C# vs C - Big performance difference

You must be comparing debug builds. I just compiled your C code, and got

Time elapsed: 0.000000

If you don't enable optimizations, any benchmarking you do is completely worthless. (And if you do enable optimizations, the loop gets optimized away. So your benchmarking code is flawed too. You need to force it to run the loop, usually by summing up the result or similar, and printing it out at the end)

It seems that what you're measuring is basically "which compiler inserts the most debugging overhead". And turns out the answer is C. But that doesn't tell us which program is fastest. Because when you want speed, you enable optimizations.

By the way, you'll save yourself a lot of headaches in the long run if you abandon any notion of languages being "faster" than each others. C# no more has a speed than English does.

There are certain things in the C language that would be efficient even in a naive non-optimizing compiler, and there are others that relies heavily on a compiler to optimize everything away. And of course, the same goes for C# or any other language.

The execution speed is determined by:

• the platform you're running on (OS, hardware, other software running on the system)
• the compiler
• your source code

A good C# compiler will yield efficient code. A bad C compiler will generate slow code. What about a C compiler which generated C# code, which you could then run through a C# compiler? How fast would that run? Languages don't have a speed. Your code does.

I'll keep it brief, it is already marked answered. C# has the great advantage of having a well defined floating point model. That just happens to match the native operation mode of the FPU and SSE instruction set on x86 and x64 processors. No coincidence there. The JITter compiles Math.Sqrt() to a few inline instructions.

Native C/C++ is saddled with years of backwards compatibility. The /fp:precise, /fp:fast and /fp:strict compile options are the most visible. Accordingly, it must call a CRT function that implements sqrt() and checks the selected floating point options to adjust the result. That's slow.

Since you never use 'root', the compiler may have been removing the call to optimize your method.

You could try to accumulate the square root values into an accumulator, print it out at the end of the method, and see what's going on.

Edit : see Jalf's answer below

my first guess is a compiler optimization because you never use root. You just assign it, then overwrite it again and again.

Edit: damn, beat by 9 seconds!

To see if the loop is being optimised away, try changing your code to

root += Math.Sqrt(i);

ans similarly in the C code, and then print the value of root outside the loop.

Maybe the c# compiler is noticing you don't use root anywhere, so it just skips the whole for loop. :)

That may not be the case, but I suspect whatever the cause is, it is compiler implementation dependent. Try compiling you C program with the Microsoft compiler (cl.exe, available as part of the win32 sdk) with optimizations and Release mode. I bet you'll see a perf improvement over the other compiler.

EDIT: I don't think the compiler can just optimize out the for loop, because it would have to know that Math.Sqrt() doesn't have any side-effects.

Whatever the time diff. may be, that "elapsed time" is invalid. It would only be a valid one if you can guarantee that both programs run under the exact same conditions.

Maybe you should try a win. equivalent to $/usr/bin/time my_cprog;/usr/bin/time my_csprog

I put together (based on your code) two more comparable tests in C and C#. These two write a smaller array using the modulus operator for indexing (it adds a little overhead, but hey, we're trying to compare performance [at a crude level]).

C code:

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <math.h>

void main()
{
    int count = (int)1e8;
    int subcount = 1000;
    double* roots = (double*)malloc(sizeof(double) * subcount);
    clock_t start = clock();
    for (int i = 0 ; i < count; i++)
    {
        roots[i % subcount] = sqrt((double)i);
    }
    clock_t end = clock();
    double length = ((double)end - start) / CLOCKS_PER_SEC;
    printf("Time elapsed: %f\n", length);
}

In C#:

using System;

namespace CsPerfTest
{
    class Program
    {
        static void Main(string[] args)
        {
            int count = (int)1e8;
            int subcount = 1000;
            double[] roots = new double[subcount];
            DateTime startTime = DateTime.Now;
            for (int i = 0; i < count; i++)
            {
                roots[i % subcount] = Math.Sqrt(i);
            }
            TimeSpan runTime = DateTime.Now - startTime;
            Console.WriteLine("Time elapsed: " + Convert.ToString(runTime.TotalMilliseconds / 1000));
        }
    }
}

These tests write data to an array (so the .NET runtime shouldn't be allowed to cull the sqrt op) although the array is significantly smaller (didn't want to use excessive memory). I compiled these in release config and run them from inside a console window (instead of starting through VS).

On my computer the C# program varies between 6.2 and 6.9 seconds, while the C version varies between 6.9 and 7.1.

If you just single-step the code at the assembly level, including stepping through the square-root routine, you will probably get the answer to your question.

No need for educated guessing.

The other factor that may be an issue here is that the C compiler compiles to generic native code for the processor family you target, whereas the MSIL generated when you compiled the C# code is then JIT compiled to target the exact processor you have complete with any optimisations that may be possible. So the native code generated from the C# may be considerably faster than the C.

It would seem to me that this is nothing to do with the languages themselves, rather it is to do with the different implementations of the square root function.

Actually guys, the loop is NOT being optimized away. I compiled John's code and examined the resulting .exe. The guts of the loop are as follows:

 IL_0005:  stloc.0
 IL_0006:  ldc.i4.0
 IL_0007:  stloc.1
 IL_0008:  br.s       IL_0016
 IL_000a:  ldloc.1
 IL_000b:  conv.r8
 IL_000c:  call       float64 [mscorlib]System.Math::Sqrt(float64)
 IL_0011:  pop
 IL_0012:  ldloc.1
 IL_0013:  ldc.i4.1
 IL_0014:  add
 IL_0015:  stloc.1
 IL_0016:  ldloc.1
 IL_0017:  ldc.i4     0x5f5e100
 IL_001c:  ble.s      IL_000a

Unless the runtime is smart enough to realize the loop does nothing and skips it?

Edit: Changing the C# to be:

 static void Main(string[] args)
 {
      DateTime startTime = DateTime.Now;
      double root = 0.0;
      for (int i = 0; i <= 100000000; i++)
      {
           root += Math.Sqrt(i);
      }
      System.Console.WriteLine(root);
      TimeSpan runTime = DateTime.Now - startTime;
      Console.WriteLine("Time elapsed: " +
          Convert.ToString(runTime.TotalMilliseconds / 1000));
 }

Results in the time elapsed (on my machine) going from 0.047 to 2.17. But is that just the overhead of adding a 100 million addition operators?