How do I choose optimization flags (-O2/-O3/-Ofast)?

When compiling C++ code, selecting the right optimization flags can significantly impact the performance of your application. The most commonly used optimization flags include -O2, -O3, and -Ofast. Each of these flags optimizes the code differently and is suited for various use cases.

-O2 Optimization

The -O2 flag enables a good level of optimization without compromising compilation speed and code size. It performs a range of optimizations, including inline functions and dead code elimination, making it suitable for most production applications.

-O3 Optimization

The -O3 flag turns on even more optimizations than -O2, focusing on performance improvement. This includes more aggressive inlining and loop unrolling. However, it may increase the binary size and compilation time significantly, so usage depends on the application's requirements for speed versus size.

-Ofast Optimization

The -Ofast flag goes a step further, enabling optimizations that may violate strict standards compliance (like IEEE floating-point rules). Use it when performance is a top priority, and you can afford to sacrifice portability or correctness in certain scenarios.

Choosing the Right Optimization Flag

To choose the best optimization flag for your project, consider the following:

• Performance Needs: If you require maximum speed, -O3 or -Ofast is the way to go.
• Code Size: If your application is sensitive to binary size, -O2 might be more appropriate.
• Development Stage: Use -O0 during development for faster compilation and easier debugging before switching to higher optimization levels for production.