TweetFollow Us on Twitter

December 94 - Balance of Power: PowerPC Branch Prediction

Balance of Power: PowerPC Branch Prediction

Dave Evans

The PowerPC processors try to predict which way your code will execute. This sounds surprisingly astrological for a digital machine, but it becomes very useful for a pipelined processor and will often speed up your code. In this column I'll go over why and how this works, focusing especially on the new PowerPC 604 processor prediction techniques, and I'll answer the question "Can a Power Macintosh really tell the future?"


Typically about one-seventh of the instructions in your code are branches, either to call subroutines or to make logical decisions in your program. The PowerPC processor would ordinarily tend to stall at branches, since it tries to work on more than one instruction at a time and it's not always sure which code it should execute after a branch. It could either take the branch or fall through, and often the processor won't know which until a couple of cycles later.

So the PowerPC processors allow for speculative execution, meaning they'll guess at the most probable direction the branch will go and then will issue those instructions. But the processor doesn't let the instructions commit until it's sure the guess was correct. Usually it guesses right, and a few instructions are already completed when the branch is decided. If the guess was wrong, it throws out those results and starts over with the correct code.

This predictive skill helps keep the processor executing successfully without stalls, and better prediction techniques will yield better overall performance. The new PowerPC 604 processor improves on earlier prediction techniques; I'll discuss all of them in detail below.

But first, a relevant astrological note: The "birthday" of the 601 makes it a Taurus, whereas then 603 is a Libra. The 604 chip had a birthday in April, so it's an Aries.


The PowerPC 601 and 603 processors use basically the same techniques to predict branches. For simple unconditional branches, for example, they both process and remove the branch early in the instruction issue stage. This operation, called branch folding, keeps the instruction stream moving without having to wait for the branch to be processed. The branch is handled early, and the new instructions are fetched from the cache immediately.

For conditional branches, both processors first try to handle the branch early in the instruction issue stage. If the condition being tested has already been evaluated, the branch is folded out of the instruction stream. But if the condition being tested is still in the pipeline, the processor must guess at the branch direction.

Prediction of guessed branches are based on two things: the direction of the branch and a software "hint" bit. If the direction is negative -- backward in your code -- the branch is taken (because loops often iterate a few times backward before falling through, and this heuristic is more often true). All other branches fall through by default. The hint bit is a way for the compiler to reverse this heuristic: if the bit is set, the prediction will be reversed.

As far as I know there are no compilers that allow you to specify the hint bit in your code, although this could be a valuable feature. Also, profilers or similar tools could take statistics on your code flow and then set the bits for you from trial runs of your software.


The PowerPC 604 has much better branch prediction, which means better performance. Because branch statements most often repeat themselves, it remembers recent branch results to make its predictions:
  • It has a cache of the last 64 branches that it has taken, and any time it sees one of these branches again it will immediately predict to the same branch destination. This technique, called dynamic branch prediction, is used on the Pentium and other processors with great results.

  • It keeps a history of all other branches and predicts based on the recent directions that branch took.
The cache technique has the advantage of being very fast. When the 604 fetches an instruction, it also sends the instruction's address to the branch cache. If the instruction is a recently executed branch, the cache will return the address of where the branch last went. This is immediately used to fetch the next instruction. Because this all occurs during the fetch of the branch instruction itself, there's no delay in fetching the first predicted instruction.

For conditional branches that aren't in the branch cache, the 604 keeps a history of recent times it saw that instruction. It keeps 512 such histories, each two bits wide, to remember whether the branch was taken during the last few executions. The processor hashes the instruction address to keep the branch histories distinct, and hash collisions are very rare.

Each history is set to one of four states: strongly taken, taken, not taken, and strongly not taken. The current state determines the branch prediction as taken or not taken. After the branch commits, the state is updated. Each update adjusts the state one step toward strongly taken or strongly not taken. The two intermediate steps are a hedge so that it will usually take two mistakes before a prediction changes. Because branches tend to repeat, this algorithm generally results in the following prediction:

  • If the branch was taken during the last two executions, the 604 predicts it will again be taken.

  • If the branch wasn't taken during both of the last two executions, the 604 predicts it again won't be taken.
Also with the 604, branches on the count register base their prediction on the current count value. This will usually predict loops correctly and yield good performance, since loops count down for a number of iterations before the final iteration causes an incorrect prediction.

But these techniques also come with a tradeoff: the 604 has an extra pipeline stage to dispatch instructions. This means instructions take longer to get through the pipe, and mispredicted branches are more expensive.


The 604 is the fastest PowerPC processor yet, and I can't talk about it here without also going into why it's such a fast engine. Besides its advanced branch prediction hardware, it has significantly more integer and floating-point hardware, which yields improved overall performance. Given that it's produced with a more advanced silicon process than the original 601, it's clocked above 80 MHz and offers blazingly fast computation for your code.

