TweetFollow Us on Twitter

Jul 94 Dialogue Box
Volume Number:10
Issue Number:7
Column Tag:Dialogue Box

Dialogue Box

By Scott T Boyd, Editor

In The Cornfield == In The Weeds?

You raised many fine points about the need (or lack thereof) to write assembly language for Power PC. I would have appreciated more detail to illustrate the points you made. For instance, I would like to know what the two lines of code that sped up the search algorithm by a factor of three did, in general. Also, a simple example of Power PC code you saw rewritten by someone else where the rewrite ran slower. Or, an example where a Power PC rewrite looks faster but did not run faster. Some of your points raised big question marks for me.

The article as a whole appeared directed toward the C programmer, but very little of how to write C that is compiled to good Power PC code was addressed.

A cursory examination of the PowerPC instruction set shows that it is not a C machine; there are instructions that do not match well to C, as well as C constructs that do not map well to PowerPC instructions. For instance, the expression: unsigned char c = f; where f is a float compiles to over 20 instructions on an RS6000 using the xlc compiler with the O3 optimization (the highest level), regardless of the rounding convention chosen. This is in part because there are no instructions that move data between the general purpose registers and the floating point registers, and in part because both the unsigned char data type and the float data type, while supported on the Power PC in theory, are not natural data types; at least the compiler does not think so.

Another example true for both 68K and Power PC is that while the processors make overflow detection easy, the C language does not provide any natural method to write code that detects arithmetic overflow in integer arithmetic. Thus, while 68K and Power PC both have instructions to support multiple word precision arithmetic easily, writing it portably is not so easy (although it is possible; QuickDraw GX supplied PowerPC with C implementations of 64-bit-wide numbers thanks to the magic of Apple engineer Rob Johnson).

You mention that bit manipulation is too hard to do in C, etc., claiming that if the arguer knew C well, there would be little or no argument. Not always so; for instance, finding the first set bit in a register is a single instruction for both PowerPC and 68K, but I think you’ll have a hard time generating a C construct to generate that instruction. Similarly, I challenge you to generate C constructs for the 68K instructions BFINS or BFEXT, or the more complicated cases of the PowerPC instruction rlwimi; for instance, some compilers will not generate a simple bit rotate no matter how clever your C is (MPW C will, but only because of requests on my part).

Sometimes the best way around these sorts of problems is to use a standard C library, since the compiler author is likely to make sure that at least the standard library calls generate the correct instructions. For instance, the fabs() function will generate the corresponding instruction on the xlc compiler. But, I tried to get the compiler to generate the fnabs instruction (negative absolute value of a double) using various permutations of

-(a > 0 ? a : -a) 

by moving the negation inside the expression and reversing the conditional, and not only did it never generate the correct instruction, each time it generated a different set of instructions for each of the identical permutations!

To suggest that the compiler is always smarter than the programmer is a bit naive. While I do not doubt that this can be the case, I suggest that a thorough understanding of the instruction set and the experimental knowledge gained from attempting to write C constructs to generate those instructions can go a long way towards high end program tuning. For instance, I had no trouble getting both MetroWerks and xlc to generate a single instruction out of the line: d = -(d * d + d); where d is a double. At first, reading your article seemed to support the argument that I hear that “all engineers should write C as well as they can without regard to the assembly that is produced, because the compiler will always be smarter than they are, and because they want to be portable, etc”. It is fine to drive a car without understanding how the engine works, but a little less savory to drive one knowing that the designer did not understand the same. In order to become expert at computer programming, you have to understand how computers work, including dirty assembly. It is difficult to gain that understanding without ever having written a line of it.

I am not suggesting writing the next version of MacWorks in assembly; far from it. I just finished working on QuickDraw GX, and the native PowerPC version has nearly no assembly at all; the 68K version has maybe 1%, and all assembly has portable C equivalents. But as I consider graphics algorithms for the next version, I immediately consider what assembly best implements the algorithms, and how that defines the high level representation for those algorithms. I rely on my experience writing very large projects completely in assembly, and expect those working with me to have a similar depth of knowledge.

Further on in the article, I get the feeling that you do expect the reader to understand assembly. But rather than heading the paragraph “How to Write the Code in Assembly Language”, I suggest “How to Verify the Algorithm Written in a High Level Language.” Disassemble it. Understand what you are asking the computer to do. In the PowerPC case, important concepts include leaf node routines, how floats and integers work, register conventions, C switch statements (not cheap because of the architecture), to mention a few.

