TweetFollow Us on Twitter

May 94 Cornfield
Volume Number:10
Issue Number:5
Column Tag:From The Corn Field

Thoughts From The Cornfield

Provocative, perhaps inflammatory, but just say no to assembly language on PowerPC

By Steve Kiene, MindVision Software, Lincoln, Nebraska

About the author

Steve, author of things like Stacker for Macintosh, cares about performance, code size, performance, portability, and performance as much as anyone we know (well, there’s always Mike Scanlin, too). Steve’s recently worked through a number of issues about porting to the PowerPC for performance, and along the way surprised himself with his conclusions. He’s curious about your reaction, so please let us know if they surprise you, too. I can just see our assistant Al holding up a placard with this in big letters: editorial@xplain.com

Writing code in assembly language instead of a high-level language to get performance is fast becoming an historic anachronism. The fierce competition in the 90’s leads to time-to-market battles that cannot be won by the company that insists on writing large chunks of their product in assembly language.

I’ve seen plenty of code whose authors have spent an inordinate amount of time tweaking assembly language instructions to get the most speed out of the code when the real problem was a slow algorithm. It’s the old problem of not seeing the forest for the trees. Careful examination of the algorithm offers more potential for improved performance than coding a bad algorithm in tightly-tuned assembly language. This generally holds true even when the improved algorithm is coded in C.

I took some code a friend had written; he had spent weeks hand-tuning assembly code. I re-examined the algorithms and found a better way to do it. I coded it up in C and the new C code ran fifty times faster on the 68K than the assembly solution had before. Now, it not only performs, it’s portable and more maintainable. After simply recompiling the code for Power Macintosh, its speed doubled. To convert the assembly code would have taken at least a couple of weeks for someone proficient not only in writing PowerPC assembly code, but also good at scheduling the assembly instructions to keep the chip as busy as possible.

Now, with all that said, there are reasons for writing PowerPC assembly language. So, if you have to write part of your program in assembly, make sure it’s the right part and be completely sure you cannot increase the speed by improving the algorithm. There’s little sense in writing assembly language for code that only amounted to 4% of the execution time, but it’s not that hard to find programs that do just that. What was gained by writing the code in assembly language rather than a high-level language?

The release of the Power Macintosh machines has sent many 68K assembly language programmers scrambling to learn the new architecture and its assembly language so that they can continue to performance-program the Macintosh. However, as they are finding out, programming in PowerPC assembly language is much harder than on the 68K Macintosh.

I’ve seen several examples of PowerPC assembly language code that at first glance looks fast but after careful examination the code turns out to run slower than expected. RISCprocessors require understanding the architecture of both the CPU and the memory bus to get good performance, and it’s simply difficult to keep all of the rules and constraints in your head while trying to be creative and write code. Compilers, on the other hand, just don’t care how many rules they have to remember.

Reasons to avoid assembly language

(1) Assembly language code is not easily ported to different instruction set architectures. There are tools which will port 68K assembly to PowerPC assembly, but you run the risk that the architectures are so different a port doesn’t get you the full potential of the new architecture.

(2) Code can be written in a shorter amount of time in a high level language than it can be in assembly. People want to argue this, claiming that bit manipulation routines are too hard to do in C, but it’s just not true. I suspect that if they knew C as well as they knew assembler there would be little or no argument.

(3) It is far easier to make mistakes in assembly than it is in a high level language. High level languages offer abstraction and structure which makes many common assembly language problems simply non-existent.

(4) Code written in assembly is harder to maintain both for the original programmer as well as a different programmer. Because of the fine-grain control you get with assembly language, it is not always easy to follow the flow of the code.

(5) The development tools available for writing assembly language are not advancing at the same rate as those for high level languages. In fact, there are many situations where the tools are getting worse. Apple’s PowerPC Assembler for MPW is not nearly as sophisticated as their 68K Assembler.

Reasons to use assembly language

(1) Highly time-critical code, such as software which interfaces with a piece of hardware which has very specific timing dependencies. Not very common.

