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A8/9 NEON 128bit registers, 64bit alu's

Sander Bogaert
Note: This was originally posted on 1st December 2011 at http://forums.arm.com

Hi,

I've been reading a lot about the neon architecture for a project but there is still this one thing I'm not entirely sure about. If I understand correctly the 128bit-view is more of an aid for the programmer since the alu's in the neon engine are only 64bit wide instructions working on a  qx register will just take double the time. I was going over the timing tables trying my best to understand them :-) and saw this confirmed in some instruction timings but in other the operation on 128bit would also just take one cycle ( add for example ).

Did I read the table wrong? If not: how is this achieved? Is this divided over 2 64bit adders which are coupled ( carry ) then?

Thanks in advance
Parents
  • Note: This was originally posted on 10th December 2011 at http://forums.arm.com

    It's true that NEON has 128-bit registers but nearly always operates on just 64-bits at a time. But the size of the registers is an Architecture (language) specification that can't change whereas the internal use of 64-bits is an implementation issue that will change over time, so you can expect that perhaps in 1 more year, ARM devices will operate on 128-bits at a time instead of 64-bits. So if you write your NEON code now for 128-bit, it will be more future proof because the same code will potentially double in speed in the future!

    Cheers,
    Shervin Emami.
    http://www.shervinem...rmAssembly.html
Reply
  • Note: This was originally posted on 10th December 2011 at http://forums.arm.com

    It's true that NEON has 128-bit registers but nearly always operates on just 64-bits at a time. But the size of the registers is an Architecture (language) specification that can't change whereas the internal use of 64-bits is an implementation issue that will change over time, so you can expect that perhaps in 1 more year, ARM devices will operate on 128-bits at a time instead of 64-bits. So if you write your NEON code now for 128-bit, it will be more future proof because the same code will potentially double in speed in the future!

    Cheers,
    Shervin Emami.
    http://www.shervinem...rmAssembly.html
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