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ARM Cortex-A72 Architecture Deep Dive

Final Thoughts

At a high-level, the A72 looks nearly identical to the A57, but at a lower level there are a significant number of changes throughout the entire pipeline that appear to make the A72 a decent upgrade. The most notable changes affecting performance are the improved branch prediction, increased dispatch bandwidth, lower-latency execution units, and higher bandwidth L2 cache. All of these enhancements, and many more which we did not discuss here, lead to better performance—between 16-50% across a range of synthetic benchmarks, according to ARM. Real-world performance gains will be less, of course, but A72 is definitely an improvement over the A57, especially with floating-point workloads.

The A72 is not a pure performance play, however. ARM is targeting a much higher power efficiency with this architecture than with any previous high-end CPU core. It’s clear from the lengthy list of optimizations discussed above that reducing power consumption was paramount; many of the changes are purely focused on power with no net performance gain.

Reducing power and area—the A72 achieves a 10% core area reduction overall—obviously has a positive effect on battery life and cost, but it has a secondary effect on performance too. Normally, reducing latency in the execution units puts pressure on the max attainable core frequency due to increased circuit complexity and tighter timing windows; however, the A72’s power and area optimizations elsewhere, not to mention the move to FinFET, actually allow the A72 to reach a slightly higher frequency. Reducing power also reduces thermal load, allowing for higher sustained performance, something the A57 struggles with at 20nm.

The Cortex-A72 may not be a revolutionary design that catapults it above Apple’s Twister CPU in the A9 SoC for single-core performance or undercuts the A53 in power consumption, but it’s a significant update nonetheless, addressing the A57’s issues by enabling higher peak and sustained performance while using less power.


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Update, 1/12/16, 10:55am PT: Clarified how the Decode/Rename/Dispatch pipeline works and added some information about the issue queues.

Matt Humrick is a Staff Editor at Tom's Hardware, covering Smartphones and Tablets. Follow him on Twitter.

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  • Aspiring techie
    I wonder what would happen if these guys dipped their toe into the desktop CPU market.
    Reply
  • utroz
    I wonder what would happen if these guys dipped their toe into the desktop CPU market.
    Well no current Windows support so you would need to run Linux or other ARMv8 compatible OS. For people that just watch netflix, check facebook and other web pages, and type up a few papers for school or work an ARM cpu would have plenty of CPU performance..
    Reply
  • InvalidError
    17300174 said:
    I wonder what would happen if these guys dipped their toe into the desktop CPU market.
    Not much.

    Changing instruction set does not magically free the architecture from process limitations nor remove bottlenecks from software architecture. If ARM designed a CPU core specifically for desktop, it would hit most of the same performance scaling bottlenecks x86 has. You would likely end up with a 50W ARM chip being roughly even with a 50W Intel chip, the main difference between the two - aside from the ISA - being that the ARM chip is $100 while the Intel chip is $400.
    Reply
  • somebodyspecial
    And I think that's his point...The PRICE and how much of the public at large such a machine could get. Myself I can't wait until they put out a full desktop chip with heatsink/fan big psu, HD or SSD, 16-32GB mem etc (hopefully with an optional slot for discrete gpu when desired). As games amp up on ARM you'd most likely only be missing WINDOWS/x86 if you use pro apps stuff. Unreal4/Unity5 etc will provide nice graphics for games on the ARM side (most engines port easily today and get even better with Vulkan coming), so only pro apps would be left off for years and some of the big ones (adobe etc) might put out full apps soon anyway. Take off $200-300 for cpu and $100 for windows and I'm guessing an ARM desktop could do quite a bit of damage to WINTEL.

    We see people already opting for chromebooks, tablets etc as PC's. Knock a chunk off desktop prices and you'll gains some users and push devs past mobile on arm. I'm hoping NV builds such a box at some point (just a much bigger Shield TV box really), but with multiple OS's (steamos, linux, and android) or at least a way to do it yourself. That would be a pretty versatile box ;) It isn't so much about if they BEAT intel, as it is about dropping the price of PC's everywhere. If ARM's side wants to grow much more they have to go to desktops. Surely everyone wants a chunk of Intel's ~13B a year.
    Reply
  • viewtyjoe
    If ARM's side wants to grow much more they have to go to desktops. Surely everyone wants a chunk of Intel's ~13B a year.

    ARM makes their money by licensing out their designs to other companies which manufacture the actual chips. The only company I'm aware of with the resources and ARM license to theoretically make something like this happen is AMD, and their use of ARM is more directed towards the server sector.
    Reply
  • TechyInAZ
    I wonder what would happen if these guys dipped their toe into the desktop CPU market.

    I doubt that would happen, having ARM AND X86 on the desktop patform is just going to cause frustration.
    Reply
  • pug_s
    If ARM's side wants to grow much more they have to go to desktops. Surely everyone wants a chunk of Intel's ~13B a year.

