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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

    Could you tell us precisely how large the image is (in pixels, an exact count) and how many times you're calling the function to get the numbers you're getting? Then we can put together some rough cycles/iteration counts and analyze the loop to see how the numbers compare with what we expect.

    It's actually interesting that the memory performance was holding you back more on the amlogic board than the i.MX51. I was actually considering using AML8276-M for a device over i.MX535.. guess there would have been a good reason not to..
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  • Note: This was originally posted on 26th July 2011 at http://forums.arm.com

    Could you tell us precisely how large the image is (in pixels, an exact count) and how many times you're calling the function to get the numbers you're getting? Then we can put together some rough cycles/iteration counts and analyze the loop to see how the numbers compare with what we expect.

    It's actually interesting that the memory performance was holding you back more on the amlogic board than the i.MX51. I was actually considering using AML8276-M for a device over i.MX535.. guess there would have been a good reason not to..
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