Ternary operator is twice as slow as an if-else block?

Question

I read everywhere that ternary operator is supposed to be faster than, or at least the same as, its equivalent if-else block.

However, I di

Answer 1

EDIT: All change... see below.

I can't reproduce your results on the x64 CLR, but I can on x86. On x64 I can see a small difference (less than 10%) between the conditional operator and the if/else, but it's much smaller than you're seeing.

I've made the following potential changes:

• Run in a console app
• Build with /o+ /debug-, and run outside the debugger
• Run both pieces of code once to JIT them, then lots of times for more accuracy
• Use Stopwatch

Results with /platform:x64 (without the "ignore" lines):

if/else with 1 iterations: 17ms
conditional with 1 iterations: 19ms
if/else with 1000 iterations: 17875ms
conditional with 1000 iterations: 19089ms

Results with /platform:x86 (without the "ignore" lines):

if/else with 1 iterations: 18ms
conditional with 1 iterations: 49ms
if/else with 1000 iterations: 17901ms
conditional with 1000 iterations: 47710ms

My system details:

• x64 i7-2720QM CPU @2.20GHz
• 64-bit Windows 8
• .NET 4.5

So unlike before, I think you are seeing a real difference - and it's all to do with the x86 JIT. I wouldn't like to say exactly what is causing the difference - I may update the post later on with more details if I can bother to go into cordbg :)

Interestingly, without sorting the array first, I end up with tests which take about 4.5x as long, at least on x64. My guess is that this is to do with branch prediction.

Code:

using System;
using System.Diagnostics;

class Test
{
    static void Main()
    {
        Random r = new Random(0);
        int[] array = new int[20000000];
        for(int i = 0; i < array.Length; i++)
        {
            array[i] = r.Next(int.MinValue, int.MaxValue);
        }
        Array.Sort(array);
        // JIT everything...
        RunIfElse(array, 1);
        RunConditional(array, 1);
        // Now really time it
        RunIfElse(array, 1000);
        RunConditional(array, 1000);
    }

    static void RunIfElse(int[] array, int iterations)
    {        
        long value = 0;
        Stopwatch sw = Stopwatch.StartNew();

        for (int x = 0; x < iterations; x++)
        {
            foreach (int i in array)
            {
                if (i > 0)
                {
                    value += 2;
                }
                else
                {
                    value += 3;
                }
            }
        }
        sw.Stop();
        Console.WriteLine("if/else with {0} iterations: {1}ms",
                          iterations,
                          sw.ElapsedMilliseconds);
        // Just to avoid optimizing everything away
        Console.WriteLine("Value (ignore): {0}", value);
    }

    static void RunConditional(int[] array, int iterations)
    {        
        long value = 0;
        Stopwatch sw = Stopwatch.StartNew();

        for (int x = 0; x < iterations; x++)
        {
            foreach (int i in array)
            {
                value += i > 0 ? 2 : 3;
            }
        }
        sw.Stop();
        Console.WriteLine("conditional with {0} iterations: {1}ms",
                          iterations,
                          sw.ElapsedMilliseconds);
        // Just to avoid optimizing everything away
        Console.WriteLine("Value (ignore): {0}", value);
    }
}