When I write a routine to test the performance of two stuffs, which optimization flags should I use? -O0, -O2, or -g ?
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When I write a routine to test the performance of two stuffs, which optimization flags should I use? -O0, -O2, or -g ?
You should test the performance of your code using each of the settings. Ideally the larger the number -O0, -O1, -O2, -O3, implies better performance as there is more/better optimization, but that is not always the case.
Likewise depending on how your code is written some of it may be removed in a way that you didnt expect from the language or the compiler or both. So not only do you need to test the performance of your code, you need to actually test the program generated from your code to see that it does what you think it does.
There is definitely not one optimization setting that provides the best performance for any code that can be compiled by that compiler. You have to test the settings and compiler on a particular system to verify that for that system the code does indeed run faster. How you test that performance is filled with many traps and other error producing problems that you can easily misunderstand the results. So you have to be careful in how you test your performance.
For gcc folks usually say -O3 is risky to use and -O2 is the best performance/safe. And for the most part that is the case -O2 is used enough to get many bugs flushed out. -O2 does not always produce the fastest code but it generally produces faster code that -O0 and -O1. Use of debuggers can defeat the optimization or remove it all together, so never test for performance with a debugger based build or using a debugger. Test on the system as the user would use the system, if the user uses a debugger when they run your program then test that way, otherwise dont.