(2) Code where space is at a minimum, such as embedded controllers. Generally not applicable to the Macintosh.

(3) Code that is proven to be an unacceptable bottleneck in a specific task.

(4) Places where parameters are passed in specific locations that are not easily accessible to a high level language. [Between the PowerPCruntime architecture, and the protocol conversion that Mixed Mode does for you, this problem essentially goes away on the Power Macintosh - Ed stb]

In all of these instances, there is a need for assembly only in specific places in the code. There is no need to code large parts in assembly.

How to speed up your code - the old way

The most common way to speed up existing code is to find the parts of the program that are slow and rewrite them in assembly. In the past that may have been a good way to gain more speed. Today, that model is not only outdated, it can backfire. I’ve seen people rewrite their code in PowerPC assembly language only to see it run SLOWER. Do not assume you know more about the processor architecture than the compiler. Unless you understand the instruction scheduling of the processor entirely, you probably can’t out-do a good compiler.

How to speed up your code - the new way

Determine which parts of your program are used the most. If a particular feature takes several minutes to run but is only used once a month, maybe it’s not as important as features which takes ten seconds but are used every five minutes. Watch your customers’ usage patterns. Ask them which parts of the program are annoyingly slow. Ask them why they think those parts are slow. Remember, slowness is subjective. What is slow to a power user may seem perfectly fine to a novice user. Who uses your product, the novice or the power user?

Once you have identified the areas of your software that seem slow, you may want to back up the results with scientific data. Run performance analysis tools to see exactly where in the code things are slow when you perform the tasks that users said were slow. THINK C and CodeWarrior have performance monitoring tools included that work well. MPW has its own performance tools which are adequate. If you are writing code that is not easily interfaced to these tools, I recommend you look at the source code provided for the performance tools in THINK C. It is very easy to adapt this code to monitor the performance of any piece of code.

One thing to remember is that the performance of your software may differ greatly when comparing Power Macintosh to the 68K Macintosh. Performance may also vary quite a bit between specific Macintosh models. Machines with a 32 bit data bus will perform memory intensive operations much faster than machines with a 16 bit data bus.

Now that you have figured out which parts of your program are slow, it is time to decide how to make them faster. The first thing to do is to examine the underlying algorithms of the code. Is there anything fundamental that you can do to speed things up? For example, if you are performing a text search, how do you search through the text? Do you use Munger? Perhaps something like a Boyer-Moore algorithm would be much faster. Remember, the key is to work smarter. Brute force is not the answer - it’s a matter of brains over brawn.

Sometimes simply a small change to your existing algorithm will make things much faster. I sped up a search algorithm I wrote years ago by a factor of three by simply adding two lines of code. Look at your algorithm and examine how it operates with common data that goes through it. Perhaps certain shortcuts can be taken when the most common data runs through it.

If you don’t have many books on fundamental computer algorithms, now is the time to stock up. I am a firm believer that you cannot have enough books on algorithms. At the end of this article I have listed several books that will help broaden and round out your algorithm skills. I highly recommend all of them.

Once you have analyzed the specific parts of your program that are bottlenecks, it is time to look at the architecture of your program as a whole. If your program is rather large you may want to look at it as several modules working together.

Is the underlying architecture of your program going to be a bottleneck? Are there time consuming tasks that can be done in the background at idle time rather than being done while the user must wait? Are you doing network communication synchronously when you could do it asynchronously and give the user their machine back? Are there tasks that need to be performed but don’t need to give immediate feedback to the user? These kinds of tasks are good candidates for idle time processing, additional user feedback, modeless dialog boxes, asynchronous programming, and other methods of helping the user feel as if they are not waiting on your program, or at least aren’t prevented from doing something else while you get your thing done. If you keep the user occupied or help them feel productive while your program is working, they’ll be more patient with whatever performance you have.

