TweetFollow Us on Twitter

Optimizing for PPC
Volume Number:12
Issue Number:5
Column Tag:Book Review

The Need for Speed

Learn the nitty-gritty of PowerPC optimization

By Mike Scanlin

Optimizing PowerPC Code:
Programming the PowerPC Chip in Assembly Language

By Gary Kacmarcik

Addison-Wesley, 1995

ISBN 0-201-40839-2, 694 pages (softback). $39.95.

I’m disappointed. It’s just no challenge any more. It took me years of careful trial, error, repeated error, and determined study, to perfect my 680x0 optimizing skills to the point where I really understood the chip from a software point of view. I was looking forward to the same kind of challenge on the PowerPC (scrounging for obscure magazine articles, surfing the net looking for example code, writing and timing code three different ways, disassembling all the programs with good performance to see how they did it, etc.). But now that I’ve read this book, all the hard theory has been taken care of, and the only thing remaining is to do a few PowerPC assembly language projects and put the theory to the test. Mr. Kacmarcik has cut short my search for knowledge by writing a book which makes plain everything about the PowerPC chip, including the subtle pipeline and cache interactions that a true optimizer wants to know.

This book is intended for programmers with some high-level experience and at least a little experience with assembly language. It does not explain what hexadecimal means, for example, but it does define concepts like “latency” and “throughput”.

The first nine of the sixteen chapters review in precise detail the entire PowerPC instruction set and architecture. The purpose of these chapters is to broaden the audience for this book. Anyone with PowerPC experience could skim these 170 pages in an hour or so. For the rest, though, it is a reasonable starting point. Unfortunately, there are too few examples for the descriptions of the individual instructions to be meaningful. It’s like someone handing you a book on how to write poetry where the first hundred pages are a dictionary explaining all the words you can use in your poems but not really giving you the context or any examples to appreciate them. It’s hard to separate the really important stuff (like everyday instructions, registers and concepts) from the stuff that was just put in for the sake of completeness. An uninitiated person who tries to understand it all will probably become overwhelmed. I can accept that these chapters are meant to be an introduction and a bit of a reference (in addition to the complete references in the appendices), but it’s a little too much, too soon, in my opinion.

The next seven chapters, and especially Appendix D, are the reason to buy this book. They contain the info that is hard to find elsewhere. The chapter titles will give you a good idea of what you’ll find:

10. Memory and Caches

11. Pipelining

12. PowerPC 601 Instruction Timing

13. Programming Model [C calling conventions]

14. Introduction to Optimizing

15. Resource Scheduling

16. More Optimization Techniques

Appendix D. Optimization Summary

The cache discussion reviews how set-associative caches work. This is good info that you can apply to designing your own caches in higher-level languages like C. It is interesting to read that cache simulations have shown nearly identical cache hit rates for caches with random line-replacement algorithms and caches with least-recently-used line-replacement algorithms. There are tidbits of useful information sprinkled throughout this chapter, such as the sentence, “According to the PowerPC ISA, the programmer should assume that the processor has a split (instruction/data) cache, and that the processor will not automatically keep the instruction cache consistent with data written via the store instructions (that is, with the data cache).” Writers of self-modifying code, beware.

Even though the cache discussion is complete, it illustrates a problem that several of the chapters have: it’s missing down-to-earth examples. For instance, it says the 601 has “a unified 32K, eight-way set associative cache”, and explains what that means technically, but it doesn’t go on to tell me how far apart two addresses need to be before they map to the same cache line. If I’m working on an image-filtering application, it is really useful to know what sizes not to use for rowBytes (to avoid thrashing the data cache) if my algorithm visits all the pixels down a vertical column.

The instruction timing chapter was one of my favorites. Here’s an example of the kind of precision you can expect:

The Multiply Low Immediate (mulli) instruction always takes five cycles in IE. The length of time that the other multiply instructions spend in IE is dependent on the data contained in rB. If the upper 16 bits of rB are all sign bits, then the instruction spends five cycles in IE, otherwise it spends nine cycles. This means that the lesser (in magnitude) of the two arguments should be placed in rB because there is a potential savings of four cycles if -2^15 <= rB < (2^15 - 1).

All your favorite timing topics are handled here along with micro-examples to illustrate each stage of the pipeline for the entire sequence of instructions. Topics include: branch prediction (taken and not taken), cache hits and misses, pipeline synchronization, pipeline stalls, misaligned data accesses, and more. Here’s another example of the kind of details you’ll find. This is from the discussion of instruction fetching:

This may seem like a strange thing to affect timing, but the address affects where the data will be stored in the cache, and the cache timing is different when the request is from the upper or lower part of a cache line. If your timings always assume that you’ll receive four or eight instructions at a time, you may be surprised when the code is timed on a real system . For a critical loop, it might be worthwhile to place a few nops before the loop so that it fits nicely into a cache line.

The programming model chapter was good. I especially liked the explanation of how leaf routines that don’t need more than 220 bytes of stack space don’t need to allocate a stack frame (because, by convention, interrupt routines know not to use the 220 bytes above the current stack pointer - known as the “Red Zone” in Inside Macintosh). This chapter also discusses why you should not use the Load and Store Multiple instructions.

I must say I was disappointed that the chapter titled “Introduction To Optimizing” was only eight pages long. I was hoping that after plowing through 300 pages of details I would finally get to see 100 lines of before and after PowerPC assembly. But I didn’t. So I kept plowing ahead and on page 317 I found out that, as a rule of thumb, I should always place two independent instructions between two branches that are taken (jumps to subroutines, perhaps). As I got further and further into the book I would find a gem like this every 20 to 50 pages. I couldn’t help but think: “These are the really useful pieces of information; why can’t he just list everything like this and give lots of examples?” Then I found Appendix D.

