I have heard from various sources (though mostly from a colleague of mine), that compiling with an optimisation level of -O3 in g++ is somehow 'dangerous', and should be avoided in general unless proven to be necessary.
Is this true, and if so, why? Should I just be sticking to -O2?
In my somewhat checkered experience, applying -O3 to an entire program almost always makes it slower (relative to -O2), because it turns on aggressive loop unrolling and inlining that make the program no longer fit in the instruction cache. For larger programs, this can also be true for -O2 relative to -Os!
The intended use pattern for -O3 is, after profiling your program, you manually apply it to a small handful of files containing critical inner loops that actually benefit from these aggressive space-for-speed tradeoffs. Newer versions of GCC have a profile-guided optimization mode that can (IIUC) selectively apply the -O3 optimizations to hot functions -- effectively automating this process.
