abrash-black-book/chapter-06.md

title	author	date	identifier	publisher	category	chapter	pages
Michael Abrash's Graphics Programming Black Book, Special Edition	Michael Abrash	1997-07-01	scheme text ISBN 1576101746	The Coriolis Group	Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development	06	125-133

Chapter 6 -- Looking Past Face Value

How Machine Instructions May Do More Than You Think

I first met Jeff Duntemann at an authors' dinner hosted by PC Tech Journal at Fall Comdex, back in 1985. Jeff was already reasonably well-known as a computer editor and writer, although not as famous as Complete Turbo Pascal, editions 1 through 672 (or thereabouts), TURBO TECHNIX, and PC TECHNIQUES would soon make him. I was fortunate enough to be seated next to Jeff at the dinner table, and, not surprisingly, our often animated conversation revolved around computers, computer writing, and more computers (not necessarily in that order).

Although I was making a living at computer work and enjoying it at the time, I nonetheless harbored vague ambitions of being a science-fiction writer when I grew up. (I have since realized that this hardly puts me in elite company, especially in the computer world, where it seems that every other person has told me they plan to write science fiction "someday." Given that probably fewer than 500—I'm guessing here—original science fiction and fantasy short stories, and perhaps a few more novels than that, are published each year in this country, I see a few mid-life crises coming.)

At any rate, I had accumulated a small collection of rejection slips, and fancied myself something of an old hand in the field. At the end of the dinner, as the other writers complained half-seriously about how little they were paid for writing for Tech Journal, I leaned over to Jeff and whispered, "You know, the pay isn't so bad here. You should see what they pay for science fiction—even to the guys who win awards!"

To which Jeff replied, "I know. I've been nominated for two Hugos."

Oh.

Had I known I was seated next to a real, live science-fiction writer—an award-nominated writer, by God!—I would have pumped him for all I was worth, but the possibility had never occurred to me. I was at a dinner put on by a computer magazine, seated next to an editor who had just finished a book about Turbo Pascal, and, gosh, it was obvious that the appropriate topic was computers.

For once, the moral is not "don't judge a book by its cover." Jeff is in fact what he appeared to be at face value: a computer writer and editor. However, he is more, too; face value wasn't full value. You'll similarly find that face value isn't always full value in computer programming, and especially so when working in assembly language, where many instructions have talents above and beyond their obvious abilities.

On the other hand, there are also a number of instructions, such as LOOP, that are designed to perform specific functions but aren't always the best instructions for those functions. So don't judge a book by its cover, either.

Assembly language for the x86 family isn't like any other language (for which we should, without hesitation, offer our profuse thanks). Assembly language reflects the design of the processor rather than the way we think, so it's full of multiple instructions that perform similar functions, instructions with odd and often confusing side effects, and endless ways to string together different instructions to do much the same things, often with seemingly minuscule differences that can turn out to be surprisingly important.

To produce the best code, you must decide precisely what you need to accomplish, then put together the sequence of instructions that accomplishes that end most efficiently, regardless of what the instructions are usually used for. That's why optimization for the PC is an art, and it's why the best assembly language for the x86 family will almost always handily outperform compiled code. With that in mind, let's look past face value—and while we're at it, I'll toss in a few examples of not judging a book by its cover.

The point to all this: You must come to regard the x86 family instructions for what they do, not what you're used to thinking they do. Yes, SHL shifts a pattern left—but a look-up table can do the same thing, and can often do it faster. ADD can indeed add two operands, but it can't put the result in a third register; LEA can. The instruction set is your raw material for writing high-performance code. By limiting yourself to thinking only in certain well-established ways about the various instructions, you're putting yourself at a substantial disadvantage every time you sit down to program.

In short, the x86 family can do much more than you think—if you'll use everything it has to offer. Give it a shot!

Memory Addressing and Arithmetic

Years ago, I saw a clip on the David Letterman show in which Letterman walked into a store by the name of "Just Lamps" and asked, "So what do you sell here?"

"Lamps," he was told. "Just lamps. Can't you read?"

"Lamps," he said. "I see. And what else?"

From that bit of sublime idiocy we can learn much about divining the full value of an instruction. To wit:

Quick, what do the x86's memory addressing modes do?

"Calculate memory addresses," you no doubt replied. And you're right, of course. But what else do they do?

They perform arithmetic, that's what they do, and that's a distinctly different and often useful perspective on memory address calculations.

For example, suppose you have an array base address in BX and an index into the array in SI. You could add the two registers together to address memory, like this:

add  bx,si
mov  al,[bx]

Or you could let the processor do the arithmetic for you in a single instruction:

mov  al,[bx+si]

The two approaches are functionally interchangeable but not equivalent from a performance standpoint, and which is better depends on the particular context. If it's a one-shot memory access, it's best to let the processor perform the addition; it's generally faster at doing this than a separate ADD instruction would be. If it's a memory access within a loop, however, it's advantageous on the 8088 CPU to perform the addition outside the loop, if possible, reducing effective address calculation time inside the loop, as in the following:

      add   bx,si
LoopTop:
      mov   al,[bx]
      inc   bx
      loop  LoopTop

Here, MOV AL,[BX] is two cycles faster than MOV AL,[BX+SI].

