On Thu, Nov 29, 2012 at 04:50:19PM +0100, Steven Bosscher wrote:
> > 2012-11-29 Marek Polacek <>
> >
> > PR middle-end/54838
> > * cprop.c (bypass_block): Set header and latch to NULL when
> > BB has more than one latch edge.
> > (n_latches): New variable.
>
> You don't have to mention a new local variable in the ChangeLog.
Ok.
> But FWIW, not all DFS back edges are latches. Maybe name it n_back_edges?
Yeah, sure.
> > @@ -1605,7 +1605,8 @@ bypass_block (basic_block bb, rtx setcc,
> > && dest != EXIT_BLOCK_PTR)
> > {
> > if (current_loops != NULL
> > - && e->src->loop_father->latch == e->src)
> > + && (e->src->loop_father->latch == e->src
> > + || n_latch_edges > 1))
> > {
> > /* ??? Now we are creating (or may create) a loop
> > with multiple entries. Simply mark it for
>
> It seems to me that this threading should just not happen. Creating
> loops with multiple entries is something to be avoided because most
> loop-based optimizations don't work on irreducible regions. So this
> affects all passes that run after CPROP, including unrolling, IRA, the
> scheduler, etc.
>
> There is already code that tries to avoid creating multi-entry loops:
>
> /* 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;
> }
>
> Apparently your test case manages to slip through, and I wonder why.
That's probably because even though BB 4 is a header, 3->4 and 9->4
are back edges (in the condition there's !(e->flags & EDGE_DFS_BACK),
which in this case is 0). Note that the comment speaks about
edges coming from outside of the loop.
Updated patch:
2012-11-29 Marek Polacek <pola...@redhat.com>
PR middle-end/54838
* cprop.c (bypass_block): Set header and latch to NULL when
BB has more than one latch edge.
* gcc.dg/pr54838.c: New test.
--- gcc/cprop.c.mp 2012-11-29 15:49:53.120524295 +0100
+++ gcc/cprop.c 2012-11-29 17:45:03.004041242 +0100
@@ -1499,6 +1499,7 @@ bypass_block (basic_block bb, rtx setcc,
int may_be_loop_header;
unsigned removed_p;
unsigned i;
+ unsigned n_back_edges;
edge_iterator ei;
insn = (setcc != NULL) ? setcc : jump;
@@ -1510,13 +1511,12 @@ bypass_block (basic_block bb, rtx setcc,
if (note)
find_used_regs (&XEXP (note, 0), NULL);
- may_be_loop_header = false;
+ n_back_edges = 0;
FOR_EACH_EDGE (e, ei, bb->preds)
if (e->flags & EDGE_DFS_BACK)
- {
- may_be_loop_header = true;
- break;
- }
+ n_back_edges++;
+
+ may_be_loop_header = n_back_edges > 0;
change = 0;
for (ei = ei_start (bb->preds); (e = ei_safe_edge (ei)); )
@@ -1605,7 +1605,8 @@ bypass_block (basic_block bb, rtx setcc,
&& dest != EXIT_BLOCK_PTR)
{
if (current_loops != NULL
- && e->src->loop_father->latch == e->src)
+ && (e->src->loop_father->latch == e->src
+ || n_back_edges > 1))
{
/* ??? Now we are creating (or may create) a loop
with multiple entries. Simply mark it for
--- 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
