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

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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Simon Craske over 12 years ago

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

If you're down to final tweaking, it might be worth experimenting with preloading ahead in the source image.
Something like:

;; now at 256 8-bit values ... VPADDL.u8 Q3,Q3; 8 adds PLD [r1,#((4*640)-256)] ... VPADAL.u8 Q3,Q7; 16 adds PLD [r2,#((4*640)-256)] ... VPADAL.u8 Q3,Q11; 16 adds PLD [r3,#((4*640)-256)] ... VPADAL.u8 Q3,Q15; 16 adds PLD [r4,#((4*640)-256)] ;; now at 32 16-bit values

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Simon Craske over 12 years ago

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

If you're down to final tweaking, it might be worth experimenting with preloading ahead in the source image.
Something like:

;; now at 256 8-bit values ... VPADDL.u8 Q3,Q3; 8 adds PLD [r1,#((4*640)-256)] ... VPADAL.u8 Q3,Q7; 16 adds PLD [r2,#((4*640)-256)] ... VPADAL.u8 Q3,Q11; 16 adds PLD [r3,#((4*640)-256)] ... VPADAL.u8 Q3,Q15; 16 adds PLD [r4,#((4*640)-256)] ;; now at 32 16-bit values

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