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NEON performance; Operators order;

Green Troll
Note: This was originally posted on 22nd October 2012 at http://forums.arm.com

Hi Guys!

I have 2 short codes, which give the equal result.

Code1
     
"vmull.s16    q8, d8, d0   \r\n"    //Col 0-3
      "vmlal.s16    q8, d9, d1   \r\n"
      "vmlal.s16    q8, d10, d2   \r\n"
      "vmlal.s16    q8, d11, d3   \r\n"

      "vmull.s16    q12, d12, d4   \r\n"  //Col 4-7
      "vmlal.s16    q12, d13, d5   \r\n"
      "vmlal.s16    q12, d14, d6   \r\n"
      "vmlal.s16    q12, d15, d7   \r\n"

      "vadd.i32  q8, q8, q12     \r\n"



Code2

      "vmull.s16    q8, d8, d0   \r\n"    //Col 0-3
      "vmull.s16    q12, d12, d4   \r\n"  //Col 4-7

      "vmlal.s16    q8, d9, d1   \r\n"
      "vmlal.s16    q12, d13, d5   \r\n"

      "vmlal.s16    q8, d10, d2   \r\n"
      "vmlal.s16    q12, d14, d6   \r\n"

      "vmlal.s16    q8, d11, d3   \r\n"
      "vmlal.s16    q12, d15, d7   \r\n"

      "vadd.i32  q8, q8, q12     \r\n"



It seems, that Code2 should be faster, because commands use different registers, so shouldn't wait previous one.
But in practise Code1 is faster (checked by timer in loop)

Could anybody explain it, please?

You are welcomed to offer better option than Code1/2 :)

Thanks in advance.

PS: The core is Cortex-A9!!!
Parents
  • Note: This was originally posted on 23rd October 2012 at http://forums.arm.com

    For instance, if you can have a register that has 1 in all of the fields you can replace that vadd at the end with another vmlal.


    Exophase, I knew there is a special forwarding path for VMLA, but I never imagined it can allow VMLA to be faster than VADD! Can you give any more info on why an extra VMLA would be faster than a VADD in this case (ie: at the end of the code shown here)? I would have expected VADD to always be faster.

    -Shervin.
Reply
  • Note: This was originally posted on 23rd October 2012 at http://forums.arm.com

    For instance, if you can have a register that has 1 in all of the fields you can replace that vadd at the end with another vmlal.


    Exophase, I knew there is a special forwarding path for VMLA, but I never imagined it can allow VMLA to be faster than VADD! Can you give any more info on why an extra VMLA would be faster than a VADD in this case (ie: at the end of the code shown here)? I would have expected VADD to always be faster.

    -Shervin.
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