Finally, I disagree that PowerPC assembly language is more difficult than 68K. I suggest it is just different. 68K code can slow down or speed up by a factor of 2 depending on how it is loaded in the cache, and another factor of 2 if the data is misaligned; you won’t detect either easily by looking at a code listing. While the scheduling and pipe-lining issues in PowerPC complicate writing good assembly, there are not a lot of rules to learn; the simplified memory model and uniformity of instruction latency makes it actually much easier than the cycle counting I used to do for 68K.

Well, I thought I was done, but I disagree with one more thing: your assessment that THINK C has adequate performance tools. By this I assume that you mean the profiler option. First, it is not easy to adopt this to monitor the performance of any piece of code (as you suggest) since the compiler must generate special callouts in the body of the code for the profiler to work at all. Secondly, it works unmodified using Ticks, not the Microsecond timer; coarse by any measure. Third, when adapted to use the Microsecond timer, the software must be calibrated to eliminate the time used by the profiler code itself. Without doing this, the software can’t tell the difference between one function callout and two. Fourth, even with the corrections, I have found that I need to either remove all network connections or turn off interrupts on my IIfx (slow enough for microseconds to be useful) to get accurate timings. And you can’t just jam a register on a PowerPC to turn off interrupts. Lastly, I have to rewrite the printouts to get useful info; the THINK standard ones just aren’t very good. Not a big deal, but work nonetheless.

I hope I wasn’t being too big of an pain in the backside with everything written above; I liked the article. And, if I am wrong about any of the points in my rebuttal, please do not hesitate to correct my shortcomings.

- Cary Clark, Apple Computer, Inc.

More 601 Assembly Feedback

I would like to comment on the prevailing “propaganda” regarding assembly language and the PowerPC. Every time I hear it, I feel that my intelligence is being insulted.

It has been expressed (primarily by Apple, but also by Metrowerks) that trying to use assembly language on the PowerPC is a bad idea. They give a number of reasons including the difficulty of porting, the difficulty of optimizing, the need to adapt to different PowerPC implementations and the quality of existing compilers. I am told that I couldn’t match the speed of compiled C/C++ code even if I tried.

In fact I agree with their reasons and have often generated optimal code simply by breaking long expressions into many pieces, using extra variables to hold intermediate values and reordering the statements to schedule well on the PowerPC.

However, there is a important point that everyone seems to be missing! The PowerPC architecture defines an instruction set that is significantly larger than what the compilers actually use. For example, compile:

x = (x << 31) + (x >> 1)// Rotate right one bit

and you will probably get at least three instructions, when one (a right longword rotate by one bit) would suffice on both the 680x0 and the PowerPC. No compiler I have ever seen (and I’ve seen quite a few) generates rotate instructions.

There are numerous other examples, including absolute value, multiply long high word, add/subtract with extend, and count leading zeros. All are useful in specialized compute-intensive operations. In addition, certain “no-op” instructions are necessary on the PowerPC 601 to keep the pipeline running at full speed.

Of course, none of them correspond to built-in operations of the C, C++, or Pascal language -- and that’s the real reason we need assembler. In fact, it’s one of the reasons that inline assembler is part of the developing ANSI C++ specification.

I would appreciate it if a more generous attitude were taken towards assembly language in the future.

- Robert P. Munafo, Malden, MA

Sometimes our mailbox gets to be a bit interactive. In a followup letter, Robert added

Thank you for your reply. I feel a lot better after hearing what you had to say about the use of assembler at Apple. [Ibasically said that most everyone was writing system software in C, with a handful of exceptions, like Mixed Mode, and the emulator - Ed stb]

I was trying not to respond exclusively to Steve’s article (“Thoughts from the Cornfield”, MacTech vol. 10, No. 5) but to each of the other times Assembler has been discouraged and the general “C++ is now sufficient for everything” propaganda. For example:

• Same issue, p. 68, column 1, last ¶

• Vol. 10, No. 3 Page 82 column 1, ¶ 3

• The place in the CodeWarrior manual where they explain why inline assembler for PowerPC is not yet supported.

