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How to efficiently sum 4 x 8bit integers with ARM or NEON

Shervin Emami
Note: This was originally posted on 17th September 2010 at http://forums.arm.com

Hi,

I am trying to write an ASM function to shrink an 8-bit greyscale image by 4, so I need to get the sum of 4 bytes very quickly. From what I have read, NEON needs atleast 32-bit integers and VFP is for floats, so it looks like I should just stick with ARM (or Thumb-2) instructions.

But I'm just a beginner so I'm wondering if there is a more efficient method of summing 4 consecutive bytes than convert each byte to a 32bit int and then sum them (and then shift right to get the average).

Its for a Cortex-A8 (ARMv7-A), and the data is aligned to 32 bytes or whatever I want.

Cheers,
Shervin Emami
[url="http://www.shervinemami.co.cc/"]http://www.shervinemami.co.cc/[/url]
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  • Arm Employee Badge
    Note: This was originally posted on 27th September 2010 at http://forums.arm.com

    Especially since my optimised C code is structured to do the exact same thing as what my ARM assembly code does, but obviously the C compiler didn't agree!


    Very few compilers generate "weird" instructions - so if you are after anything a little special in the instruction set the odds are you will either need to use intrinsics for that instruction or fall back to assembler.
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  • Arm Employee Badge
    Note: This was originally posted on 27th September 2010 at http://forums.arm.com

    Especially since my optimised C code is structured to do the exact same thing as what my ARM assembly code does, but obviously the C compiler didn't agree!


    Very few compilers generate "weird" instructions - so if you are after anything a little special in the instruction set the odds are you will either need to use intrinsics for that instruction or fall back to assembler.
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