DSP优化C循环的例子

#define WORD_ALIGNED(x) (_nassert(((int)(x) & 0x3) == 0))

/*输入参数加上关键字restrict，内存独立声明，xptr yptr指向的区域不重合*/

void lesson3_c(short * restrict xptr, short * restrict yptr, short *zptr,

               short *w_sum, int N)

{

    int i, w_vec1, w_vec2;

    short w1, w2;

     

    /*保证xptr yptr是四字节对齐的，编程者负责要保证这一点*/                            

    WORD_ALIGNED(xptr);

    WORD_ALIGNED(yptr);                                  

                                      

    w1 = zptr[0];

    w2 = zptr[1];

 

    /*最小循环20次，循环的次数是2的倍数*/

    #pragma MUST_ITERATE(20, , 2);

    for (i = 0; i < N; i++)

    {

        w_vec1 =  xptr * w1;

        w_vec2 =  yptr * w2;

        w_sum = (w_vec1 + w_vec2) >> 15;

    }

}