arm_fir_fast_q15.c

/* ----------------------------------------------------------------------    
* Copyright (C) 2010-2014 ARM Limited. All rights reserved.    
*    
* $Date:        19. March 2015
* $Revision: 	V.1.4.5
*    
* Project: 	    CMSIS DSP Library    
* Title:        arm_fir_fast_q15.c    
*    
* Description:  Q15 Fast FIR filter processing function.    
*    
* Target Processor: Cortex-M4/Cortex-M3
*  
* Redistribution and use in source and binary forms, with or without 
* modification, are permitted provided that the following conditions
* are met:
*   - Redistributions of source code must retain the above copyright
*     notice, this list of conditions and the following disclaimer.
*   - Redistributions in binary form must reproduce the above copyright
*     notice, this list of conditions and the following disclaimer in
*     the documentation and/or other materials provided with the 
*     distribution.
*   - Neither the name of ARM LIMITED nor the names of its contributors
*     may be used to endorse or promote products derived from this
*     software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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* ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE. 
* -------------------------------------------------------------------- */

#include "arm_math.h"

/**    
 * @ingroup groupFilters    
 */

/**    
 * @addtogroup FIR    
 * @{    
 */

/**    
 * @param[in] *S points to an instance of the Q15 FIR filter structure.    
 * @param[in] *pSrc points to the block of input data.    
 * @param[out] *pDst points to the block of output data.    
 * @param[in] blockSize number of samples to process per call.    
 * @return none.    
 *    
 * <b>Scaling and Overflow Behavior:</b>    
 * \par    
 * This fast version uses a 32-bit accumulator with 2.30 format.    
 * The accumulator maintains full precision of the intermediate multiplication results but provides only a single guard bit.    
 * Thus, if the accumulator result overflows it wraps around and distorts the result.    
 * In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits.    
 * The 2.30 accumulator is then truncated to 2.15 format and saturated to yield the 1.15 result.    
 *    
 * \par    
 * Refer to the function <code>arm_fir_q15()</code> for a slower implementation of this function which uses 64-bit accumulation to avoid wrap around distortion.  Both the slow and the fast versions use the same instance structure.    
 * Use the function <code>arm_fir_init_q15()</code> to initialize the filter structure.    
 */

void arm_fir_fast_q15(
  const arm_fir_instance_q15 * S,
  q15_t * pSrc,
  q15_t * pDst,
  uint32_t blockSize)
{
  q15_t *pState = S->pState;                     /* State pointer */
  q15_t *pCoeffs = S->pCoeffs;                   /* Coefficient pointer */
  q15_t *pStateCurnt;                            /* Points to the current sample of the state */
  q31_t acc0, acc1, acc2, acc3;                  /* Accumulators */
  q15_t *pb;                                     /* Temporary pointer for coefficient buffer */
  q15_t *px;                                     /* Temporary q31 pointer for SIMD state buffer accesses */
  q31_t x0, x1, x2, c0;                          /* Temporary variables to hold SIMD state and coefficient values */
  uint32_t numTaps = S->numTaps;                 /* Number of taps in the filter */
  uint32_t tapCnt, blkCnt;                       /* Loop counters */


  /* S->pState points to state array which contains previous frame (numTaps - 1) samples */
  /* pStateCurnt points to the location where the new input data should be written */
  pStateCurnt = &(S->pState[(numTaps - 1u)]);

  /* Apply loop unrolling and compute 4 output values simultaneously.      
   * The variables acc0 ... acc3 hold output values that are being computed:      
   *      
   *    acc0 =  b[numTaps-1] * x[n-numTaps-1] + b[numTaps-2] * x[n-numTaps-2] + b[numTaps-3] * x[n-numTaps-3] +...+ b[0] * x[0]      
   *    acc1 =  b[numTaps-1] * x[n-numTaps] +   b[numTaps-2] * x[n-numTaps-1] + b[numTaps-3] * x[n-numTaps-2] +...+ b[0] * x[1]      
   *    acc2 =  b[numTaps-1] * x[n-numTaps+1] + b[numTaps-2] * x[n-numTaps] +   b[numTaps-3] * x[n-numTaps-1] +...+ b[0] * x[2]      
   *    acc3 =  b[numTaps-1] * x[n-numTaps+2] + b[numTaps-2] * x[n-numTaps+1] + b[numTaps-3] * x[n-numTaps]   +...+ b[0] * x[3]      
   */

  blkCnt = blockSize >> 2;

