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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 23rd March 2013 at http://forums.arm.com

    Hi,

    I executed NEON operation test on Linux platform board. I am doing  for 4*4 matrix multiplication using arm & neon instructions.
    (1)   Matrix multiplication: Method of  calculating one bye one.Here I have used only S registers. (Normal ARM instructions)
      Here I am loading the float array content to S registers (32-bit)  using "vldmia" and then "vmul.f32" and "vmla.f32" to perform matrix  multiplication using S registers as operand and to to hold the result.

    (2)   Matrix multiplication: Since 128  bit calculation is done, the  number of instructions will become 1/4 compared to  (1). Here I have  used Q and D registers. (Neon instructions)
    Here  I am loading the complete float array content to Q registers(128  bit) using "vldmia" and then "vmul.f32" and "vmla.f32" to perform matrix  multiplication using Q(128 bit) and D (64 bit) registers which will  obviously reduce the number of instructions ( Load, store,  multiplication) to 1/4 th of (1) code.

    I am using linux 3.0.35  and test code is executed on Linux platform (Cortex-a9 architecture) .
    But there is no speed difference between (1) and (2).

    In my Linux kernel configuration following options enabled
    CONFIG_VFP=y
    CONFIG_VFPv3=y
    CONFIG_NEON=y

    Following gcc command I have used to build the NEON application and  gcc compiler version is gcc 4.6.2
    gcc  -march=armv7-a -mtune=cortex-a9 -mfpu=neon -ftree-vectorize -ffast-math -mfloat-abi=hard  -o test.out test.c

    Why I dint find any performance difference between normal ARM and NEON codes?
    I have tested same code with the Cortex-A8 and I am able to achieve the performance difference.

    Thanks in advance
Reply
  • Note: This was originally posted on 23rd March 2013 at http://forums.arm.com

    Hi,

    I executed NEON operation test on Linux platform board. I am doing  for 4*4 matrix multiplication using arm & neon instructions.
    (1)   Matrix multiplication: Method of  calculating one bye one.Here I have used only S registers. (Normal ARM instructions)
      Here I am loading the float array content to S registers (32-bit)  using "vldmia" and then "vmul.f32" and "vmla.f32" to perform matrix  multiplication using S registers as operand and to to hold the result.

    (2)   Matrix multiplication: Since 128  bit calculation is done, the  number of instructions will become 1/4 compared to  (1). Here I have  used Q and D registers. (Neon instructions)
    Here  I am loading the complete float array content to Q registers(128  bit) using "vldmia" and then "vmul.f32" and "vmla.f32" to perform matrix  multiplication using Q(128 bit) and D (64 bit) registers which will  obviously reduce the number of instructions ( Load, store,  multiplication) to 1/4 th of (1) code.

    I am using linux 3.0.35  and test code is executed on Linux platform (Cortex-a9 architecture) .
    But there is no speed difference between (1) and (2).

    In my Linux kernel configuration following options enabled
    CONFIG_VFP=y
    CONFIG_VFPv3=y
    CONFIG_NEON=y

    Following gcc command I have used to build the NEON application and  gcc compiler version is gcc 4.6.2
    gcc  -march=armv7-a -mtune=cortex-a9 -mfpu=neon -ftree-vectorize -ffast-math -mfloat-abi=hard  -o test.out test.c

    Why I dint find any performance difference between normal ARM and NEON codes?
    I have tested same code with the Cortex-A8 and I am able to achieve the performance difference.

    Thanks in advance
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