GCC OPTIMIZATION

[Manan Chopra <mchopra@xxxxxxxx>]

Hi all,
  I am a Physics student.  We in our research group have been 
developing a molecular simulation software.
So to make our code run fast we are using gcc optimizations but i have 
got a case in which gcc shows different behaviour when run with or 
without optimization.
And i am not very sure how to correct this situation.

Below is the part of code in which i am seeing this behaviour. THIS IS 
BUGGY LINE is the exact line which gives different results with and 
without optimization.
The value of  x[i] at that point is 1.5(exact) value of box_xh is  2.1 
and rnx is 0.6 so icelx should evaluate to 6. which it does if i dont 
use optimization but when i switch optimization it gives icelx = 5. 
which is not correct. One moe thing i should mention real is double in 
my code.

So can some please tell me what i am doing wrong in this code.
I will really appreciate your help.
Thanks
-Manan



void make_clist(cl *clist,cord position,sim simul,syst sys,out 
*output,files all_files)
{
  //This code makes neighbour list
  real box_x = simul.box_x;
  real box_y = simul.box_y;
  real box_z = simul.box_z;

  real box_xh = simul.box_xh;
  real box_yh = simul.box_yh;
  real box_zh = simul.box_zh;

  int nx,ny,nz;
  nx = clist->nx;
  ny = clist->ny;
  nz = clist->nz;

  real rnx = box_x / nx;//width of cell in x direction
  real rny = box_y / ny;//width of cell in y direction
  real rnz = box_z / nz;//width of cell in z direction

  clist->rnx = rnx;
  clist->rny = rny;
  clist->rnz = rnz;


  int icel,i,icelx,icely,icelz,ncell;
  real *x,*y,*z;

  int *hoc,*fcell_list,*rcell_list;
  x = position.x;
  y = position.y;
  z = position.z;

  hoc = clist->hoc;
  fcell_list = clist->fcell_list;
  rcell_list = clist->rcell_list;
  ncell = clist->ncell;

  for(icel = 0;icel < ncell;icel++) hoc[icel] = -1;//All the cells have 
-1 on there top initially
  for(i = 0;i < simul.nsites;i++) fcell_list[i] = rcell_list[i] = 
-1;//Initialize the neighbour list
  for( i = 0;i < simul.nsites;i++)
  {
      icelx = floor((x[i] + box_xh) / rnx);//This is because the cells 
span fom -box_h to box_h //THIS IS BUGY LINE
      icely = floor((y[i] + box_yh) / rny);
      icelz = floor((z[i] + box_zh) / rnz);
      icel = icelx + icely * nx + icelz * nx * ny;
      //printf("%d\n",icelx);

#ifdef DEBUG2
      output->log_ptr += sprintf(output->log_ptr,"%d\t",icel+1);
      ioflush(output,all_files,false);
#endif

      fcell_list[i] = hoc[icel];
      if(hoc[icel] != -1) rcell_list[hoc[icel]] = i;
      hoc[icel] = i;
  }

#ifdef DEBUG2
  output->log_ptr += sprintf(output->log_ptr,"Forward nlist : ");
  ioflush(output,all_files,false);

  for(i = 0;i < simul.nsites;i++) output->log_ptr += 
sprintf(output->log_ptr,"%d  ",fcell_list[i]+1);
  ioflush(output,all_files,false);
    output->log_ptr += sprintf(output->log_ptr,"\n");
  ioflush(output,all_files,false);
    output->log_ptr += sprintf(output->log_ptr,"Backward nlist : ");
  ioflush(output,all_files,false);
    for(i = 0;i < simul.nsites;i++) output->log_ptr += 
sprintf(output->log_ptr,"%d  ",rcell_list[i]+1);
  ioflush(output,all_files,false);
    output->log_ptr += sprintf(output->log_ptr,"\n");
  ioflush(output,all_files,false);
    output->log_ptr += sprintf(output->log_ptr,"hoc list :");
  ioflush(output,all_files,false);
  for(i = 0;i < ncell;i++) output->log_ptr += 
sprintf(output->log_ptr,"%d  ",hoc[i]+1);
  ioflush(output,all_files,false);
    output->log_ptr += sprintf(output->log_ptr,"\n");
  ioflush(output,all_files,false);

#endif           }