Appendix D begins on page 677 and ends on page 678. But those are the two best pages in the whole book. If you want to apply the 90-10 rule to reading this book and you only have time to read two pages, then you better make it these two - they are the “rules of thumb” to follow when writing PowerPC assembly code. If you do these things right then a large portion of your optimizing job will be done.

This is a great book. I was frustrated that I had to read almost 700 pages before I found the summary of tricks that I was looking for. But there are lots of little bits sprinkled throughout, such as the table on page 347 that shows how to multiply something by 3 through 10 with no more than 3 integer shifts, adds and subtracts. Mechanically, the book is beautiful to read. It is nicely typeset with fonts, font sizes and diagrams well chosen.

My biggest complaint is that I want to see real-world code examples (i.e. more than five instruction sequences) in action. I’d like the author to provide some high-resolution timer code so that I can time my own code and know if I’ve made a difference (how about a performance workbench to experiment with?). And I’d like to see things like a C program calling some performance bottleneck written in assembly so I could get a bigger picture of how all this code fits together in a real program. Nevertheless, if you have any interest in writing fast PowerPC code, you should buy this book.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

How to become the ultimate robot warrior...
Chrono Strike is a delightfully immersive beat ‘em up with a sense of humor (any game with a good Sims reference gets points in my book). [Read more] | Read more »
Tips and tricks to get a higher score in...
Snow Roll is a devilish endless runner very much in the vein of Flappy Bird. It revels in its dastardly level of difficulty, and doesn’t really care how angry you get at it as it knows you’ll keep coming back for more. [Read more] | Read more »
How to win big in Slots Deluxe
Cheating while gambling is illegal and morally wrong, and in some parts of the world it leads to men with names like Vinnie "Six Knuckles" Manchenzo beating you to a pulp in a dark alley. [Read more] | Read more »
How to take over the world in Dictator 2
Running a country isn't easy - especially when you're a dictator who wants to take over the world and crush everyone in your path while you do it. [Read more] | Read more »
Tips and tricks to get a higher score in...
Tank.iois - you guessed it! - another multiplayer arena battler likeAgar.io and Slither.io. It does differentiate itself by putting you in a tiny tank though, so it's not exactly the same. To help you get that all-important high score, we've got a... | Read more »
How to unlock characters in One Tap Tenn...
As the title suggests, One Tap Tennis requires only a single tap to play its particular brand of tennis, and rewards you with a ton of unlockable characters if you perform well. Fortunately for you, we at 148Apps have got a few tips and tricks to... | Read more »
Grab it now: Game Craft’s Legend of War...
The real time strategy game is now available for you to sink your teeth into, through the App Store and Google Play. Combining elements of skill, strategy and empire building, Legend of War is a real gamers’ game. [Read more] | Read more »
Skateboard Party 3 ft. Greg Lutzka (Gam...
Skateboard Party 3 ft. Greg Lutzka 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Skateboard Party is back! This third edition of the popular sports franchise features professional skater... | Read more »
Cubious (Games)
Cubious 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Cubious – How smart are you? How high is your IQube? Solve the impossible puzzles to find out, and help a lost little cube find his... | Read more »
Goat Simulator Waste of Space (Games)
Goat Simulator Waste of Space 1.1 Device: iOS Universal Category: Games Price: $4.99, Version: 1.1 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICESiPhone 4S, iPad 2, iPod Touch 5 or better.** | Read more »

Price Scanner via MacPrices.net

Enterprise Workers Pick Technology Over Perks...
New Adobe study shows surprising attitudes about office jobs and where the future of work is heading. Adobe has released survey findings revealing that a surprising 70 percent of U.S. office workers... Read more
Memorial Day Weekend Sale: $50-$100 off 11-in...
B&H Photo has 13″ and 11″ MacBook Airs with 256GB SSDs on sale for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/256GB MacBook Air: $999 $100 off MSRP... Read more
Memorial Day Weekend Sales: Apple MacBook Pro...
B&H Photo has 13″ and 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz... Read more
Memorial Day Weekend Sales: Apple iMacs and M...
Take up to $150 off the price of a new iMac or Mac mini at the following Apple resellers this Memorial Day weekend: iMacs: B&H Photo has 21″ and 27″ iMacs on sale for up to $150 off MSRP... Read more
Apple refurbished Retina MacBook Pros availab...
Apple has Certified Refurbished 2015 13″ and 15″ Retina MacBook Pros available for up to $380 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free... Read more
Apple refurbished 11-inch MacBook Airs availa...
Apple has Certified Refurbished 11″ MacBook Airs (the latest models), available for up to $170 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is... Read more
Goal Zero and OtterBox Partner to Expand iPh...
Goal Zero, specialists in portable power, have announced a partnership with OtterBox, brand smartphone case protection, to offer the Slide and Slide Plus Batteries as modules compatible with the new... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $250 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799... Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more

Jobs Board

Senior *Apple* Engineer - Signature Technol...
One-year contract for an Apple consultant. The resource should be an Apple Certified Technical Coordinator or get the certification within 60 days of starting Read more
*Apple* Architect - AECOM (United States)
**Requisition/Vacancy No.** 132759BR **Position Title** Apple Architect **Job Category** Information Technology **Business Line** Government **Country** United Read more
*Apple* Solutions Consultant - APPLE (United...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
*Apple* Project Engineer - Smart Source Inc...
SmartSource is in need of an Apple Project Engineer for a 12 month contract opportunity in Pittsburg, PA. Role: Apple Project Engineer Location: Pittsburg, PA Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.