As a backbone for the chip, the instruction issuing and control logic allow the 604 to issue up to four instructions per clock, compared to the 601's and 603's effective three. As mentioned above, however, its pipeline has one extra decode stage and branches are issued and handled in their own branch unit. To help it speculatively execute more instructions than the other chips, it also comes with twice the number of "rename" registers than the 603. Twelve extra general-purpose and eight extra floating-point registers are available to hold speculatively produced results until a branch commits. The 604 is also the first PowerPC processor that can speculatively execute two branches at once. This, combined with advanced branch prediction, should keep the processor screaming even through complex code flow.

What most people will notice, however, is the additional integer math performance on the 604. At any one time, the 604 can have two add-subtract instructions and one multiply-divide instruction completing in a cycle. IBM says that it therefore has three integer units, but the multiply-divide hardware is also used for logical and bit manipulation operations. The bottom line is much better integer performance than the Power Macintosh 8100/80. As an example of this, the following code should execute nearly twice as fast on the 604 than on the 601:

do {
   unsigned long   datapoint;
   datapoint = *(dataarray + datasize);
   if (datapoint > kThreshold) {
      if (datapoint > kMaxLong - accumulate)
      accumulate += datapoint;
      samplecount += 1;
   } while (datasize--);
Looking at this code, we see a few integer operations that will be dual-issued on the 604. As long as the datapoint values aren't too erratic, the 604 will better predict the first if statement's branch: it will assume that the current datapoint is on the same side of the threshold as on the previous iteration, which in fact is where it will tend to be. And the second if statement, which checks for an overflow, will (barring an exception) get predicted correctly out of the loop. The 601 or 603 may predict it incorrectly. So even though one integer unit will be busy doing the math, the overflow checking will effectively occur without stalling the pipeline.

The floating-point hardware was also supercharged. On the 601 and 603 processors, a single-precision floating-point instruction can issue and complete each cycle, but double-precision numbers take twice as long. The 604 allows one full double-precision multiply-add instruction to be issued and one to complete each cycle. The chip is twice as fast as the 601 and 603 for these double-precision calculations.


So can Power Macintosh tell your future? It certainly tries to with the prediction techniques described above, and in doing so yields better performance. With the simple methods of the 601 and 603, or the dynamic prediction of the 604, your Power Macintosh will speculatively execute your code with seemingly psychic results.

What about the future of the Power Macintosh? The PowerPC architecture allows excellent growth. When I saw the specifications for the first processor, the 601, I was very impressed. It's an excellent design and it has proven to be a potent engine for the Macintosh. When I saw the specifications for the follow-on chips, however, I was really blown away. The 603 and 604 offer incredible performance for the price, and prove that the PowerPC architecture scales well both into low-cost/low-energy solutions and to the cutting edge in performance. And the technology applied to the 604 can be expanded in future chips, adding more execution units and advanced caches at higher clock speeds. The latest IBM POWER2 processors can issue two load/store, two logic/branch, two floating-point, and two integer instructions per cycle. These processors point to the future of PowerPC performance.

So without any additional tuning on your part, PowerPC will continue to improve your performance in the future. I also feel compelled to reiterate this advice from my previous columns: tune your critical code. Tuning often trades performance for code readability and maintainability, so carefully choose which code to tune and use code profilers (and the stars?) to guide your way.

DAVE EVANS (Aquarius, January 20-February 18) Look for opportunities to communicate. You are bound to have fun. Love is in the air; don't work too much or you'll miss it. Apple continues to hold promise for you. Compatible with Sagittarius.

Thanks to Phil Sohn, Peter Steinauer, and Eric Traut for reviewing this column.

This page was last modified on Sunday, April 06 1997 04:24

Community Search:
MacTech Search:

