One neat thing about Common Lisp is that you can specify compilation directives in an extremely granular way: per file, per function, and even in a scope-delimi

One neat thing about Common Lisp is that you can specify compilation directives in an extremely granular way: per file, per function, and even in a scope-delimited fashion within a part of a function. You can do this with Lisp's DECLAIM, DECLARE, and LOCALLY DECLARE syntax respectively. Compiler directives are given as numerical levels of speed, safety, compilation speed, code size, and debuggability. There are other more specialized directives beyond those. If you're building a web app and the backend has 2 or 3 really hot code paths, you can do something like: (defun hot-path (request) (declare (optimize (speed 3) (safety 0) (debug 0))) ...) This roughly reads: "For ONLY the function HOT-PATH, compile it only with execution speed in mind. Eliminate extra runtime checks like array bounds checking or overflow, because I have meticulously checked and proved that to not be an issue. While you're at it, don't mind how debuggable it is at runtime; don't record anything about the call or it's internal workings, perform tail call elimination, etc." I'll usually abstract away the DECLARE with more semantically meaningful terms: OPTIMIZE-DANGEROUSLY-FAST, OPTIMIZE-FOR-SCRUTINY, etc. In general, in Lisp, I would say it's even considered bad hygiene to flip the global optimization switch; as djb and the article say, most of your large programs don't need it. You also often sacrifice debuggability in the entirety of your program if you ask the compiler to do what it can to make it fast.