I do appreciate the section on “How to write the code in assembly language” inasmuch as it points out the advantages of using the compiler’s output as a first step, and benchmarking your efforts to see if they really speed things up, etc.

I think that even if you are not going to write any assembler, you still need to be rather familiar with the operation of the PowerPC chip in order to optimize your C or C++ code.

It would be very interesting indeed if compilers could be improved a bit as a result of this discussion! Even GCC and G++, (the GNU compilers) which are widely regarded as pretty much the best thing going, do not generate rotate instructions. However, it is quite easy to generate rotate instructions if you have already implemented a peephole optimizer.

- Robert Munafo

You Can Beat Mpw’s Code Generator

You must have been up late for you to have thought “people like Symantec’s quick turnaround tools and MPW’s code generation.” It’s OK, ’cuz we know it was probably a simple case of getting the object files swapped - and, alas, it was very late. <grin> I think MPW’s code generation is brain-dead; Think’s code is significantly more intelligent. Is it not smaller, too?

Thanks for mentioning your visit to the Software Developer’s Conference. Going behind enemy lines is good for our side. Maybe as a result you will broaden the horizons of Mac tool developers; causing the number and the quality of Mac development tools to grow. Miracles are possible...

- dk smith, mtn. view, ca

I must have been asleep when the balance changed. Besides, I never said that MPW’s code generation wasn’t brain-dead - Ed stb

But You Can’t Beat A Good Algorithm

Your article on performance and the misconceptions about the use of assembly (Thoughts from the Cornfield, May 1994) was excellent. The importance of a good algorithm cannot be overlooked. The thought of thousands of lines of 68K code being ported to run native on the PowerPC architecture is scary - there's a lot of Toolbox/OS code in this category. Let’s hope those porting 68K code to C don’t blindly port the assembly language program's structure, interfaces, and algorithms. This could waste the PowerPC’s speed and nullify our hardware performance advantages over Pentium.

- dk smith, mtn. view, ca

Free the SDKs

This is an open letter to decision makers at Apple in which I request that the policy of charging extra for crucial SDKs be discontinued.

Why are SDKs important? Software Development Kits (SDKs) contain critical information that enables developers to support specific components of the Macintosh OS and User Interface.

As a small developer, Ifind it difficult enough to budget for necessities like E.T.O., the Developer Program, and WWDC, but the added pain of buying an SDK for every feature Iwant to add to my application is burdensome in terms both of time and money.

Apple is fond of comparing itself to Microsoft. So let’s make a rough comparison of the yearly cost of the Apple and Microsoft developer programs, and see how Apple’s policy of charging for SDKs dramatically alters the cost equation.

The comparisons assume I want:

1. Access to all the SDKs I’ll need for a platform.

2. Programming tools to use the SDKs.

3. System software to test with.

4. Programmer’s documentation.

APPLE

Associates Program (& dev. CDs) $350 Yearly

E.T.O (development tools) $400 Yearly

(initial $1295) ---

$750 Base

Add a few SDKs (a sampling for reference only):

QuickTime SDK $195

Easy Open SDK $150

AppleScript SDK $199

---

“Some SDKs” $544

---

Add it all up to get $1294 Total

MICROSOFT

Developer Network Level 2 $495 yearly

Visual C++ $99 update

initial $599 ---

$594 Total

I’ll point out areas in which the above comparison favors Apple:

1. As SDKs are updated, Apple charges update fees in the ~$100 range per SDK (reference AppleScript SDK and QuickTime SDK). So though I’ve listed Apple’s initial SDK charge, there’s a recurring cost component as well. Microsoft ships updates to SDKs on each quarterly Developer Network CD. Apple forces developers to purchase these updates (once they realize something is not up-to-date).

2. We’ll also ignore the fact that Microsoft ships the Windows programming documentation with Visual C++. Apple developers need to spend untold extra $$$ hundreds $$$ for $Inside $Mac.

3. The items bought above from Apple do not guarantee access to all SDKs and all copies of system software that we need from Apple. The list above contains only a few. I’m omitting some little things...like AOCE :-). The items from Microsoft include ALL WINDOWS SDKs and OS versions. Including NT. And soon Chicago. It’s coming...

So what am I bitching about, and why?

My major gripe is that Apple’s policy of charging for SDKs adds substantial cost and time overhead to Macintosh development that hinders support of new features.