    ARM makes their money by licensing out their designs to other companies which manufacture the actual chips. The only company I'm aware of with the resources and ARM license to theoretically make something like this happen is AMD, and their use of ARM is more directed towards the server sector.
    And I think that's his point...The PRICE and how much of the public at large such a machine could get. Myself I can't wait until they put out a full desktop chip with heatsink/fan big psu, HD or SSD, 16-32GB mem etc (hopefully with an optional slot for discrete gpu when desired). As games amp up on ARM you'd most likely only be missing WINDOWS/x86 if you use pro apps stuff. Unreal4/Unity5 etc will provide nice graphics for games on the ARM side (most engines port easily today and get even better with Vulkan coming), so only pro apps would be left off for years and some of the big ones (adobe etc) might put out full apps soon anyway. Take off $200-300 for cpu and $100 for windows and I'm guessing an ARM desktop could do quite a bit of damage to WINTEL.

    We see people already opting for chromebooks, tablets etc as PC's. Knock a chunk off desktop prices and you'll gains some users and push devs past mobile on arm. I'm hoping NV builds such a box at some point (just a much bigger Shield TV box really), but with multiple OS's (steamos, linux, and android) or at least a way to do it yourself. That would be a pretty versatile box ;) It isn't so much about if they BEAT intel, as it is about dropping the price of PC's everywhere. If ARM's side wants to grow much more they have to go to desktops. Surely everyone wants a chunk of Intel's ~13B a year.
    And I think that's his point...The PRICE and how much of the public at large such a machine could get. Myself I can't wait until they put out a full desktop chip with heatsink/fan big psu, HD or SSD, 16-32GB mem etc (hopefully with an optional slot for discrete gpu when desired). As games amp up on ARM you'd most likely only be missing WINDOWS/x86 if you use pro apps stuff. Unreal4/Unity5 etc will provide nice graphics for games on the ARM side (most engines port easily today and get even better with Vulkan coming), so only pro apps would be left off for years and some of the big ones (adobe etc) might put out full apps soon anyway. Take off $200-300 for cpu and $100 for windows and I'm guessing an ARM desktop could do quite a bit of damage to WINTEL.

    We see people already opting for chromebooks, tablets etc as PC's. Knock a chunk off desktop prices and you'll gains some users and push devs past mobile on arm. I'm hoping NV builds such a box at some point (just a much bigger Shield TV box really), but with multiple OS's (steamos, linux, and android) or at least a way to do it yourself. That would be a pretty versatile box ;) It isn't so much about if they BEAT intel, as it is about dropping the price of PC's everywhere. If ARM's side wants to grow much more they have to go to desktops. Surely everyone wants a chunk of Intel's ~13B a year.

    It is possible to make a hardware ARM chip that rivals Intel. The only problem is software as there is no software that would take advantage of it. Maybe in the distant future that Android OS and its apps would morph to a desktop OS that would rival to Microsoft, but not yet. Microsoft's closest adaptation of ARM soc's are windows 10 developer edition running in the Raspberry Pi 2. All of the software and games have to be written to ARM compatible. Even so, Intel's main focus now are now on low powered and mobile chips that is competing with ARM itself. Who knows, maybe by then AMD would get its act together and compete with Intel on the low powered space when they have access to 14-16nm technologies.
    Reply
  • MichaelWest
    With regard to the discussion of when someone is going to provide ARM type cpu's for desktops for ARM gaming etc we already pretty much have the beginning of this with all the many Media streaming TV boxes on the market. They mainly run Android which is not ideal for desktop applications yet and this could take many years to see this improve. Hardware wise they are more than fast enough for all the current ARM android games and there is room for them to get even faster. They don't face the same power usage limitations mobile devices face. They may be targeted for use on TV's with remotes but they can work just as well with HDMI monitors, keyboards, mice and game controllers. If all you wanted was a simple cheap desktop for email/internet and ARM gaming then they already fit the bill.

    Reply
  • cbxbiker61
    It is possible to make a hardware ARM chip that rivals Intel. The only problem is software as there is no software that would take advantage of it. Maybe in the distant future that Android OS and its apps would morph to a desktop OS that would rival to Microsoft, but not yet. Microsoft's closest adaptation of ARM soc's are windows 10 developer edition running in the Raspberry Pi 2. All of the software and games have to be written to ARM compatible. Even so, Intel's main focus now are now on low powered and mobile chips that is competing with ARM itself. Who knows, maybe by then AMD would get its act together and compete with Intel on the low powered space when they have access to 14-16nm technologies.

    That is only true when you define "software" being "Windows binaries".

    More and more people every day are waking up to the fact that "software" is really source code, which can be compiled on any architecture for which there is a compiler. You just have to use an open platform with an open compiler, i.e. Linux/BSD.
    Reply
  • bit_user
    17300684 said:
    Changing instruction set does not magically free the architecture from process limitations nor remove bottlenecks from software architecture. If ARM designed a CPU core specifically for desktop, it would hit most of the same performance scaling bottlenecks x86 has. You would likely end up with a 50W ARM chip being roughly even with a 50W Intel chip
    If that were true, Intel could've made an Atom that's at least as efficient as competing ARM cores. But ISA does actually count for something. x86 is significantly harder to decode, and occupies more space in ICaches.

    It would be interesting if someone designed an ARM v8 core for optimal single-thread performance. I'm pretty sure it could provide superior performance at the same power, and use less power at the same performance as Skylake (assuming similar design resources & process node as Intel). But this is a tall order, and there's not yet a big enough market. Maybe in 5 years, once ARM has grabbed a significant chunk of server market share, there'll be enough interest in building workstation-oriented ARM cores.
    Reply