Re: [PATCH] Don't bypass blocks with multiple latch edges (PR middle-end/54838)

[Marek Polacek <polacek at redhat dot com>]

On Fri, Nov 30, 2012 at 11:00:28PM +0100, Eric Botcazou wrote:
> OK, let's tweak the patch as follows:
>  1) when current_loops is not NULL, we compute may_be_loop_header and whether 
> the loop has more than 1 latch edge exactly,
>  2) when current_loops is NULL, we use your above method to do the same,
>  3) once this is done, we return from the function before entering the loop if 
> this is a (potential) header with more than 1 (potential) latch edge.  The 
> comment can say that threading through a loop header with more than 1 latch 
> edge is delicate and cite tree-threadupdate.c:thread_through_loop_header.

Like this?  Regtested/bootstrapped on x86_64-linux, ok for trunk?

2012-12-01  Marek Polacek  <polacek@redhat.com>

	PR middle-end/54838
	* cprop.c (bypass_block): Determine number of latches.  Return
	when there is 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	2012-12-01 16:14:59.387335461 +0100
@@ -1510,13 +1510,28 @@ bypass_block (basic_block bb, rtx setcc,
   if (note)
     find_used_regs (&XEXP (note, 0), NULL);
 
-  may_be_loop_header = false;
-  FOR_EACH_EDGE (e, ei, bb->preds)
-    if (e->flags & EDGE_DFS_BACK)
-      {
-	may_be_loop_header = true;
-	break;
-      }
+  /* Determine whether there are more latch edges.  Threading through
+     a loop header with more than one latch is delicate, see e.g.
+     tree-ssa-threadupdate.c:thread_through_loop_header.  */
+  if (current_loops)
+    {
+      may_be_loop_header = bb == bb->loop_father->header;
+      if (may_be_loop_header
+	  && bb->loop_father->latch == NULL)
+	return 0;
+    }
+  else
+    {
+      unsigned n_back_edges = 0;
+      FOR_EACH_EDGE (e, ei, bb->preds)
+	if (e->flags & EDGE_DFS_BACK)
+	  n_back_edges++;
+
+      may_be_loop_header = n_back_edges > 0;
+
+      if (n_back_edges > 1)
+        return 0;
+    }
 
   change = 0;
   for (ei = ei_start (bb->preds); (e = ei_safe_edge (ei)); )
--- gcc/testsuite/gcc.dg/pr54838.c.mp	2012-11-26 14:48:43.783980854 +0100
+++ gcc/testsuite/gcc.dg/pr54838.c	2012-11-26 14:49:51.051158719 +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