How to write the code in Assembly Language

If, after careful examination, you have determined a bottleneck in your program, analyzed the algorithms as best you can, rewritten them to be as efficient as possible, and still it is not fast enough, perhaps it is time to code a small part in assembly. The best place to start is to disassemble compiler- generated code for the routine you want to code in assembly. Look at the code. What is inefficient about it? Are registers constantly being reloaded? Are the registers being used efficiently? Are the instructions scheduled for maximum pipelining? Very often you can take the disassembled code, make a few minor modifications to it and see a very nice speed increase.

Perform accurate timing tests on the code you are optimizing. Unless you completely understand the PowerPC Architecture Manual and the PowerPC 601 User’s Guide, more often than not you will make PowerPC code slower than a good compiler. The bottom line is that it must run faster, not look faster.

Maintain an exact high-level equivalent of the assembly code, and keep it right there in the same file. This way if you port your code to a different architecture, you’ve got what you need to get up and running quickly. In many cases the bottleneck on one machine will not be a bottleneck on another.

In Conclusion

This article has discussed some alternate methods of speeding up your program execution that are in many ways better than traditional methods used by many programmers. The goal is to maximize your gain and minimize your effort. By working smarter rather than harder, you can have a faster program in less time.

Recommended Books

[1] Thomas H. Cormen, Charles E. Leiserson, and Ronald L. Rivest. Introduction to Algorithms. MIT Press, 1990.

[2] Alfred V. Aho, John E. Hopcroft, and Jeffrey D. Ullman. The Design and Analysis of Computer Algorithms. Addison-Wesley, 1974.

[3] Saumyendra Sengupta and Paul Edwards. Data Structures in ANSI C. Academic Press, 1991.

[4] Donald Knuth. The Art of Computer Programming, Volumes 1-3. Addison-Wesley, 1973

[5] Daniel H. Greene and Donald E. Knuth. Mathematics for the Analysis of Algorithms., Third Edition Birkhäuser, 1990.

[6] P. D. Eastman. Go, Dog, Go! Random House, 1961.

[7] Manoochehr Azmoodeh. Abstract Data Types and Algorithms, Second Edition. Macmillan, 1990.

 
AAPL
$111.78
Apple Inc.
-0.87
MSFT
$47.66
Microsoft Corpora
+0.14
GOOG
$516.35
Google Inc.
+5.25

MacTech Search:
Community Search:

Software Updates via MacUpdate

calibre 2.13 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Mellel 3.3.7 - Powerful word processor w...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
ScreenFlow 5.0.1 - Create screen recordi...
Save 10% with the exclusive MacUpdate coupon code: AFMacUpdate10 Buy now! ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your... Read more
Simon 4.0 - Monitor changes and crashes...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
BBEdit 11.0.2 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
ExpanDrive 4.2.1 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Adobe After Effects CC 2014 13.2 - Creat...
After Effects CC 2014 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous After Effects customer). After Effects CS6 is still available... Read more
Evernote 6.0.5 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
Command-C 1.1.7 - Clipboard sharing tool...
Command-C is a revolutionary app which makes easy to share your clipboard between iOS and OS X using your local WiFi network, even if the app is not currently opened. Copy anything (text, pictures,... Read more
Tidy Up 4.0.2 - Find duplicate files and...
Tidy Up is a complete duplicate finder and disk-tidiness utility. With Tidy Up you can search for duplicate files and packages by the owner application, content, type, creator, extension, time... Read more

