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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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  • Note: This was originally posted on 27th September 2010 at http://forums.arm.com

    I just installed NASM only to discover that it doesn't support ARM. So I guess I'll stick with the XCode default "gcc-4.2 -x assembler-with-cpp" and not have NEON alignment. And I finally figured out why you only gave alignment on some rows and not others: because you thought I had a 480 pixel wide image and since 480 is not divisible by 64 or 128, only some rows would have good alignment.

    Like I said, thanks a lot both of you for your help with jump starting me on ARM and NEON development.
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  • Note: This was originally posted on 27th September 2010 at http://forums.arm.com

    I just installed NASM only to discover that it doesn't support ARM. So I guess I'll stick with the XCode default "gcc-4.2 -x assembler-with-cpp" and not have NEON alignment. And I finally figured out why you only gave alignment on some rows and not others: because you thought I had a 480 pixel wide image and since 480 is not divisible by 64 or 128, only some rows would have good alignment.

    Like I said, thanks a lot both of you for your help with jump starting me on ARM and NEON development.
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