second case, it does not have to. So the second case is guaranteed to be at least as effective.

Often, the compiler will be able to optimize the first case into the second case, but sometimes it may not be able to.

Anyway, we're talking about highly negligible impact.

But on more complicated objects such as iterators-like objects, having a temporary state may be pretty slower if iterated millions of times.

Rule of thumb

Use prefix version unless you specifically want the postfix semantics.