GCC Optimization.

[Manan <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. Red 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.

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
       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           
}