On a 286 or 386, however, the balance shifts. MOV AL,[BX+SI] takes no longer than MOV AL,[BX] on these processors because effective address calculations generally take no extra time at all. (According to the MASM manual, one extra clock is required if three memory addressing components, as in MOV AL,[BX+SI+1], are used. I have not been able to confirm this from Intel publications, but then I haven't looked all that hard.) If you're optimizing for the 286 or 386, then, you can take advantage of the processor's ability to perform arithmetic as part of memory address calculations without taking a performance hit.

The 486 is an odd case, in which the use of an index register or the use of a base register that's the destination of the previous instruction may slow things down, so it is generally but not always better to perform the addition outside the loop on the 486. All memory addressing calculations are free on the Pentium, however. I'll discuss 486 performance issues in Chapters 12 and 13, and the Pentium in Chapters 19 through 21.

Math via Memory Addressing

You're probably not particularly wowed to hear that you can use addressing modes to perform memory addressing arithmetic that would otherwise have to be performed with separate arithmetic instructions. You may, however, be a tad more interested to hear that you can also use addressing modes to perform arithmetic that has nothing to do with memory addressing, and with a couple of advantages over arithmetic instructions, at that.

How?

With LEA, the only instruction that performs memory addressing calculations but doesn't actually address memory. LEA accepts a standard memory addressing operand, but does nothing more than store the calculated memory offset in the specified register, which may be any general-purpose register. The operation of LEA is illustrated in Figure 6.1, which also shows the operation of register-to-register ADD, for comparis on.

What does that give us? Two things that ADD doesn't provide: the ability to perform addition with either two or three operands, and the ability to store the result in any register, not just in one of the source operands.

Imagine that we want to add BX to DI, add two to the result, and store the result in AX. The obvious solution is this:

mov  ax,bx
add  ax,di
add  ax,2

(It would be more compact to increment AX twice than to add two to it, and would probably be faster on an 8088, but that's not what we're after at the moment.) An elegant alternative solution is simply:

lea  ax,[bx+di+2]

Likewise, either of the following would copy SI plus two to DI

mov  di,si
add  di,2

or:

lea  di,[si+2]

Mind you, the only components LEA can add are BX or BP, SI or DI, and a constant displacement, so it's not going to replace ADD most of the time. Also, LEA is considerably slower than ADD on an 8088, although it is just as fast as ADD on a 286 or 386 when fewer than three memory addressing components are used. LEA is 1 cycle slower than ADD on a 486 if the sum of two registers is used to point to memory, but no slower than ADD on a Pentium. On both a 486 and Pentium, LEA can also be slowed down by addressing interlocks.

The Wonders of LEA on the 386

LEA really comes into its own as a "super-ADD" instruction on the 386, 486, and Pentium, where it can take advantage of the enhanced memory addressing modes of those processors. (The 486 and Pentium offer the same modes as the 386, so I'll refer only to the 386 from now on.) The 386 can do two very interesting things: It can use any 32-bit register (EAX, EBX, and so on) as the memory addressing base register and/or the memory addressing index register, and it can multiply any 32-bit register used as an index by two, four, or eight in the process of calculating a memory address, as shown in Figure 6.2. Let's see what that's good for.

Well, the obvious advantage is that any two 32-bit registers, or any 32-bit register and any constant, or any two 32-bit registers and any constant, can be added together, with the result stored in any register. This makes the 32-bit LEA much more generally useful than the standard 16-bit LEA in the role of an ADD with an independent destination.

But what else can LEA do on a 386, besides add?

It can multiply any register used as an index. LEA can multiply only by the power-of-two values 2, 4, or 8, but that's useful more often than you might imagine, especially when dealing with pointers into tables. Besides, multiplying by 2, 4, or 8 amounts to a left shift of 1, 2, or 3 bits, so we can now add up to two 32-bit registers and a constant, and shift (or multiply) one of the registers to some extent—all with a single instruction. For example,

lea  edi,TableBase[ecx+edx*4]

replaces all this

mov  edi,edx
shl  edi,2
add  edi,ecx
add  edi,offset TableBase

when pointing to an entry in a doubly indexed table.

Multiplication with LEA Using Non-Powers of Two

Are you impressed yet with all that LEA can do on the 386? Believe it or not, one more feature still awaits us. LEA can actually perform a fast multiply of a 32-bit register by some values other than powers of two. You see, the same 32-bit register can be both base and index on the 386, and can be scaled as the index while being used unchanged as the base. That means that you can, for example, multiply EBX by 5 with:

lea ebx,[ebx+ebx*4]

Without LEA and scaling, multiplication of EBX by 5 would require either a relatively slow MUL, along with a set-up instruction or two, or three separate instructions along the lines of the following

mov  edx,ebx
shl  ebx,2
add  ebx,edx

and would in either case require the destruction of the contents of another register.

Multiplying a 32-bit value by a non-power-of-two multiplier in just 2 cycles is a pretty neat trick, even though it works only on a 386 or 486.

The full list of values that LEA can multiply a register by on a 386 or 486 is: 2, 3, 4, 5, 8, and 9. That list doesn't include every multiplier you might want, but it covers some commonly used ones, and the performance is hard to beat.

I'd like to extend my thanks to Duane Strong of Metagraphics for his help in brainstorming uses for the 386 version of LEA and for pointing out the complications of 486 instruction timings.