  /* 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)
  {
    /* Copy four new input samples into the state buffer.      
     ** Use 32-bit SIMD to move the 16-bit data.  Only requires two copies. */
    *pStateCurnt++ = *pSrc++;
    *pStateCurnt++ = *pSrc++;
    *pStateCurnt++ = *pSrc++;
    *pStateCurnt++ = *pSrc++;


    /* Set all accumulators to zero */
    acc0 = 0;
    acc1 = 0;
    acc2 = 0;
    acc3 = 0;

    /* Typecast q15_t pointer to q31_t pointer for state reading in q31_t */
    px = pState;

    /* Typecast q15_t pointer to q31_t pointer for coefficient reading in q31_t */
    pb = pCoeffs;

    /* Read the first two samples from the state buffer:  x[n-N], x[n-N-1] */
    x0 = *__SIMD32(px)++;

    /* Read the third and forth samples from the state buffer: x[n-N-2], x[n-N-3] */
    x2 = *__SIMD32(px)++;

    /* Loop over the number of taps.  Unroll by a factor of 4.      
     ** Repeat until we've computed numTaps-(numTaps%4) coefficients. */
    tapCnt = numTaps >> 2;

    while(tapCnt > 0)
    {
      /* Read the first two coefficients using SIMD:  b[N] and b[N-1] coefficients */
      c0 = *__SIMD32(pb)++;

      /* acc0 +=  b[N] * x[n-N] + b[N-1] * x[n-N-1] */
      acc0 = __SMLAD(x0, c0, acc0);

      /* acc2 +=  b[N] * x[n-N-2] + b[N-1] * x[n-N-3] */
      acc2 = __SMLAD(x2, c0, acc2);

      /* pack  x[n-N-1] and x[n-N-2] */
#ifndef ARM_MATH_BIG_ENDIAN
      x1 = __PKHBT(x2, x0, 0);
#else
      x1 = __PKHBT(x0, x2, 0);
#endif

      /* Read state x[n-N-4], x[n-N-5] */
      x0 = _SIMD32_OFFSET(px);

      /* acc1 +=  b[N] * x[n-N-1] + b[N-1] * x[n-N-2] */
      acc1 = __SMLADX(x1, c0, acc1);

      /* pack  x[n-N-3] and x[n-N-4] */
#ifndef ARM_MATH_BIG_ENDIAN
      x1 = __PKHBT(x0, x2, 0);
#else
      x1 = __PKHBT(x2, x0, 0);
#endif

      /* acc3 +=  b[N] * x[n-N-3] + b[N-1] * x[n-N-4] */
      acc3 = __SMLADX(x1, c0, acc3);

      /* Read coefficients b[N-2], b[N-3] */
      c0 = *__SIMD32(pb)++;

      /* acc0 +=  b[N-2] * x[n-N-2] + b[N-3] * x[n-N-3] */
      acc0 = __SMLAD(x2, c0, acc0);

      /* Read state x[n-N-6], x[n-N-7] with offset */
      x2 = _SIMD32_OFFSET(px + 2u);

      /* acc2 +=  b[N-2] * x[n-N-4] + b[N-3] * x[n-N-5] */
      acc2 = __SMLAD(x0, c0, acc2);

      /* acc1 +=  b[N-2] * x[n-N-3] + b[N-3] * x[n-N-4] */
      acc1 = __SMLADX(x1, c0, acc1);

      /* pack  x[n-N-5] and x[n-N-6] */
#ifndef ARM_MATH_BIG_ENDIAN
      x1 = __PKHBT(x2, x0, 0);
#else
      x1 = __PKHBT(x0, x2, 0);
#endif

