True Unsafe Code Performance

c# performance unsafe
Miguel picture Miguel · Mar 21, 2011 · Viewed 25.8k times · Source

I understand unsafe code is more appropriate to access things like the Windows API and do unsafe type castings than to write more performant code, but I would like to ask you if you have ever noticed any significant performance improvement in real-world applications by using it when compared to safe c# code.

Answer

Thomas Bratt picture Thomas Bratt · Jul 10, 2012

Some Performance Measurements

The performance benefits are not as great as you might think.

I did some performance measurements of normal managed array access versus unsafe pointers in C#.

Results from a build run outside of Visual Studio 2010, .NET 4, using an Any CPU | Release build on the following PC specification: x64-based PC, 1 quad-core processor. Intel64 Family 6 Model 23 Stepping 10 GenuineIntel ~2833 Mhz.

Linear array access
 00:00:07.1053664 for Normal
 00:00:07.1197401 for Unsafe *(p + i)

Linear array access - with pointer increment
 00:00:07.1174493 for Normal
 00:00:10.0015947 for Unsafe (*p++)

Random array access
 00:00:42.5559436 for Normal
 00:00:40.5632554 for Unsafe

Random array access using Parallel.For(), with 4 processors
 00:00:10.6896303 for Normal
 00:00:10.1858376 for Unsafe

Note that the unsafe *(p++) idiom actually ran slower. My guess this broke a compiler optimization that was combining the loop variable and the (compiler generated) pointer access in the safe version.

Source code available on github.

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