Compilers are often critical components in various development toolchains that boosts developer productivity. A compiler is normally used as a monolithic black box that consumes a high-level source program and produces a semantically-equivalent low-level one. It is still structured inside though; what flows between internal layers are called intermediate representations (IRs). IRs are critical to compilers. Like there are many compilers, there are also many IRs in use. I’m fortunate to have direct experience with three major schools of IRs or infrastructures thus far—LLVM IR, SPIR-V, MLIR, particularly extensively for the last two, where I both joined development in an early stage. So I’d like to write a series of blog posts to log down my understanding of compilers and IRs. Hopefully it could be beneficial to others.