When to use -O2 flag for gcc?

Question

If I use "-O2" flag, the performance improves, but the compilation time gets longer.

How can I decide, whether to use it or not?

Maybe O2 makes the most difference in some certain types of code (e.g. math calculations?), and I should use it only for those parts of the project?

EDIT: I want to emphasize the fact that setting -O2 for all components of my project changes the total compilation time from 10 minutes to 30 minutes.

Answer 1

I would recommend using -O2 most of the time, benefits include:

• Usually reduces size of generated code (unlike -O3).
• More warnings (some warnings require analysis that is only done during optimization)
• Often measurably improved performance (which may not matter).

If release-level code will have optimization enabled, it's best to have optimization enabled throughout the development/test cycle.

Source-level debugging is more difficult with optimizations enabled, occasionally it is helpful to disable optimization when debugging a problem.

Answer 2

I'm in bioinformatics so my advice may be biased. That said, I always use the -O3 switch (for release and test builds, that is; not usually for debugging). True, it has certain disadvantages, namely increasing compile-time and often the size of the executable.

However, the first factor can be partially mitigated by a good build strategy and other tricks reducing the overall build time. Also, since most of the compilation is really I/O bound, the increase of compile time is often not that pronounced.

The second disadvantage, the executable's size, often simply doesn't matter at all.

Answer 3

Never.

Use -O3 -Wall -Werror -std=[whatever your code base should follow]