On Tue, Jul 31, 2012 at 12:38 PM, Jan Hubicka <hubi...@ucw.cz> wrote: >> On Tue, Jul 31, 2012 at 12:20 PM, Dehao Chen <de...@google.com> wrote: >> > 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; >> > >> > 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) >> >> Hmm, so the issue is that loop->latch does not belong to loop? That looks >> like a bogus loop structure. Indeed we have the loop header of the inner >> loop as latch of the outer loop. It still looks ok to predict this as >> unlikely >> as the edge is not only the latch edge of the outer loop but also an exit >> of the inner loop. >> >> Easier for profile would be to force canonicalization via >> >> loop_optimizer_init (LOOPS_NORMAL); >> >> instead of >> >> loop_optimizer_init (0); >> if (dump_file && (dump_flags & TDF_DETAILS)) >> flow_loops_dump (dump_file, NULL, 0); >> >> mark_irreducible_loops (); > > Yeah, this may also work. The reason it is not done is that > 1) it seemed expensive to force CFG changes just to compute profile decade > ago > 2) cfgcleanup afterwards will anyway remove the headers again. > So I originally hoped to do the right thing without normalization.
Ok ... then you should pass AVOID_CFG_MODIFICATIONS instead. And be prepared for odd situations like this ;) > Honza
