• Locked Locked
  • Replies 19 replies
  • Subscribers 119 subscribers
  • Views 10954 views
  • Users 0 members are here
2025 survey

We are running a survey to help us improve the experience for all of our members. If you see the survey appear, please take the time to tell us about your experience if you can.

Options
Related
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Cortex-A8 NEON strange performance issue

Jake Lee
Note: This was originally posted on 10th July 2013 at http://forums.arm.com

Hi all,

As an experienced assembly programmer, I'm currently working on NEON tutorials.

First example done, it beats the original C function by a huge margin :


pld [r1, #64*0]
pld [r1, #64*1]
pld [r1, #64*2]
pld [r1, #64*3]

    ldr  r12, [sp]
    vdup.16 d0, r2 //coeff
    vdup.16 d1, r3 //intercept

1:
        pld  [r1, #64*4]
        vld1.16  {d28-d31}, [r1,:256]!

        vmull.s16   q12, d28, d0[0]
        vmull.s16   q13, d29, d0[0]
        vmull.s16   q14, d30, d0[0]
        vmull.s16   q15, d31, d0[0]

        vaddw.s16   q12, q12, d1
        vaddw.s16   q13, q13, d1
        vaddw.s16   q14, q14, d1
        vaddw.s16   q15, q15, d1

        vqrshrun.s32    d24, q12, #8
        vqrshrun.s32    d25, q13, #8
        vqrshrun.s32    d26, q14, #8
        vqrshrun.s32    d27, q15, #8

        vst1.16  {d24-d27}, [r0,:256]!

        subs    r12, r12, #16
    bgt  1b
    bx      lr

However, there are some pipeline stalls here and there, and the dual-issuing isn't fully in effect. So I decided to optimize it :



pld  [r1, #64*0]
    pld  [r1, #64*1]
    pld  [r1, #64*2]
    pld  [r1, #64*3]
    pld  [r1, #64*4]
    ldr  r12, [sp]
    vdup.16 d0, r2 //coeff
    vdup.16 d1, r3 //intercept
    vld1.16  {d28-d31}, [r1,:256]!
    vmull.s16   q12, d28, d0[0]
    vmull.s16   q13, d29, d0[0]
    vmull.s16   q14, d30, d0[0]
    vmull.s16   q15, d31, d0[0]
    1:
        vaddw.s16   q12, q12, d1
        vld1.16  {d20-d23}, [r1,:256]!
        vaddw.s16   q13, q13, d1
        vaddw.s16   q14, q14, d1
        vaddw.s16   q15, q15, d1
        vqrshrun.s32    d24, q12, #8
        vqrshrun.s32    d25, q13, #8
        vqrshrun.s32    d26, q14, #8
        vqrshrun.s32    d27, q15, #8
        vmull.s16   q8, d20, d0[0]
        vmull.s16   q9, d21, d0[0]
        vmull.s16   q10, d22, d0[0]
        vmull.s16   q11, d23, d0[0]
        vst1.16  {d24-d27}, [r0,:256]!
        vaddw.s16   q8, q8, d1
        vaddw.s16   q9, q9, d1
        vaddw.s16   q10, q10, d1
        vaddw.s16   q11, q11, d1
        subs    r12, r12, #32
        vqrshrun.s32    d16, q8, #8
        vqrshrun.s32    d17, q9, #8
        vqrshrun.s32    d18, q10, #8
        ble  2f
        pld  [r1, #64*4]
        vld1.16  {d28-d31}, [r1,:256]!
        vqrshrun.s32    d19, q11, #8
        vmull.s16   q12, d28, d0[0]
        vmull.s16   q13, d29, d0[0]
        vmull.s16   q14, d30, d0[0]
        vst1.16  {d16-d19}, [r0,:256]!
        vmull.s16   q15, d31, d0[0]
    b 1b
    2:
    vqrshrun.s32    d19, q11, #8
    vst1.16  {d16-d19}, [r0,:256]!
    bx      lr

There is no pipeline stall at all in this optimized version, and all the memory accessing instructions are surrounded by data processing ones, so they get dual issued twice - at top and bottom. - looks good~

Then the bitter surprise comes while benchmarking on my 4th gen iPod touch and iPhone4 though (iOS6.13 if it matters) : The "optimized" version is about 5% slower than the initial one.


(r12 gets the value of a full HD resolution(1920*1080), and both functions are called several thousand times in a row)

I've been trying several variations, repositioning subs and pld around, nothing matters, the optimized version is ALWAYS slower.

And I removed the pld instruction completely, both functions become much slower as expected, but voila, the optimized version is now about 5% faster.


It seems I did the right thing in removing pipeline stalls and unfolding dual issuing capability, but I must have done something the L2 cache doesn't like in the optimized version.

Can someone give me some insight regarding this? I'm really curious, desperate, frustrated or whatever since without knowing the reason of this strange behavior, all the efforts I put in mastering NEON would be in vain.


Thanks in advance.
Parents
  • Note: This was originally posted on 13th July 2013 at http://forums.arm.com


    Yes I think Webshaker's idea will give you good results, because it is easy to read ARM & NEON docs and think that counting the CPU clock cycles for your instructions will be enough to estimate the total time of your loop, when in fact most code in mobile spends more time loading & saving memory than performing CPU calculations. So your main goal should be to minimize the amount of time wasted while waiting for data to load & store. This usually means accessing memory in good cache-friendly ways, using main memory the least you can inside your loop, AND trying to increase the number of useful operations that happen between loading a value from memory and actually using it in another instruction.

    Also, having longer loops can sometimes reduce the speed of your code, so this might also be influencing your perf.


    I've been trying about 20 variations, long loops, short loops,,,,, including what webshaker suggested and what you are talking about.

    However, the unoptimized original is ALWAYS the fastest as soon as PLD is enabled.

    There must be still something we aren't aware of....
Reply
  • Note: This was originally posted on 13th July 2013 at http://forums.arm.com


    Yes I think Webshaker's idea will give you good results, because it is easy to read ARM & NEON docs and think that counting the CPU clock cycles for your instructions will be enough to estimate the total time of your loop, when in fact most code in mobile spends more time loading & saving memory than performing CPU calculations. So your main goal should be to minimize the amount of time wasted while waiting for data to load & store. This usually means accessing memory in good cache-friendly ways, using main memory the least you can inside your loop, AND trying to increase the number of useful operations that happen between loading a value from memory and actually using it in another instruction.

    Also, having longer loops can sometimes reduce the speed of your code, so this might also be influencing your perf.


    I've been trying about 20 variations, long loops, short loops,,,,, including what webshaker suggested and what you are talking about.

    However, the unoptimized original is ALWAYS the fastest as soon as PLD is enabled.

    There must be still something we aren't aware of....
Children
No data