请问循环展开（loop unrolling）相对于M0 与 M3 有何不同

只看该作者 · 2017-8-28 11:05

在与一个朋友讨论时，发现 CMSIS DSP_lib 的函数中，对于 M3核进行了循环展开，而M0并没有
请问 M3核哪些特性循环展开后效率相对于M0核来说效率更高？

-------------------------  以下是 CMSIS DSP_lib 的一个函数  ------------------------------------------------

/* ----------------------------------------------------------------------
* Copyright (C) 2010 ARM Limited. All rights reserved.
*
* $Date:       15. July 2011
* $Revision: V1.0.10
*
* Project:       CMSIS DSP Library
* Title:       arm_mult_f32.c
*
* Description: Floating-point vector multiplication.
*
* Target Processor: Cortex-M4/Cortex-M3/Cortex-M0
*
* Version 1.0.10 2011/7/15
* Big Endian support added and Merged M0 and M3/M4 Source code.
*
* Version 1.0.3 2010/11/29
* Re-organized the CMSIS folders and updated documentation.
*
* Version 1.0.2 2010/11/11
* Documentation updated.
*
* Version 1.0.1 2010/10/05
* Production release and review comments incorporated.
*
* Version 1.0.0 2010/09/20
* Production release and review comments incorporated.
*
* Version 0.0.5  2010/04/26
* incorporated review comments and updated with latest CMSIS layer
*
* Version 0.0.3  2010/03/10
* Initial version
* -------------------------------------------------------------------- */

#include "arm_math.h"

/**
* @ingroup groupMath
*/

/**
* @defgroup BasicMult Vector Multiplication
*
* Element-by-element multiplication of two vectors.
*
* <pre>
*    pDst[n] = pSrcA[n] * pSrcB[n], 0 <= n < blockSize.
* </pre>
*
* There are separate functions for floating-point, Q7, Q15, and Q31 data types.
*/

/**
* @addtogroup BasicMult
* @{
*/

/**
* @brief Floating-point vector multiplication.
* @param[in]    *pSrcA points to the first input vector
* @param[in]    *pSrcB points to the second input vector
* @param[out]    *pDst points to the output vector
* @param[in]    blockSize number of samples in each vector
* @return none.
*/

void arm_mult_f32(
  float32_t * pSrcA,
  float32_t * pSrcB,
  float32_t * pDst,
  uint32_t blockSize)
{
  uint32_t blkCnt;                            /* loop counters */

#ifndef ARM_MATH_CM0

/* Run the below code for Cortex-M4 and Cortex-M3 */

  /* 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 * B */
/* Multiply the inputs and store the results in output buffer */
*pDst++ = (*pSrcA++) * (*pSrcB++);
*pDst++ = (*pSrcA++) * (*pSrcB++);
*pDst++ = (*pSrcA++) * (*pSrcB++);
*pDst++ = (*pSrcA++) * (*pSrcB++);

/* Decrement the blockSize 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;

#else

  /* Run the below code for Cortex-M0 */

  /* Initialize blkCnt with number of samples */
  blkCnt = blockSize;

#endif /* #ifndef ARM_MATH_CM0 */

  while(blkCnt > 0u)
  {
/* C = A * B */
/* Multiply the inputs and store the results in output buffer */
*pDst++ = (*pSrcA++) * (*pSrcB++);

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

}

/**
* @} end of BasicMult group
*/