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Differences between NEON in Cortex-A8 and A9

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.

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Etienne SOBOLE over 12 years ago

Note: This was originally posted on 27th July 2011 at http://forums.arm.com

So 16 cycles like predicted. Note that you'd get a lot better performance if you unrolled this loop to fill up the latency after the last multiply and shift. Doing it 4 times should be sufficient.

or by trying this code
http://pulsar.webshaker.net/ccc/result.php?lng=fr&sample=3
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Etienne SOBOLE over 12 years ago

Note: This was originally posted on 27th July 2011 at http://forums.arm.com

So 16 cycles like predicted. Note that you'd get a lot better performance if you unrolled this loop to fill up the latency after the last multiply and shift. Doing it 4 times should be sufficient.

or by trying this code
http://pulsar.webshaker.net/ccc/result.php?lng=fr&sample=3
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