WHAT ARE THE WAYS COMPILERS RECOGNIZE COMPLEX PATTERNS?

Summary from the TLDR newsletter

While there are a lot of techniques that could be used to apply to certain types of optimization, sometimes compilers will opt to look for a more-or-less hardcoded pattern. Compilers must consider many tradeoffs when choosing what optimizations to perform and how to perform them. Extremely sophisticated algorithms may be avoided because they can be expensive, too complex, or not broadly applicable. Hardcoded optimizations are fairly cheap and the maintenance burden is very low.