On Fri, Nov 30, 2012 at 10:01:37AM +0100, Richard Biener wrote:
> RTL cprop seems to run both before and after RTL loop optimizers (currently
> after RTL loop optimizers we throw away loops - an arbitrary chosen point
> before IRA across which I could not get things to work). Thus you could do
>
> if (current_loops)
> is_loop_header = bb == bb->loop_father->header;
> else
> {
> may_be_loop_header = false;
> FOR_EACH_EDGE (e, ei, bb->preds)
> if (e->flags & EDGE_DFS_BACK)
> {
> may_be_loop_header = true;
> break;
> }
> }
I can do this as a followup.
> I don't understand
>
> /* The irreducible loops created by redirecting of edges entering the
> loop from outside would decrease effectiveness of some of the
> following optimizations, so prevent this. */
> if (may_be_loop_header
> && !(e->flags & EDGE_DFS_BACK))
> {
> ei_next (&ei);
> continue;
> }
>
> why isn't this simply
>
> if (may_be_loop_header)
> {
> ei_next (&ei);
> continue;
> }
>
> ? It looks like the code tries to allow "rotating" a loop - but that's only
> good if bb has exactly two predecessors (one entry and one latch edge).
> And even then it requires to manually update the loop structures (update
> what the new header and latch blocks are).
>
> That said, removing the !(e->flags & EDGE_DFS_BACK) condition seems
> to fix the ICE. Threading across a loop header is in fact complicated
> (see the special routine tree-ssa-threadupdate.c:thread_through_loop_header
> necessary for that). Let's declare the GIMPLE level did all interesting
> threadings through headers.
Agreed. This is the fix I had some time ago, but at that time it
didn't seem like such a great idea. Done this time around.
Regtested/bootstrapped on x86_64-linux, ok for trunk?
2012-11-30 Marek Polacek <pola...@redhat.com>
PR middle-end/54838
* cprop.c (bypass_block): Skip header edges.
* gcc.dg/pr54838.c: New test.
--- gcc/cprop.c.mp 2012-11-29 15:49:53.120524295 +0100
+++ gcc/cprop.c 2012-11-30 10:30:23.509501957 +0100
@@ -1539,8 +1539,7 @@ bypass_block (basic_block bb, rtx setcc,
/* The irreducible loops created by redirecting of edges entering the
loop from outside would decrease effectiveness of some of the
following optimizations, so prevent this. */
- if (may_be_loop_header
- && !(e->flags & EDGE_DFS_BACK))
+ if (may_be_loop_header)
{
ei_next (&ei);
continue;
--- gcc/testsuite/gcc.dg/pr54838.c.mp 2012-11-26 14:48:43.783980854 +0100
+++ gcc/testsuite/gcc.dg/pr54838.c 2012-11-29 17:43:19.397737779 +0100
@@ -0,0 +1,24 @@
+/* PR middle-end/54838 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fno-forward-propagate -ftracer" } */
+
+void bar (void);
+
+void
+foo (void *b, int *c)
+{
+again:
+ switch (*c)
+ {
+ case 1:
+ if (!b)
+ {
+ bar ();
+ return;
+ }
+ goto again;
+ case 3:
+ if (!b)
+ goto again;
+ }
+}
Marek
