Fix thunk generation for functions returning DECL_BY_REFERENCE

[Jan Hubicka <hubicka at ucw dot cz>]

Hi,
the Chromium miscompilation is caused by introducing extra copy when returning
non-POD object (as tested in the testcase).   This is bug in
cgraph_node::expand_thunk not setting gimple_call_set_return_slot_opt (I copied
same code from ipa-split).

This bug should reproduce on all release branches for target not defining
assembler thunks.

Bootstrapped/regtested x86_64-linux, comitted, earlier version was tested by
Martin to fix chromium.

Honza

	PR ipa/65236
	* g++.dg/ipa/ipa-icf-6.C: New testcase.
	* cgraphunit.c (cgraph_node::expand_thunk): Enable return slot
	opt.

Index: testsuite/g++.dg/ipa/ipa-icf-6.C
===================================================================
--- testsuite/g++.dg/ipa/ipa-icf-6.C	(revision 0)
+++ testsuite/g++.dg/ipa/ipa-icf-6.C	(revision 0)
@@ -0,0 +1,37 @@
+/* { dg-do run } */
+/* { dg-options "-O3 -fdump-ipa-icf"  } */
+
+struct A {                                                                      
+  A() {ptr=&b;}                                                                 
+  A(const A &a) {ptr = &b;}                                                     
+  void test() { if (ptr != &b) __builtin_abort ();}                             
+  int b;                                                                        
+  int *ptr;                                                                     
+};                                                                              
+
+A test1(A a)
+{
+  a.test();
+  return a;
+}
+A test2(A a)
+{
+  a.test();
+  return a;
+}
+__attribute__ ((noinline))
+static void
+test_me (A (*t)(A))
+{
+  struct A a, b=t(a);
+  b.test ();
+}
+int
+main()
+{
+  test_me (test1);
+  test_me (test2);
+  return 0;
+}
+/* { dg-final { scan-ipa-dump-times "Unified; Wrapper has been created" 1 "icf"  } } */
+/* { dg-final { cleanup-ipa-dump "icf" } } */
Index: cgraphunit.c
===================================================================
--- cgraphunit.c	(revision 221076)
+++ cgraphunit.c	(working copy)
@@ -1680,6 +1680,14 @@ cgraph_node::expand_thunk (bool output_a
       callees->call_stmt = call;
       gimple_call_set_from_thunk (call, true);
       gimple_call_set_with_bounds (call, instrumentation_clone);
+
+      /* Return slot optimization is always possible and in fact requred to
+         return values with DECL_BY_REFERENCE.  */
+      if (aggregate_value_p (resdecl, TREE_TYPE (thunk_fndecl))
+	  && (!is_gimple_reg_type (TREE_TYPE (resdecl))
+	      || DECL_BY_REFERENCE (resdecl)))
+        gimple_call_set_return_slot_opt (call, true);
+
       if (restmp && !alias_is_noreturn)
 	{
           gimple_call_set_lhs (call, restmp);