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Differences between NEON in Cortex-A8 and A9

Kun Feng
Note: This was originally posted on 25th July 2011 at http://forums.arm.com

Currently i am working on a Cortex-A9 single-core chip(AML8726-m if you want to know more), and in the datasheet it's said there is a neon in it. But when i test the code here(http://hilbert-space.de/?p=22), i cannot find any acceleration on it, sometimes the neon-assembly- optimized code runs even slower than the arm-c-code. At the same time, the same code can get a pretty good acceleration on my i.MX515 which is a Cortex-A8 chip.


I am using the Android NDK to build a test app running on Android, can it be the reason?
Can anyone tell me why it happens? 


Here is some results:
#####On A8#####
arm c code: 116.*** ms
neon c code: 83.*** ms
neon asm code: 51.*** ms
#####On A9#####
arm c code: 107.*** ms
neon c code: 106-107.*** ms
neon asm code: 106-107.*** ms

Android is Linux based OS, so I can call gettimeofday() to get a precise time period in us level. The results on A9 are not identical but almost the same and I didn't run the same binary 3 times, I'm sure.

Thanks and looking forward to any useful suggestions.


Parents
  • Note: This was originally posted on 26th July 2011 at http://forums.arm.com


    Hum. I can't believe that this is the problem.
    It does not explain why on Cortex A8 the time are different...

    Except if in it's lowcost soc, the is not cache.

    May be you're right !


    I fix it now, it's the memory latency caused this problem
    And I think I am a little bit "lucky" to get almost three identical time on my A9 platform.
    So my A9 has a poor memory performance which I have to take care in future.
    Thanks for your help!!!
Reply
  • Note: This was originally posted on 26th July 2011 at http://forums.arm.com


    Hum. I can't believe that this is the problem.
    It does not explain why on Cortex A8 the time are different...

    Except if in it's lowcost soc, the is not cache.

    May be you're right !


    I fix it now, it's the memory latency caused this problem
    And I think I am a little bit "lucky" to get almost three identical time on my A9 platform.
    So my A9 has a poor memory performance which I have to take care in future.
    Thanks for your help!!!
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