Why is this so much slower in C++?

Question

I have converted this simple method from C# to C++. It reads a path table and populates a list of lists of ints (or a vector of vectors of ints).

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Answer 1

Based on your update it is pretty clear that the function you posted by itself is not causing the performance problem, so while there are many ways in which you can optimize it it seems that is not going to help.

I presume you can reproduce this performance problem every time you run your code, correct? Then I would like to suggest that you do the following tests:

• if you are compiling your program in debug mode (i.e. no optimizations), then recompile for release (full optimizations, favoring speed) and see if that makes a difference.

• To check if the extra time is spent on this suspected function you can add printf statements at the start and end of the function that include timestamps. If this is not a console app but a GUI app and printfs are not going anywhere, then write to a log file. If you are on Windows, you can alternatively use OutputDebugString and use a debugger to capture the printfs. If you are on Linux, you can write to the system log using syslog.

• Use a source code profiler to determine where is all that time spent. If the difference between calling this function or not is several minutes, then a profiler will surely give a clue as to what is happening. If you are on Windows, then Very Sleepy is a good choice, and if you are on Linux you can use OProfile.

Update: So you say that a release build is fast. That likely means that the library functions that you use in this function have slow debug implementations. The STL is know to be that way.

I'm sure you need to debug other parts of your application and you don't want to wait all those minutes for this function to complete in debug mode. The solution to this problem is to build your project in release mode, but change the release configuration in the following way:

• disable optimizations only for the files you want to debug (make sure optimizations remain enabled at least for the file that has the slow function). To disable optimizations on a file, select the file in the Solution Explorer, right click, select Properties, then go to Configuration Properties|C/C++/Optimization. Look at how all the items in that page are set for the Debug build, and copy all of those in your Release build. Repeat for all the files that you want to be available to the debugger.

• enable debugging info (the pdb file) to be generated. To do this, select the Project at the top of the Solution Explorer, right click, select Properties. Then go to Configuration Properties|Linker|Debugging and copy all the settings from the Debug build into the Release build.

With the above changes you will be able to debug the parts of the release binary that were configured as above just like you do it in the debug build.

Once you are done debugging you will need to reset all those settings back, of course.

I hope this helps.