On Tue, Apr 19, 2016 at 12:09 PM, Jan Hubicka <hubi...@ucw.cz> wrote:
>> > 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.
Thanks for explanation.
Thanks,
bin
