If vs. Switch, test and analysis

I remember a long time ago, a buddy in the blog Park complained that the source code of. Net was too bad and switch was everywhere. I did a test at that time, proving that the switch has a much higher performance than if. I saw this topic again today. Well, let's test it again.

Code:

Code
Using System;
Using System. Collections. Generic;
Using System. diagnostics;
Using System. text;

Namespace Leleapplication1
{
Class Program
{
Static Int32 count =   100000000 ;

Static Int32 testifelse (int32 value)
{
Int32 I =   0 ;


Stopwatch SW =   New Stopwatch ();
Sw. Start ();
For ( Int J =   0 ; J < Count; j ++ )
{
If (Value =   1 )
I + =   1 ;
Else   If (Value =   2 )
I + =   2 ;
Else   If (Value =   3 )
I + =   3 ;
Else   If (Value =   4 )
I + =   4 ;
Else   If (Value =   5 )
I + =   5 ;
}
Sw. Stop ();
Console. writeline ( " Testifelse: "   + Sw. elapsedmilliseconds );
Return I;
}

Static Int32 testswitch (int32 value)
{
Int32 I =   0 ;

Stopwatch SW =   New Stopwatch ();
Sw. Start ();

For ( Int J =   0 ; J < Count; j ++ )
{
Switch (Value)
{
Case   1 :
I + = 1 ;
Break ;
Case   2 :
I + = 2 ;
Break ;
Case   3 :
I + = 3 ;
Break ;
Case   4 :
I + = 4 ;
Break ;
Case   5 :
I + = 5 ;
Break ;
Default :
Break ;
}
}
Sw. Stop ();
Console. writeline ( " Testswitch: "   + Sw. elapsedmilliseconds );

Return I;
}

Static   Void Main ( String [] ARGs)
{
Testifelse (5);
Testswitch (5);
Console. Read ();
}
}
}

 

Release compilation. Test results:

Testifelse: 613
Testswitch: 165

Performance gap about 4 times. If we look at Il through decompilation, we will find that the testswitch method has the following sentence:

Switch (l_0032, l_0038, l_003e, l_0044, l_004a)

This statement implements a jump table.

As a front-line worker said, this switch command is an index jump, and if... else is an index-free jump. If... else is O (n) level, switch... case is O (1) level.

If we increase the number of branches of the above test code to 10, and test the performance of testswitch (10) and testifelse (10), we will find that the former is almost 7-8 times faster than the latter.

For more information, see a chapter in understanding computer systems. (If you forget which chapter is not around, the difference between switch and if is explained ). You can also refer to this articleArticle: Http://www.9php.com/faq/cxsjl/c/2008/10/1435098132356.html .. For more information, see: http://www.cnblogs.com/yeah/archive/2009/02/16/1392094.html.

If switch (string...) is tested, switch has the same performance as if... else. I thought it was impossible to generate a jump table. That is to say, in this case, switch case is still at the O (1) level branch statement.