Latest Forum Discussions

See All

Make your own Tribez Figures (and More)...
Make your own Tribez Figures (and More) with Toyze Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
So Many Holiday iOS Sales Oh My Goodness...
The holiday season is in full-swing, which means a whole lot of iOS apps and games are going on sale. A bunch already have, in fact. Naturally this means we’re putting together a hand-picked list of the best discounts and sales we can find in order... | Read more »
It’s Bird vs. Bird in the New PvP Mode f...
It’s Bird vs. Bird in the New PvP Mode for Angry Birds Epic Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Telltale Games and Mojang Announce Minec...
Telltale Games and Mojang Announce Minecraft: Story Mode – A Telltale Games Series Posted by Jessica Fisher on December 19th, 2014 [ permalink ] | Read more »
WarChest and Splash Damage Annouce Their...
WarChest and Splash Damage Annouce Their New Game: Tempo Posted by Jessica Fisher on December 19th, 2014 [ permalink ] WarChest Ltd and Splash Damage Ltd are teaming up again to work | Read more »
BulkyPix Celebrates its 6th Anniversary...
BulkyPix Celebrates its 6th Anniversary with a Bunch of Free Games Posted by Jessica Fisher on December 19th, 2014 [ permalink ] BulkyPix has | Read more »
Indulge in Japanese cuisine in Cooking F...
Indulge in Japanese cuisine in Cooking Fever’s new sushi-themed update Posted by Simon Reed on December 19th, 2014 [ permalink ] Lithuanian developer Nordcurrent has yet again updated its restaurant simulat | Read more »
Badland Daydream Level Pack Arrives to C...
Badland Daydream Level Pack Arrives to Celebrate 20 Million Downloads Posted by Ellis Spice on December 19th, 2014 [ permalink ] | Read more »
Far Cry 4, Assassin’s Creed Unity, Desti...
Far Cry 4, Assassin’s Creed Unity, Destiny, and Beyond – AppSpy Takes a Look at AAA Companion Apps Posted by Rob Rich on December 19th, 2014 [ permalink ] These day | Read more »
A Bunch of Halfbrick Games Are Going Fre...
A Bunch of Halfbrick Games Are Going Free for the Holidays Posted by Ellis Spice on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »

Price Scanner via MacPrices.net

The Apple Store offering free next-day shippi...
The Apple Store is now offering free next-day shipping on all in stock items if ordered before 12/23/14 at 10:00am PT. Local store pickup is also available within an hour of ordering for any in stock... Read more
It’s 1992 Again At Sony Pictures, Except For...
Techcrunch’s John Biggs interviewed a Sony Pictures Entertainment (SPE) employee, who quite understandably wished to remain anonymous, regarding post-hack conditions in SPE’s L.A office, explaining “... Read more
Holiday sales this weekend: MacBook Pros for...
 B&H Photo has new MacBook Pros on sale for up to $300 off MSRP as part of their Holiday pricing. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1699... Read more
Holiday sales this weekend: MacBook Airs for...
B&H Photo has 2014 MacBook Airs on sale for up to $120 off MSRP, for a limited time, for the Thanksgiving/Christmas Holiday shopping season. Shipping is free, and B&H charges NY sales tax... Read more
Holiday sales this weekend: iMacs for up to $...
B&H Photo has 21″ and 27″ iMacs on sale for up to $200 off MSRP including free shipping plus NY sales tax only. B&H will also include a free copy of Parallels Desktop software: - 21″ 1.4GHz... Read more
Holiday sales this weekend: Mac minis availab...
B&H Photo has new 2014 Mac minis on sale for up to $80 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 1.4GHz Mac mini: $459 $40 off MSRP - 2.6GHz Mac mini: $629 $70 off MSRP... Read more
Holiday sales this weekend: Mac Pros for up t...
B&H Photo has Mac Pros on sale for up to $500 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2599, $400 off MSRP - 3.5GHz 6-core Mac Pro: $3499, $... Read more
Save up to $400 on MacBooks with Apple Certif...
The Apple Store has Apple Certified Refurbished 2014 MacBook Pros and MacBook Airs available for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Save up to $300 on Macs, $30 on iPads with Ap...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
iOS and Android OS Targeted by Man-in-the-Mid...
Cloud services security provider Akamai Technologies, Inc. has released, through the company’s Prolexic Security Engineering & Research Team (PLXsert), a new cybersecurity threat advisory. The... Read more

Jobs Board

*Apple* Store Leader Program (US) - Apple, I...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on experience, Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.