Re-apply reverted niter change 1/4

[Jan Hubicka]

> > Index: tree-ssa-loop-ivopts.c
> > ===================================================================
> > --- tree-ssa-loop-ivopts.c      (revision 235064)
> > +++ tree-ssa-loop-ivopts.c      (working copy)
> > @@ -121,7 +121,11 @@ avg_loop_niter (struct loop *loop)
> >  {
> >    HOST_WIDE_INT niter = estimated_stmt_executions_int (loop);
> >    if (niter == -1)
> > -    return AVG_LOOP_NITER (loop);
> > +    {
> > +      niter = max_stmt_executions_int (loop);
> > +      if (niter == -1 || niter > AVG_LOOP_NITER (loop))
> > +        return AVG_LOOP_NITER (loop);
> Any reason why AVG_LOOP_NITER is still used if niter gives larger number?

if you have a loop like this

int a[1000000];

for (i=0li<1000000;i++)
  if (a[i])
    break
max_stmt_executions_int will be 1000000 but that is just upper bound, not realistic
estimate and thus you can not assume that average number of iterations is 1000000.
It is anywhere between 0 and 1000000 and I assume the constant of 5 which AVG_LOOP_NITER
expands into was chosen to avoid ivopts to give resonable balance between setup cost
and iteration cost. For example, string manipulation loops tends to get large buffers
and terminate in very few iterations.

(I do not recall the data precisely, it has been a decade. The average number
of iterations of random loop can be measured from profile feedback, it is
somewhere between 3 adn 10 for SPEC or GCC).
This is why:
/* Loopback edge is taken.  */                                                  
DEF_PREDICTOR (PRED_LOOP_BRANCH, "loop branch", HITRATE (86),                   
               PRED_FLAG_FIRST_MATCH)                                           
                                                                                
/* Edge causing loop to terminate is probably not taken.  */                    
DEF_PREDICTOR (PRED_LOOP_EXIT, "loop exit", HITRATE (91),                       
               PRED_FLAG_FIRST_MATCH)                                           

is set accordingly.

Honza
> 
> Thanks,
> bin
> > +    }
> >
> >    return niter;
> >  }