• Locked Locked
  • Replies 2 replies
  • Subscribers 119 subscribers
  • Views 2953 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

CMSIS confusion

Pawel Surowka
Note: This was originally posted on 27th October 2012 at http://forums.arm.com

Hello all
I have few questions about CMSIS. I have reviewed some libs with basic functions and there is no loops for rest of the samples (Cortex M3/4), for example (Abs function q31):

/*loop Unrolling */
  blkCnt = blockSize >> 2u;

  /* First part of the processing with loop unrolling.  Compute 4 outputs at a time.  
   ** a second loop below computes the remaining 1 to 3 samples. */
  while(blkCnt > 0u)
  {
    /* C = |A| */
    /* Calculate absolute of input (if -1 then saturated to 0x7fffffff) and then store the results in the destination buffer. */
    in = *pSrc++;
    *pDst++ = (in > 0) ? in : ((in == 0x80000000) ? 0x7fffffff : -in);
    in = *pSrc++;
    *pDst++ = (in > 0) ? in : ((in == 0x80000000) ? 0x7fffffff : -in);
    in = *pSrc++;
    *pDst++ = (in > 0) ? in : ((in == 0x80000000) ? 0x7fffffff : -in);
    in = *pSrc++;
    *pDst++ = (in > 0) ? in : ((in == 0x80000000) ? 0x7fffffff : -in);

    /* Decrement the loop counter */
    blkCnt--;
  }

  /* If the blockSize is not a multiple of 4, compute any remaining output samples here.  
   ** No loop unrolling is used. */
  blkCnt = blockSize % 0x4u;


I think there should be a loop for remaining samples. Can you explain why there is 4 operations in one loop iteration? Is it related with time optimization?

Best Regards
Parents
  • Note: This was originally posted on 27th October 2012 at http://forums.arm.com

    Hi Surix,
    The DSP functions in the CMSIS library rely on loop unrolling for optimization.  The Q31 absolute value function is unrolled by a factor of 4.  Unrolling helps to reduce run time by:  (1) allowing memory accesses to be grouped together, (2) reducing the associated loop overhead (it takes 3 cycles to do a loop on the M4).  The first loop in the function arm_abs_q31.c does processing on multiples of 4 samples.  If you look further down in the code you'll see code that does any remaining samples:


      while(blkCnt > 0u)
      {
        /* C = |A| */
        /* Calculate absolute value of the input (if -1 then saturated to 0x7fffffff) and then store the results in the destination buffer. */
        in = *pSrc++;
        *pDst++ = (in > 0) ? in : ((in == 0x80000000) ? 0x7fffffff : -in);

        /* Decrement the loop counter */
        blkCnt--;
      }

    Structured this way, the function can handle any length vector.

    -Paul
Reply
  • Note: This was originally posted on 27th October 2012 at http://forums.arm.com

    Hi Surix,
    The DSP functions in the CMSIS library rely on loop unrolling for optimization.  The Q31 absolute value function is unrolled by a factor of 4.  Unrolling helps to reduce run time by:  (1) allowing memory accesses to be grouped together, (2) reducing the associated loop overhead (it takes 3 cycles to do a loop on the M4).  The first loop in the function arm_abs_q31.c does processing on multiples of 4 samples.  If you look further down in the code you'll see code that does any remaining samples:


      while(blkCnt > 0u)
      {
        /* C = |A| */
        /* Calculate absolute value of the input (if -1 then saturated to 0x7fffffff) and then store the results in the destination buffer. */
        in = *pSrc++;
        *pDst++ = (in > 0) ? in : ((in == 0x80000000) ? 0x7fffffff : -in);

        /* Decrement the loop counter */
        blkCnt--;
      }

    Structured this way, the function can handle any length vector.

    -Paul
Children
No data