WRITE GREAT CODE,: Writing High-Level

There are many aspects of great code—far too many to describe properly in a single book. Therefore, this second volume of the Write Great Code series concentrates on one important part of great code: performance. As computer systems have increased in performance from MHz, to hundreds of MHz, to GHz, the performance of computer software has taken a back seat to other concerns. Today, it is not at all uncommon for software engineers to exclaim, “You should never optimize your code!” Funny, you don’t hear too many computer application users making such statements. Although this book describes how to write efficient code, it is not a book about optimization. Optimization is a phase near the end of the software development cycle in which software engineers determine why their code does not meet performance specifications and then massage the code to achieve those specifications. But unfortunately, if no thought is put into the performance of the application until the optimization phase, it’s unlikely that optimization will prove practical. The time to ensure that an application xviii Introduction has reasonable performance characteristics is at the beginning, during the design and implementation phases. Optimization can fine-tune the performance of a system, but it can rarely deliver a miracle. Although the quote is often attributed to Donald Knuth, who popularized it, it was Tony Hoare who originally said, “Premature optimization is the root of all evil.” This statement has long been the rallying cry of software engineers who avoid any thought of application performance until the very end of the software-development cycle—at which point the optimization phase is typically ignored for economic or time-to-market reasons. However, Hoare did not say, “Concern about application performance during the early stages of an application’s development is the root of all evil.” He specifically said premature optimization, which, back then, meant counting cycles and instructions in assembly language code—not the type of coding you want to do during initial program design, when the code base is rather fluid. So, Hoare’s comments were on the mark. The following excerpt from a short essay by Charles Cook (www.cookcomputing.com/blog/archives/ 000084.html) describes the problem with reading too much into this statement: I’ve always thought this quote has all too often led software designers into serious mistakes because it has been applied to a different problem domain to what was intended. The full version of the quote is “We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.” and I agree with this. It’s usually not worth spending a lot of time micro-optimizing code before it’s obvious where the performance bottlenecks are. But, conversely, when designing software at a system level, performance issues should always be considered from the beginning. A good software developer will do this automatically, having developed a feel for where performance issues will cause problems. An inexperienced developer will not bother, misguidedly believing that a bit of fine tuning at a later stage will fix any problems. Hoare was really saying that software engineers should worry about other issues, like good algorithm design and good implementations of those algorithms, before they worry about traditional optimizations, like how many CPU cycles a particular statement requires for execution. Although you could certainly apply many of this book’s concepts during an optimization phase, most of the techniques here really need to be done during initial coding. If you put them off until you reach “code complete,” it’s unlikely they will ever find their way into your software. It’s just too much work to implement these ideas after the fact.