http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53695
--- Comment #20 from rguenther at suse dot de <rguenther at suse dot de> 2012-08-23 11:00:29 UTC --- On Thu, 23 Aug 2012, rguenther at suse dot de wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53695 > > --- Comment #17 from rguenther at suse dot de <rguenther at suse dot de> > 2012-08-23 09:19:04 UTC --- > On Thu, 23 Aug 2012, steven at gcc dot gnu.org wrote: > > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53695 > > > > --- Comment #16 from Steven Bosscher <steven at gcc dot gnu.org> 2012-08-23 > > 08:53:04 UTC --- > > (In reply to comment #15) > > > Makes me wonder why the loop isn't recognized in the original test case... > > > > Ah, maybe because bb3 has an abnormal predecessor and is therefore ignored > > as a > > potential loop header? > > > > /* If we have an abnormal predecessor, do not consider the > > loop (not worth the problems). */ > > if (bb_has_abnormal_pred (header)) > > continue; > > > > Which brings things back to my question why this kind of loop header is > > rejected! :-) > > Because gimple_split_edge doesn't like to split abnormal edges, > called via force_single_succ_latches (). So we do definitely > not allow abnormal latch -> header edges. Still abnormal loop entries > should be fine. So, > > Index: gcc/cfgloop.c > =================================================================== > --- gcc/cfgloop.c (revision 190613) > +++ gcc/cfgloop.c (working copy) > @@ -400,24 +400,21 @@ flow_loops_find (struct loops *loops) > { > edge_iterator ei; > > - /* If we have an abnormal predecessor, do not consider the > - loop (not worth the problems). */ > - if (bb_has_abnormal_pred (header)) > - continue; > - > FOR_EACH_EDGE (e, ei, header->preds) > { > basic_block latch = e->src; > > - gcc_assert (!(e->flags & EDGE_ABNORMAL)); > - > /* Look for back edges where a predecessor is dominated > by this block. A natural loop has a single entry > node (header) that dominates all the nodes in the > loop. It also has single back edge to the header > from a latch node. */ > if (latch != ENTRY_BLOCK_PTR > - && dominated_by_p (CDI_DOMINATORS, latch, header)) > + && dominated_by_p (CDI_DOMINATORS, latch, header) > + /* We cannot make latches simple by splitting the > + latch -> header edge if the latch edge is abnormal. */ > + && (single_succ_p (latch) > + || !(e->flags & EDGE_ABNORMAL))) > { > /* Shared headers should be eliminated by now. */ > SET_BIT (headers, header->index); > > should "work". But doesn't fix the testcase (of course). Btw, another idea would be to make labels that are target of abnormal edges end a basic-block. That way you'd have the edges "pre-split". Richard.
