Apple's Swift: Pipeline Depth & Memory Latency

Section by Anand Shimpi

For the first time since the iPhone's introduction in 2007, Apple is shipping a smartphone with a CPU clock frequency greater than 1GHz. The Cortex A8 in the iPhone 3GS hit 600MHz, while the iPhone 4 took it to 800MHz. With the iPhone 4S, Apple chose to maintain the same 800MHz operating frequency as it moved to dual-Cortex A9s. Staying true to its namesake, Swift runs at a maximum frequency of 1.3GHz as implemented in the iPhone 5's A6 SoC. Note that it's quite likely the 4th generation iPad will implement an even higher clocked version (1.5GHz being an obvious target).

Clock speed alone doesn't tell us everything we need to know about performance. Deeper pipelines can easily boost clock speed but come with steep penalties for mispredicted branches. ARM's Cortex A8 featured a 13 stage pipeline, while the Cortex A9 moved down to only 8 stages while maintining similar clock speeds. Reducing pipeline depth without sacrificing clock speed contributed greatly to the Cortex A9's tangible increase in performance. The Cortex A15 moves to a fairly deep 15 stage pipeline, while Krait is a bit more conservative at 11 stages. Intel's Atom has the deepest pipeline (ironically enough) at 16 stages.

To find out where Swift falls in all of this I wrote two different codepaths. The first featured an easily predictable branch that should almost always be taken. The second codepath featured a fairly unpredictable branch. Branch predictors work by looking at branch history - branches with predictable history should be, well, easy to predict while the opposite is true for branches with a more varied past. This time I measured latency in clocks for the main code loop:

Branch Prediction Code
  Apple A3 (Cortex A8 @ 600MHz Apple A5 (2 x Cortex A9 @ 800MHz Apple A6 (2 x Swift @ 1300MHz
Easy Branch 14 clocks 9 clocks 12 clocks
Hard Branch 70 clocks 48 clocks 73 clocks

The hard branch involves more compares and some division (I'm basically branching on odd vs. even values of an incremented variable) so the loop takes much longer to execute, but note the dramatic increase in cycle count between the Cortex A9 and Swift/Cortex A8. If I'm understanding this data correctly it looks like the mispredict penalty for Swift is around 50% longer than for ARM's Cortex A9, and very close to the Cortex A8. Based on this data I would peg Swift's pipeline depth at around 12 stages, very similar to Qualcomm's Krait and just shy of ARM's Cortex A8.

Note that despite the significant increase in pipeline depth Apple appears to have been able to keep IPC, at worst, constant (remember back to our scaled Geekbench scores - Swift never lost to a 1.3GHz Cortex A9). The obvious explanation there is a significant improvement in branch prediction accuracy, which any good chip designer would focus on when increasing pipeline depth like this. Very good work on Apple's part.

The remaining aspect of Swift that we have yet to quantify is memory latency. From our iPhone 5 performance preview we already know there's a tremendous increase in memory bandwidth to the CPU cores, but as the external memory interface remains at 64-bits wide all of the changes must be internal to the cache and memory controllers. I went back to Nirdhar's iOS test vehicle and wrote some new code, this time to access a large data array whose size I could vary. I created an array of a finite size and added numbers stored in the array. I increased the array size and measured the relationship between array size and code latency. With enough data points I should get a good idea of cache and memory latency for Swift compared to Apple's implementation of the Cortex A8 and A9.

At relatively small data structure sizes Swift appears to be a bit quicker than the Cortex A8/A9, but there's near convergence around 4 - 16KB. Take a look at what happens once we grow beyond the 32KB L1 data cache of these chips. Swift manages around half the latency for running this code as the Cortex A9 (the Cortex A8 has a 256KB L2 cache so its latency shoots up much sooner). Even at very large array sizes Swift's latency is improved substantially. Note that this data is substantiated by all of the other iOS memory benchmarks we've seen. A quick look at Geekbench's memory and stream tests show huge improvements in bandwidth utilization:

Couple the dedicated load/store port with a much lower latency memory subsystem and you get 2.5 - 3.2x the memory performance of the iPhone 4S. It's the changes to the memory subsystem that really enable Swift's performance.

 

Apple's Swift: Visualized Six Generations of iPhones: Performance Compared
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  • mjh483 - Sunday, December 02, 2012 - link

    This is by far the best in-depth review of iPhone 5. I really appreciate your effort in creating such a long review. Thank you very much and hope to see more of these reviews in future! Reply
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  • galaxus1122 - Friday, December 28, 2012 - link

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  • WaltFrench - Saturday, March 09, 2013 - link

    I appreciate all the good thinking (and work) in this post, but really think you're being unrealistic in saying, <i>“In that case you could see no improvement or even a regression in battery life.”</i>

    Batteries exist to power actual usage. My old wristwatch — haven't worn it for a couple of years now — still keeps working just great. By the standard you mention, my watch is a far superior smartphone: no, I can't do as much browsing, but the battery life is better.

    I'm one who finds that LTE data speeds mean I spend MORE time each day browsing (chasing down links from Twitter, mostly) and the battery still lasts me a whole day. By an integrated test of how much good you can get from a phone in a day, the 5 has a <b>hugely better</b> on-the-go CPU/battery/screen combo than the iPhone4 it replaced.
    Reply
  • Amit kumar - Friday, March 22, 2013 - link

    Apple has done a splendid job with the iphone 5. It’s design outstanding. I checked full features of this device this site as well. http://www.gadtecho.com/iphone-5-specs/ Reply
  • ToniPan - Tuesday, April 30, 2013 - link

    3 Months with my new iPhone 5 and i always enjoy the sound quality when I capture videos or sound.
    The only problem is that everyone walking around with the same iPhone after 3GS.. I purchased a carbon sticker from http://www.coolcasing.com
    and i don't get bored from the same exactly design over and over. Why peoples talk about oranges(Galaxy S3-S4) when we talk about Apple?
    Reply
  • Andy_K1982 - Friday, August 09, 2013 - link

    Can anyone tell me how much data will it take to use FaceTime with someone? Reply
  • rabidpeach - Friday, September 13, 2013 - link

    Howdy,

    Did you ever do an article on why apple destroys android competition in GL graphics tests? Seems like no matter how many cores they throw in they can't quite catch apple.

    Thanks, sorry if I missed something.
    Reply

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