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  1. double vectors_dot_prod(const double *x, const double *y, int n)
  2. {
  3.     double res = 0.0;
  4.     int i;
  5.     for (i = 0; i < n; i++)
  6.     {
  7.         res += x[i] * y[i];
  8.     }
  9.     return res;
  10. }
  11.  
  12. void matrix_vector_mult(const double **mat, const double *vec,
  13.                         double *result, int rows, int cols)
  14. { // in matrix form: result = mat * vec;
  15.     int i;
  16.     for (i = 0; i < rows; i++)
  17.     {
  18.         result[i] = vectors_dot_prod(mat[i], vec, cols);
  19.     }
  20. }
  21.  
  22. double vectors_dot_prod2(const double *x, const double *y, int n)
  23. {
  24.     double res = 0.0;
  25.     int i = 0;
  26.     for (; i <= n-4; i+=4)
  27.     {
  28.         res += (x[i] * y[i] +
  29.                 x[i+1] * y[i+1] +
  30.                 x[i+2] * y[i+2] +
  31.                 x[i+3] * y[i+3]);
  32.     }
  33.     for (; i < n; i++)
  34.     {
  35.         res += x[i] * y[i];
  36.     }
  37.     return res;
  38. }
  39.  
  40. void matrix_vector_mult2(const double **mat, const double *vec,
  41.                          double *result, int rows, int cols)
  42. { // in matrix form: result = mat * vec;
  43.     int i;
  44.     for (i = 0; i < rows; i++)
  45.     {
  46.         result[i] = vectors_dot_prod2(mat[i], vec, cols);
  47.     }
  48. }
  49.  
  50. #include <time.h>
  51. #include <stdio.h>
  52.  
  53. int main(int argc, const char *argv[])
  54. {
  55.     static double mat[300][50];
  56.     for (int i=0; i<300; i++)
  57.         for (int j=0; j<50; j++)
  58.             mat[i][j] = (i+j);
  59.     static const double *matp[300];
  60.     for (int i=0; i<300; i++)
  61.         matp[i] = &mat[i][0];
  62.  
  63.     static double vector[50];
  64.     for (int i=0; i<50; i++)
  65.         vector[i] = i*i;
  66.  
  67.     static double result[300];
  68.  
  69.     clock_t start = clock();
  70.     for (int n=0; n<100000; n++)
  71.     {
  72.         matrix_vector_mult(matp, vector, result, 300, 50);
  73.     }
  74.     clock_t stop = clock();
  75.     printf("Computing time = %0.3fus\n",
  76.            double(stop - start)/CLOCKS_PER_SEC/100000*1000000);
  77.     return 0;
  78. }
  79.  
Success #stdin #stdout 2.95s 2844KB
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Standard input is empty
stdout
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Computing time = 29.700us
https://ideone.com/g55tV
language:
C++ (gcc 8.3)
created:
11 years ago
visibility:
public

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