tree-optimization/109469 - SLP with returns-twice region start

The following avoids an SLP region starting with a returns-twice
call where we cannot insert stmts at the head.

	PR tree-optimization/109469
	* tree-vect-slp.cc (vect_slp_function): Skip region starts with
	a returns-twice call.

	* gcc.dg/torture/pr109469.c: New testcase.

2 files changed, 31 insertions, 3 deletions

diff --git a/gcc/testsuite/gcc.dg/torture/pr109469.c b/gcc/testsuite/gcc.dg/torture/pr109469.c
new file mode 100644
index 0000000..d05a93b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr109469.c
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+
+__attribute__((returns_twice)) int foo();
+
+struct xio myproc;
+struct xio {
+  void (*read_proc)();
+  void (*write_proc)();
+};
+
+void dummy_write_proc() {
+  switch (foo())
+  default:
+    myproc.read_proc = myproc.write_proc = dummy_write_proc;
+}
diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
index 356bdfb..d73deae 100644
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -7671,10 +7671,23 @@ vect_slp_function (function *fun)
 	{
 	  r |= vect_slp_bbs (bbs, NULL);
 	  bbs.truncate (0);
-	  bbs.quick_push (bb);
 	}
-      else
-	bbs.safe_push (bb);
+
+      /* We need to be able to insert at the head of the region which
+	 we cannot for region starting with a returns-twice call.  */
+      if (bbs.is_empty ())
+	if (gcall *first = safe_dyn_cast <gcall *> (first_stmt (bb)))
+	  if (gimple_call_flags (first) & ECF_RETURNS_TWICE)
+	    {
+	      if (dump_enabled_p ())
+		dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+				 "skipping bb%d as start of region as it "
+				 "starts with returns-twice call\n",
+				 bb->index);
+	      continue;
+	    }
+
+      bbs.safe_push (bb);
 
       /* When we have a stmt ending this block and defining a
 	 value we have to insert on edges when inserting after it for