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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]
Parents
  • Note: This was originally posted on 25th September 2010 at http://forums.arm.com

    Hi guys,

    After spending about 1 week to optimise my 2 resizing functions and learn NEON and create NEON versions, I have some timing results that you guys might be interested in knowing. As part of my project I made separate functions to divide the number of pixels by 4 (half width and half height) or to divide the number of pixels by 16 (quarter width and quarter height), where the resizing is done by adding all the 2x2 or 4x4 pixel values and dividing by 4 or 16.

    To shrink a 480x360 pixel greyscale image by 16 (quarter width and quarter height) on a Cortex-A8 (iPhone 3GS):

    Resize function in OpenCV C library (GCC4.2 with -O3) takes about 38 msec
    My optimised C code without intrinsics (GCC4.2 with -O3) takes about 27 msec
    My hand-optimised ARMv7 Assembly code takes about 7.8 msec
    My hand-optimised NEON Assembly code takes about 0.9 msec


    So I'm very happy to see the resizing function that was the bottleneck of my program is now 4200% faster! 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!

    Cheers,
    Shervin Emami.
    [url="http://www.shervinemami.co.cc/"]http://www.shervinemami.co.cc/[/url]
Reply
  • Note: This was originally posted on 25th September 2010 at http://forums.arm.com

    Hi guys,

    After spending about 1 week to optimise my 2 resizing functions and learn NEON and create NEON versions, I have some timing results that you guys might be interested in knowing. As part of my project I made separate functions to divide the number of pixels by 4 (half width and half height) or to divide the number of pixels by 16 (quarter width and quarter height), where the resizing is done by adding all the 2x2 or 4x4 pixel values and dividing by 4 or 16.

    To shrink a 480x360 pixel greyscale image by 16 (quarter width and quarter height) on a Cortex-A8 (iPhone 3GS):

    Resize function in OpenCV C library (GCC4.2 with -O3) takes about 38 msec
    My optimised C code without intrinsics (GCC4.2 with -O3) takes about 27 msec
    My hand-optimised ARMv7 Assembly code takes about 7.8 msec
    My hand-optimised NEON Assembly code takes about 0.9 msec


    So I'm very happy to see the resizing function that was the bottleneck of my program is now 4200% faster! 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!

    Cheers,
    Shervin Emami.
    [url="http://www.shervinemami.co.cc/"]http://www.shervinemami.co.cc/[/url]
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