[pornin]

In older times (around the GCC 2.7 series, I think), "-O9" was documented to be the "future-proof" setting that selects the maximum optimization level, whatever it is. In practice, GCC never defined anything beyond -O3, but I still use -O9 when I want to make GCC do its best (or worst). Call it force of habit from a lengthening experience. (With Clang I use -O3 because if I write -O9 it screams at me.)

(Note that -O3 is usually not that good an idea: aggressive loop unrolling can sometimes decrease performance because of cache issues, for instance. To optimize code properly, you have to make benchmarks and adjust both the code and compiler flags accordingly, in a well-thought feedback cycle. The use of aggressive optimizations in the blog post is to make the examples trip UB more clearly.)