Software Updates via MacUpdate

The best games like Animal Crossing on m...
Animal Crossing amiibo Festival is out right now for the Wii U, reminding us of just how much fun that world can be. Or at least to go back and check in on our villages once in a while. [Read more] | Read more »
Between 2 Taps - Tap for Tap interview M...
Hello, and welcome back to Between 2 Taps, Tap for Tap’s Indie Dev interview series. [Read more] | Read more »
Facility 47 (Games)
Facility 47 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: You wake up alone and freezing in an icy cell. You try the cell door but it’s locked, it seems that you are stuck with no... | Read more »
The best Photoshop alternative on iPad
Instagram and Lightroom are great and all, but sometimes people need to get extra creative with their image editing.Like, Photoshop creative. If you're one of these people, take a look at our pick for the best mobile Photoshop experience on iPad... | Read more »
The Walking Dead: No Man’s Land guide -...
A new update for The Walking Dead: No Man’s Land was released last week, making it the perfect time for you to head back to your base and take out some walkers. Here’s the lowdown on what’s new to the game, and how to take advantage. [Read more] | Read more »
Goat Rider guide - Tips and tricks to st...
We've all been there. One second, we're riding high on a crazed goat, and the next, we've been tossed off it like someone who's no good at goat ridin'. [Read more] | Read more »
Real Boxing 2 CREED: How to become a gre...
Just in time for Rocky fans who can’t wait to see CREED, the latest movie, we have the official tie-in game,Real Boxing 2 CREED. It builds on the success of its predecessor and there’s lots to take in so we at 148apps thought we’d run you through... | Read more »
CoinOp Heroes 2 guide - How to build an...
CoinOp Heroes 2 justlaunched and, like all clickers, it's dangerously addictive stuff. You have to furiously tap your screen to defeat wave after wave of foes and earn an insane amount of cash to spend on character upgrades and an army of minions... | Read more »
Dr. Panda Firefighters (Education)
Dr. Panda Firefighters 1.0.1 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0.1 (iTunes) Description: FIGHT FIRES AND SAVE THE DAY!Work together with Dr. Panda and his firefighting team to rescue his trapped... | Read more »
Puddle + (Games)
Puddle + 1.0 Device: iOS iPhone Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Puddle is back in a new "+" edition featuring enhanced graphics, new videos and Apple TV support ! No IAP and No Ads. Dive into Puddle... | Read more »

Price Scanner via

Save $30-$40 on new Apple TVs after rebate
Adorama has new Apple TVs on sale for up to $40 off MSRP after mail-in rebate, good through December 15th. Shipping is free, and Adorama charges NY & NJ sales tax only: - 32GB Apple TV: $119.99... Read more
13-Inch Haswell MacBook Air At Two Years – Th...
The 13-inch mid-2013 “Haswell” MacBook Air I ordered in Apple’s November 2013 Black Friday sale was my first new Mac in four and a half years — the longest interval I’ve gone between system upgrades... Read more
Target Black Friday Early Access deals: $100...
Target is offering early access to their Black Friday deals on Apple products on their online store for today, the 25th, only. Choose free shipping or free local store pickup (if available): - Apple... Read more
BlackBerry Q3 Mobility Index Report Finds iOS...
BlackBerry has announced results of its thirteenth Good Mobility Index Report, showing that organizations are increasingly building custom secure apps. Among Good Powered by BlackBerry (formerly Good... Read more
Wednesday roundup of early Black Friday Mac s...
Save up to $500 on a new Mac with these early Black Friday deals from Apple resellers, currently the lowest prices available for these models: (1) B&H Photo has all new Macs on sale for up to $... Read more
iPod nano on sale for $119, $30 off MSRP
Walmart has the 16GB iPod nano (various colors) on sale for $119.20 on their online store for a limited time. That’s $30 off MSRP. Choose free shipping or free local store pickup (if available). Sale... Read more
Adorama Black Friday deals: Up to $400 off Ma...
Adorama has released their Black Friday deals for 2015. Save up to $400 on MacBook Pros, $200 on MacBooks and MacBook Airs, and $270 on iMacs. Use code RYBFDEAL during checkout to see these prices.... Read more
B&H Photo Deals: $200 off 12-inch 1.2GHz...
In addition to the B&H Photo Black Friday week sales we posted yesterday, B&H has lowered their price on two products to $200 off MSRP: - 12″ 1.2GHz Gray Retina MacBook: $1399 save $200 - 13... Read more
Best Buy Early Access: Today only, Up to $125...
Best Buy has iPad Air 2s on sale for up to $125 off MSRP and Apple Watch models on sale on their online store for up to $100 off MSRP with special codes through midnight CT tonight. Choose free... Read more
UPPERCASE DESIGNS Premium Ultra Thin Keyboard...
UPPERCASE Designs today announced its new Premium Ultra Thin Keyboard Protector and its Palm Rest Protector Set for the 12-inch MacBook. The accessories provide durable protection for the 12-inch... Read more

Jobs Board

Hardware Systems Integration Engineer - *App...
# Hardware Systems Integration Engineer - Apple Watch Job Number: 39380139 Santa Clara Valley, Califo ia, United States Posted: Apr. 23, 2015 Weekly Hours: **Job Read more
Sr. Technical/Project Manager, *Apple* Educ...
# Sr. Technical/Project Manager, Apple Education Job Number: 36588557 New York City, New York, United States Posted: Jul. 30, 2015 Weekly Hours: 40.00 **Job Summary** 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
Merchant Operations Manager: *Apple* Pay -...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more than Read more
*Apple* Pay QA Manager - Apple Inc. (United...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more than Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.