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### The Flexible Mind

Is the never-ending collection of information all there is to the assembly optimization, then? Hardly. Knowledge is simply a necessary base on which to build. Let’s take a moment to examine the objectives of good assembly programming, and the remainder of the forces that act on assembly optimization will fall into place.

Basically, there are only two possible objectives to high-performance assembly programming: Given the requirements of the application, keep to a minimum either the number of processor cycles the program takes to run, or the number of bytes in the program, or some combination of both. We’ll look at ways to achieve both objectives, but we’ll more often be concerned with saving cycles than saving bytes, for the PC generally offers relatively more memory than it does processing horsepower. In fact, we’ll find that two-to-three times performance improvements over already tight assembly code are often possible if we’re willing to spend additional bytes in order to save cycles. It’s not always desirable to use such techniques to speed up code, due to the heavy memory requirements—but it is almost always possible.

You will notice that my short list of objectives for high-performance assembly programming does not include traditional objectives such as easy maintenance and speed of development. Those are indeed important considerations—to persons and companies that develop and distribute software. People who actually buy software, on the other hand, care only about how well that software performs, not how it was developed nor how it is maintained. These days, developers spend so much time focusing on such admittedly important issues as code maintainability and reusability, source code control, choice of development environment, and the like that they often forget rule #1: From the user’s perspective, performance is fundamental.

| Comment your code, design it carefully, and write non-time-critical portions in a high-level language, if you wish—but when you write the portions that interact with the user and/or affect response time, performance must be your paramount objective, and assembly is the path to that goal.

Knowledge of the sort described earlier is absolutely essential to fulfilling either of the objectives of assembly programming. What that knowledge doesn’t do by itself is meet the need to write code that both performs to the requirements of the application at hand and also operates as efficiently as possible in the PC environment. Knowledge makes that possible, but your programming instincts make it happen. And it is that intuitive, on-the-fly integration of a program specification and a sea of facts about the PC that is the heart of the Zen-class assembly optimization.

As with Zen of any sort, mastering that Zen of assembly language is more a matter of learning than of being taught. You will have to find your own path of learning, although I will start you on your way with this book. The subtle facts and examples I provide will help you gain the necessary experience, but you must continue the journey on your own. Each program you create will expand your programming horizons and increase the options available to you in meeting the next challenge. The ability of your mind to find surprising new and better ways to craft superior code from a concept—the flexible mind, if you will—is the linchpin of good assembler code, and you will develop this skill only by doing.

Never underestimate the importance of the flexible mind. Good assembly code is better than good compiled code. Many people would have you believe otherwise, but they’re wrong. That doesn’t mean that high-level languages are useless; far from it. High-level languages are the best choice for the majority of programmers, and for the bulk of the code of most applications. When the best code—the fastest or smallest code possible—is needed, though, assembly is the only way to go.

Simple logic dictates that no compiler can know as much about what a piece of code needs to do or adapt as well to those needs as the person who wrote the code. Given that superior information and adaptability, an assembly language programmer can generate better code than a compiler, all the more so given that compilers are constrained by the limitations of high-level languages and by the process of transformation from high-level to machine language. Consequently, carefully optimized assembly is not just the language of choice but the only choice for the 1percent to 10 percent of code—usually consisting of small, well-defined subroutines—that determines overall program performance, and it is the only choice for code that must be as compact as possible, as well. In the run-of-the-mill, non-time-critical portions of your programs, it makes no sense to waste time and effort on writing optimized assembly code—concentrate your efforts on loops and the like instead; but in those areas where you need the finest code quality, accept no substitutes.

Note that I said that an assembly programmer can generate better code than a compiler, not will generate better code. While it is true that good assembly code is better than good compiled code, it is also true that bad assembly code is often much worse than bad compiled code; since the assembly programmer has so much control over the program, he or she has virtually unlimited opportunities to waste cycles and bytes. The sword cuts both ways, and good assembly code requires more, not less, forethought and planning than good code written in a high-level language.

The gist of all this is simply that good assembly programming is done in the context of a solid overall framework unique to each program, and the flexible mind is the key to creating that framework and holding it together.

#### Where to Begin?

To summarize, the skill of assembly language optimization is a combination of knowledge, perspective, and a way of thought that makes possible the genesis of absolutely the fastest or the smallest code. With that in mind, what should the first step be? Development of the flexible mind is an obvious step. Still, the flexible mind is no better than the knowledge at its disposal. The first step in the journey toward mastering optimization at that exalted level, then, would seem to be learning how to learn.

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Graphics Programming Black Book © 2001 Michael Abrash