From: Vincent Ma <vinm...@hotmail.com>
Subject: Re: Why C is much slower than Fortran
Date: 1999/04/12
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To: "Peter S. Shenkin" <shen...@schrodinger.com>
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Dear friends,

Thanks for the reply. Here are more detailed information:

-------------------------------------------------------------
#include <stdio.h>

void main()
{
float  E[36][21][52];
float  H1[36][21][52];
float  H2[36][21][52];

 short int    ID[43][31][52];
 float  A1[10];
 float  A2[10];
 float  A3[10];

    int    i,j,k,nlp,nmax;
    int    imax, jmax, kmax;

/* ============= set parameters ============== */

        imax           =   35;
        jmax           =   20;
        kmax           =   50;

/* ============== input nmax ================= */

        nmax = 1000;

/* -----------  set initial conditions ---------- */


 for(k = 1; k <= kmax-1; k++)
 {
  for(j = 1; j <= jmax-1; j++)
  {
   for(i = 1; i <= imax-1; i++)
   {

    ID[i][j][k] = 0;
    E[i][j][k] = 4.;
    H1[i][j][k] = 5.;
    H2[i][j][k] = 6.;
    A1[0] = 0.;
    A2[0] = 2.;
    A3[0] = 3.;
           }
  }
 }

/* ############################## */

        for (nlp = 0; nlp <= nmax; nlp++)
  {
            for(i = 0; i <= imax-1; i++)
   {
               for(j = 1; j <= jmax-1; j++)
      {
                  for(k = 1; k <= kmax-1; k++)
      {
                     E[i][j][k] = A1[ID[i][j][k]]*E[i][j][k]  + A2[ID[i][j][k]]
* (H1[i][j][k] - H1[i][j-1][k] + A3[ID[i][j][k]] * (H2[i][j][k] - H2[i][j][k-1]
);
                  }
               }
            }

  }

        return;

}

-----------------------------------------------------------------------------------------------------------------------

Fortran test code is same. I ran both on Pentium 133 with 64 meg RAM with MS
Powerstation and Visual C++ 5 with full optimization on. The execution time are

Fortran -- 11 sec (i loop innermost)
C -- 101 sec (i loop innermost)
C -- 62 sec   (k innermost).

The difference is still a lot (6 times). I've tried many variations, it seems no
matter what I did, C is always slower. My previous experience on workstation
told me the same story that fortran is much faster than C (at least for this
scientific computing case).