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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 30th July 2011 at http://forums.arm.com


    or by trying this code
    http://pulsar.websha...lng=fr&sample=3


    I had tried the code you gave me before I posted this question.
    Last time, with the huge image, the second assembly code run even slower than the first one. I think it must be the memory latency problem again.
    This time, with a small image, 128*128 resolution, the time is shorten from 16.7ms to 11.3ms on my i.MX51.
    But on my A9, the improvement is so tiny, just 1ms, from 20ms to 19ms.
    So I'm confused again.
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  • Note: This was originally posted on 30th July 2011 at http://forums.arm.com


    or by trying this code
    http://pulsar.websha...lng=fr&sample=3


    I had tried the code you gave me before I posted this question.
    Last time, with the huge image, the second assembly code run even slower than the first one. I think it must be the memory latency problem again.
    This time, with a small image, 128*128 resolution, the time is shorten from 16.7ms to 11.3ms on my i.MX51.
    But on my A9, the improvement is so tiny, just 1ms, from 20ms to 19ms.
    So I'm confused again.
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