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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


    Yes the two implementations of NEON are different, so I'd expect different performance numbers between the two cores.

    Can you give as an example of an algorithm you are trying, and how you are building it? The fact you see absolutely no performance difference is "suspicious" - I'd expect some difference, even if only small. Check you are not running the same binary 3 times - it seems like the obvious conclusion to three identical performance numbers =)


    I am completely sure that I didn't run the same binary 3 times because I build an apk file then installed the same apk file into Cortex-A8 and A9 platform respectively. And the time was not identical the same, it's about 106-107ms, but no significant differences.
Reply
  • Note: This was originally posted on 26th July 2011 at http://forums.arm.com


    Yes the two implementations of NEON are different, so I'd expect different performance numbers between the two cores.

    Can you give as an example of an algorithm you are trying, and how you are building it? The fact you see absolutely no performance difference is "suspicious" - I'd expect some difference, even if only small. Check you are not running the same binary 3 times - it seems like the obvious conclusion to three identical performance numbers =)


    I am completely sure that I didn't run the same binary 3 times because I build an apk file then installed the same apk file into Cortex-A8 and A9 platform respectively. And the time was not identical the same, it's about 106-107ms, but no significant differences.
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