http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52756
--- Comment #7 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-04-02
14:05:31 UTC ---
With that patch gcc.dg/tree-ssa/ssa-dom-thread-2.c regresses as we do not
perform the threading through the header edge anymore. Note that using
thread_block (header, true); for the rest of the header edges does not
work either, as it still happily creates the multiple entry loop with
the bogus header.
The following is another possibility - remove the remaining threadings
that would be harmful but allow those that would be safe.
Index: gcc/tree-ssa-threadupdate.c
===================================================================
--- gcc/tree-ssa-threadupdate.c (revision 186082)
+++ gcc/tree-ssa-threadupdate.c (working copy)
@@ -826,6 +826,17 @@ determine_bb_domination_status (struct l
return (bb_reachable ? DOMST_DOMINATING : DOMST_LOOP_BROKEN);
}
+/* Return true if BB is part of the new pre-header that is created
+ when threading the latch to DATA. */
+
+static bool
+def_split_header_continue_p (const_basic_block bb, const void *data)
+{
+ const_basic_block new_header = (const_basic_block) data;
+ return (bb->loop_father == new_header->loop_father
+ && bb != new_header);
+}
+
/* Thread jumps through the header of LOOP. Returns true if cfg changes.
If MAY_PEEL_LOOP_HEADERS is false, we avoid threading from entry edges
to the inside of the loop. */
@@ -990,11 +1001,46 @@ thread_through_loop_header (struct loop
if (latch->aux)
{
+ basic_block *bblocks;
+ unsigned nblocks, i;
+
/* First handle the case latch edge is redirected. */
loop->latch = thread_single_edge (latch);
gcc_assert (single_succ (loop->latch) == tgt_bb);
loop->header = tgt_bb;
+ /* Remove the new pre-header blocks from our loop. */
+ bblocks = XCNEWVEC (basic_block, loop->num_nodes);
+ nblocks = dfs_enumerate_from (header, 0, def_split_header_continue_p,
+ bblocks, loop->num_nodes, tgt_bb);
+ for (i = 0; i < nblocks; i++)
+ {
+ remove_bb_from_loops (bblocks[i]);
+ add_bb_to_loop (bblocks[i], loop_outer (loop));
+ }
+ free (bblocks);
+
+ /* Cancel remaining threading requests that would make the
+ loop a multiple entry loop. */
+ FOR_EACH_EDGE (e, ei, header->preds)
+ {
+ edge e2;
+ if (e->aux == NULL)
+ continue;
+
+ if (THREAD_TARGET2 (e))
+ e2 = THREAD_TARGET2 (e);
+ else
+ e2 = THREAD_TARGET (e);
+
+ if (e->src->loop_father != e2->dest->loop_father
+ && e2->dest != loop->header)
+ {
+ free (e->aux);
+ e->aux = NULL;
+ }
+ }
+
/* Thread the remaining edges through the former header. */
thread_block (header, false);
}