      /* acc3 +=  b[N-2] * x[n-N-5] + b[N-3] * x[n-N-6] */
      acc3 = __SMLADX(x1, c0, acc3);

      /* Update state pointer for next state reading */
      px += 4u;

      /* Decrement tap count */
      tapCnt--;

    }

    /* If the filter length is not a multiple of 4, compute the remaining filter taps.       
     ** This is always be 2 taps since the filter length is even. */
    if((numTaps & 0x3u) != 0u)
    {

      /* Read last two coefficients */
      c0 = *__SIMD32(pb)++;

      /* Perform the multiply-accumulates */
      acc0 = __SMLAD(x0, c0, acc0);
      acc2 = __SMLAD(x2, c0, acc2);

      /* pack state variables */
#ifndef ARM_MATH_BIG_ENDIAN
      x1 = __PKHBT(x2, x0, 0);
#else
      x1 = __PKHBT(x0, x2, 0);
#endif

      /* Read last state variables */
      x0 = *__SIMD32(px);

      /* Perform the multiply-accumulates */
      acc1 = __SMLADX(x1, c0, acc1);

      /* pack state variables */
#ifndef ARM_MATH_BIG_ENDIAN
      x1 = __PKHBT(x0, x2, 0);
#else
      x1 = __PKHBT(x2, x0, 0);
#endif

      /* Perform the multiply-accumulates */
      acc3 = __SMLADX(x1, c0, acc3);
    }

    /* The results in the 4 accumulators are in 2.30 format.  Convert to 1.15 with saturation.       
     ** Then store the 4 outputs in the destination buffer. */

#ifndef ARM_MATH_BIG_ENDIAN

    *__SIMD32(pDst)++ =
      __PKHBT(__SSAT((acc0 >> 15), 16), __SSAT((acc1 >> 15), 16), 16);

    *__SIMD32(pDst)++ =
      __PKHBT(__SSAT((acc2 >> 15), 16), __SSAT((acc3 >> 15), 16), 16);

#else

    *__SIMD32(pDst)++ =
      __PKHBT(__SSAT((acc1 >> 15), 16), __SSAT((acc0 >> 15), 16), 16);

    *__SIMD32(pDst)++ =
      __PKHBT(__SSAT((acc3 >> 15), 16), __SSAT((acc2 >> 15), 16), 16);


#endif /*      #ifndef ARM_MATH_BIG_ENDIAN       */

    /* Advance the state pointer by 4 to process the next group of 4 samples */
    pState = pState + 4u;

    /* 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;
  while(blkCnt > 0u)
  {
    /* Copy two samples into state buffer */
    *pStateCurnt++ = *pSrc++;

    /* Set the accumulator to zero */
    acc0 = 0;

    /* Use SIMD to hold states and coefficients */
    px = pState;
    pb = pCoeffs;

    tapCnt = numTaps >> 1u;

    do
    {

      acc0 += (q31_t) * px++ * *pb++;
	  acc0 += (q31_t) * px++ * *pb++;

      tapCnt--;
    }
    while(tapCnt > 0u);

    /* The result is in 2.30 format.  Convert to 1.15 with saturation.      
     ** Then store the output in the destination buffer. */
    *pDst++ = (q15_t) (__SSAT((acc0 >> 15), 16));

    /* Advance state pointer by 1 for the next sample */
    pState = pState + 1u;

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

  /* Processing is complete.      
   ** Now copy the last numTaps - 1 samples to the satrt of the state buffer.      
   ** This prepares the state buffer for the next function call. */

  /* Points to the start of the state buffer */
  pStateCurnt = S->pState;

  /* Calculation of count for copying integer writes */
  tapCnt = (numTaps - 1u) >> 2;

  while(tapCnt > 0u)
  {
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;

    tapCnt--;

  }

  /* Calculation of count for remaining q15_t data */
  tapCnt = (numTaps - 1u) % 0x4u;

  /* copy remaining data */
  while(tapCnt > 0u)
  {
    *pStateCurnt++ = *pState++;

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

}

/**    
 * @} end of FIR group    
 */