Not only do I need to spend the money to purchase the additional SDKs, but I need to take the time to order from APDA and wait for the material before I can start to implement new features. This extra pain makes it less likely that I (or others) will add support for new system SW features.

What’s the point, Apple?

Do you want us to support new System 7.x features like scripting, QuickTime, Easy Open?

Are you trying to fund development by charging developers for SDKs?

Wouldn’t you rather do everything you can to encourage developers to continue to support the Mac and to add Macintosh-specific features to cross-platform programs in order to maintain the shrinking differentiation between the platforms so that you can maintain or improve market share?

I’ve heard folks from Apple argue that the charge for SDKs is simply to cover the cost of their delivery. Surely you’re joking, Mr. Spindler!!! Apple claimed that the E.T.O. (and/or, at one time or another, The Developer CD Series) was the one-stop source for developer tools and information for Macintosh development. Okay, we paid for it. But where are those SDKs? Surely this is the place?

The recent re-shuffle of the Developer CD series into Tools, Systems Software, and Reference Library was to make more room on the CDs. So where are those SDKs? Surely you can fit on the Easy Open SDK ($150 for a floppy and 100 pages of docs - isn’t that just pure greed?). And surely there’s room for AppleScript docs, headers, and samples?

Go ahead, make my day - Here’s what I want:

Put all SDKs, System versions, System Extensions, and DocViewer copies of associated documentation on the Developer CD Series. If I want paper documentation, I’ll pay extra. Or print it.

So why should Icare?

I’ve supported Apple for a long time. I’ve been writing commercial Macintosh Software since 1984. In 1984 Apple did everything it could to encourage development on the Mac. 1994 is like 1984 in that this is a hard sell. 1994 is not like 1984 in that Apple is doing less to support developers. Let’s make 1994 more like 1984. I want the Mac to succeed, and to trample Windows. But on a recent project (Houdini) I took a trip to the dark side. And discovered some truths: Microsoft does far better than Apple in providing access to its system technologies.. Go ahead, make my day. Make it easier for me to make the Macintosh shine.

- James Berry, jberry@teleport.com

Apple Responds!

As Ike Nassi, Apple’s Vice President of Development Products, said at the WWDC in May, Apple is committed to improving the way in which we currently distribute SDKs to developers. In fact, we are - right now - finalizing a new plan that will allow us to deliver a complete set of system software SDKs to developers on a regular basis for a very attractive price. We expect to be able to present the details of this plan within the next month or so. We’re confident that our new approach to SDK distribution will address most of the concerns of Mr. Berry and other developers with whom we’ve discussed similar issues in the past few months.

- Gary Little, Product Manager,

Macintosh Development Tools

Apple Computer, Inc.

 
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

LibreOffice 4.3.5.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
CleanApp 5.0.0 Beta 5 - Application dein...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Monolingual 1.6.2 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
NetShade 6.1 - Browse privately using an...
NetShade is an Internet security tool that conceals your IP address on the web. NetShade routes your Web connection through either a public anonymous proxy server, or one of NetShade's own dedicated... Read more
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

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

Kodak Returns to CES With New Consumer Produ...
Former photography colossus Kodak is returning to CES for the first time in three years where the Kodak booth (#21818 South Hall 1) will showcase a wide range of innovative, imaging-related products... Read more
Invaluable Launches New Eponymously -Named A...
Invaluable, the world’s largest online live auction marketplace, hhas announced the official launch of the Invaluable app for iPad, now available for download in the iTunes App Store. Invaluable... Read more
IDC Reveals Worldwide Mobile Enterprise Appli...
International Data Corporation (IDC) last week hosted the IDC FutureScape: Worldwide Mobile Enterprise Applications and Solutions 2015 Predictions Web conference. The session provided organizations... Read more
Hello Vino Wine App Launches “Safe Ride Home”...
Hello Vino has announced addition of a new “Get a Safe Ride Home” feature in its Food & Drink app with a direct connection to Uber, the technology platform that connects users with rides. The... Read more
DEVON-technologies Releases DEVONthink To Go...
Coeur d’Alene, Idaho based DEVON-technologies, LLC has updated DEVONthink To Go, its mobile companion to DEVONthink, to version 1.5. The update includes an iOS 8 extension, compatibility with the... Read more
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

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.