DSP/Source/FilteringFunctions/arm_fir_fast_q31.c

/* ----------------------------------------------------------------------
 * Project:      CMSIS DSP Library
 * Title:        arm_fir_fast_q31.c
 * Description:  Processing function for the Q31 Fast FIR filter
 *
 * $Date:        27. January 2017
 * $Revision:    V.1.5.1
 *
 * Target Processor: Cortex-M cores
 * -------------------------------------------------------------------- */
/*
 * Copyright (C) 2010-2017 ARM Limited or its affiliates. All rights reserved.
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Licensed under the Apache License, Version 2.0 (the License); you may
 * not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an AS IS BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "arm_math.h"

/**
 * @ingroup groupFilters
 */

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

/**
 * @param[in] *S points to an instance of the Q31 structure.
 * @param[in] *pSrc points to the block of input data.
 * @param[out] *pDst points to the block output data.
 * @param[in] blockSize number of samples to process per call.
 * @return none.
 *
 * <b>Scaling and Overflow Behavior:</b>
 *
 * \par
 * This function is optimized for speed at the expense of fixed-point precision and overflow protection.
 * The result of each 1.31 x 1.31 multiplication is truncated to 2.30 format.
 * These intermediate results are added to a 2.30 accumulator.
 * Finally, the accumulator is saturated and converted to a 1.31 result.
 * The fast version has the same overflow behavior as the standard version and provides less precision since it discards the low 32 bits of each multiplication result.
 * In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits.
 *
 * \par
 * Refer to the function <code>arm_fir_q31()</code> for a slower implementation of this function which uses a 64-bit accumulator to provide higher precision.  Both the slow and the fast versions use the same instance structure.
 * Use the function <code>arm_fir_init_q31()</code> to initialize the filter structure.
 */

IAR_ONLY_LOW_OPTIMIZATION_ENTER
void arm_fir_fast_q31(
  const arm_fir_instance_q31 * S,
  q31_t * pSrc,
  q31_t * pDst,
  uint32_t blockSize)
{
  q31_t *pState = S->pState;                     /* State pointer */
  q31_t *pCoeffs = S->pCoeffs;                   /* Coefficient pointer */
  q31_t *pStateCurnt;                            /* Points to the current sample of the state */
  q31_t x0, x1, x2, x3;                          /* Temporary variables to hold state */
  q31_t c0;                                      /* Temporary variable to hold coefficient value */
  q31_t *px;                                     /* Temporary pointer for state */
  q31_t *pb;                                     /* Temporary pointer for coefficient buffer */
  q31_t acc0, acc1, acc2, acc3;                  /* Accumulators */
  uint32_t numTaps = S->numTaps;                 /* Number of filter coefficients in the filter */
  uint32_t i, tapCnt, blkCnt;                    /* Loop counters */

  /* S->pState points to buffer 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 */
    *pStateCurnt++ = *pSrc++;
    *pStateCurnt++ = *pSrc++;
    *pStateCurnt++ = *pSrc++;
    *pStateCurnt++ = *pSrc++;

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

    /* Initialize state pointer */
    px = pState;

    /* Initialize coefficient pointer */
    pb = pCoeffs;

    /* Read the first three samples from the state buffer:
     *  x[n-numTaps], x[n-numTaps-1], x[n-numTaps-2] */
    x0 = *(px++);
    x1 = *(px++);
    x2 = *(px++);

    /* Loop unrolling.  Process 4 taps at a time. */
    tapCnt = numTaps >> 2;
    i = tapCnt;

    while (i > 0U)
    {
      /* Read the b[numTaps] coefficient */
      c0 = *pb;

      /* Read x[n-numTaps-3] sample */
      x3 = *px;

      /* acc0 +=  b[numTaps] * x[n-numTaps] */
      multAcc_32x32_keep32_R(acc0, x0, c0);

      /* acc1 +=  b[numTaps] * x[n-numTaps-1] */
      multAcc_32x32_keep32_R(acc1, x1, c0);

