> Are you suggesting a patch like this:
>
> Index: gcc/predict.c
> ===================================================================
> --- gcc/predict.c (revision 189835)
> +++ gcc/predict.c (working copy)
> @@ -1319,6 +1319,7 @@
> tree loop_bound_var = NULL;
> tree loop_iv_base = NULL;
> gimple stmt = NULL;
> + int header_found = 0;
We should use bool these days.
>
> exits = get_loop_exit_edges (loop);
> n_exits = VEC_length (edge, exits);
> @@ -1387,9 +1388,20 @@
>
> bbs = get_loop_body (loop);
>
> + /* Loop branch heuristics - predict an edge back to a
> + loop's head as taken. */
> + if (loop->latch && loop->latch->loop_father == loop)
> + {
> + edge e = find_edge (loop->latch, loop->header);
> + if (e)
> + {
> + header_found = 1;
> + predict_edge_def (e, PRED_LOOP_BRANCH, TAKEN);
> + }
> + }
> +
> for (j = 0; j < loop->num_nodes; j++)
> {
> - int header_found = 0;
> edge e;
> edge_iterator ei;
>
> @@ -1402,21 +1414,9 @@
> if (predicted_by_p (bb, PRED_CONTINUE))
> continue;
>
> - /* Loop branch heuristics - predict an edge back to a
> - loop's head as taken. */
> - if (bb == loop->latch)
> - {
> - e = find_edge (loop->latch, loop->header);
> - if (e)
> - {
> - header_found = 1;
> - predict_edge_def (e, PRED_LOOP_BRANCH, TAKEN);
> - }
> - }
> -
> /* Loop exit heuristics - predict an edge exiting the loop if the
> conditional has no loop header successors as not taken. */
> - if (!header_found
> + if (!(header_found && bb == loop->latch)
Yes, this seems resonable to me.
> > which also makes header_found initialized before we visit loop blocks.
> >
> > Instead the code
> >
> > /* Loop exit heuristics - predict an edge exiting the loop if the
> > conditional has no loop header successors as not taken. */
> > if (!header_found
> > /* If we already used more reliable loop exit predictors, do
> > not
> > bother with PRED_LOOP_EXIT. */
> > ...
> > FOR_EACH_EDGE (e, ei, bb->succs)
> > if (e->dest->index < NUM_FIXED_BLOCKS
> > || !flow_bb_inside_loop_p (loop, e->dest))
> > predict_edge (e, PRED_LOOP_EXIT, probability);
> >
> > looks wrong for bb's that are parts of an inner loop of loop - assuming we
> > only want to predicate exits from loop and not exits from an inner loop
> > that also happen to exit loop (we will do that when predicating the inner
> > loop).
>
> You are right. And if we want to change this, we'd also need to modify
> get_loop_exit_edges to only count edges whose src is in the same loop
> level. However, this is relatively minor issue because it only
> predicts inaccurate bias ratio, while in the testcase I gave,
> LOOP_BRANCH is predicting in the wrong direction.
Indeed, it is not the most important thing around.
Patch seems OK. We should get the statistic about branch prediction
effectivity working again.
It is broken since we moved tree-profile early. I will try to look into it.
Honza
>
> Thanks,
> Dehao
>
>
> >
> > Is that what you experienced?
> >
> > Thanks,
> > Richard.