      /* acc2 +=  b[numTaps] * x[n-numTaps-2] */
      multAcc_32x32_keep32_R(acc2, x2, c0);

      /* acc3 +=  b[numTaps] * x[n-numTaps-3] */
      multAcc_32x32_keep32_R(acc3, x3, c0);

      /* Read the b[numTaps-1] coefficient */
      c0 = *(pb + 1U);

      /* Read x[n-numTaps-4] sample */
      x0 = *(px + 1U);

      /* Perform the multiply-accumulates */
      multAcc_32x32_keep32_R(acc0, x1, c0);
      multAcc_32x32_keep32_R(acc1, x2, c0);
      multAcc_32x32_keep32_R(acc2, x3, c0);
      multAcc_32x32_keep32_R(acc3, x0, c0);

      /* Read the b[numTaps-2] coefficient */
      c0 = *(pb + 2U);

      /* Read x[n-numTaps-5] sample */
      x1 = *(px + 2U);

      /* Perform the multiply-accumulates */
      multAcc_32x32_keep32_R(acc0, x2, c0);
      multAcc_32x32_keep32_R(acc1, x3, c0);
      multAcc_32x32_keep32_R(acc2, x0, c0);
      multAcc_32x32_keep32_R(acc3, x1, c0);

      /* Read the b[numTaps-3] coefficients */
      c0 = *(pb + 3U);

      /* Read x[n-numTaps-6] sample */
      x2 = *(px + 3U);

      /* Perform the multiply-accumulates */
      multAcc_32x32_keep32_R(acc0, x3, c0);
      multAcc_32x32_keep32_R(acc1, x0, c0);
      multAcc_32x32_keep32_R(acc2, x1, c0);
      multAcc_32x32_keep32_R(acc3, x2, c0);

      /* update coefficient pointer */
      pb += 4U;
      px += 4U;

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

    /* If the filter length is not a multiple of 4, compute the remaining filter taps */

    i = numTaps - (tapCnt * 4U);
    while (i > 0U)
    {
      /* Read coefficients */
      c0 = *(pb++);

      /* Fetch 1 state variable */
      x3 = *(px++);

      /* Perform the multiply-accumulates */
      multAcc_32x32_keep32_R(acc0, x0, c0);
      multAcc_32x32_keep32_R(acc1, x1, c0);
      multAcc_32x32_keep32_R(acc2, x2, c0);
      multAcc_32x32_keep32_R(acc3, x3, c0);

      /* Reuse the present sample states for next sample */
      x0 = x1;
      x1 = x2;
      x2 = x3;

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

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

    /* The results in the 4 accumulators are in 2.30 format.  Convert to 1.31
     ** Then store the 4 outputs in the destination buffer. */
    *pDst++ = (q31_t) (acc0 << 1);
    *pDst++ = (q31_t) (acc1 << 1);
    *pDst++ = (q31_t) (acc2 << 1);
    *pDst++ = (q31_t) (acc3 << 1);

    /* Decrement the samples 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 % 4U;

  while (blkCnt > 0U)
  {
    /* Copy one sample at a time into state buffer */
    *pStateCurnt++ = *pSrc++;

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

    /* Initialize state pointer */
    px = pState;

    /* Initialize Coefficient pointer */
    pb = (pCoeffs);

    i = numTaps;

    /* Perform the multiply-accumulates */
    do
    {
      multAcc_32x32_keep32_R(acc0, (*px++), (*(pb++)));
      i--;
    } while (i > 0U);

    /* The result is in 2.30 format.  Convert to 1.31
     ** Then store the output in the destination buffer. */
    *pDst++ = (q31_t) (acc0 << 1);

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

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

  /* Processing is complete.
   ** Now copy the last numTaps - 1 samples to the start 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;

  /* Calculate remaining number of copies */
  tapCnt = (numTaps - 1U);

  /* Copy the remaining q31_t data */
  while (tapCnt > 0U)
  {
    *pStateCurnt++ = *pState++;

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


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