Do not redirect calls to cxa_pure_virtual into bultin_unreachable in some cases

[Jan Hubicka <hubicka at ucw dot cz>]

Hi,
currently we optimize calls to pure virtual functions to __builtin_unreachable.
While this is correct it does make code harder to debug.  This patch makes
ipa-devirt to treat cxa_pure_virtual specially and devirtualize to it when
it is the only posible target.

Bootstrapped/regtested x86_64-linux. There are no calls to cxa_pure_virtual
in firefox, so I suppose dropping this optimization is quite cheap.

Honza

	PR ipa/66223
	* ipa-devirt.c (is_cxa_pure_virtual_p): New function.
	(maybe_record_node): Record cxa_pure_virtual as the only possible
	target if there are not ohter candidates.
	(possible_polymorphic_call_target_p): Accept cxa_pure_virtual.

	* g++.dg/ipa/devirt-50.C: New testcase.
Index: ipa-devirt.c
===================================================================
--- ipa-devirt.c	(revision 232555)
+++ ipa-devirt.c	(working copy)
@@ -2327,6 +2327,17 @@ referenced_from_vtable_p (struct cgraph_
   return found;
 }
 
+/* Return if TARGET is cxa_pure_virtual.  */
+
+static bool
+is_cxa_pure_virtual_p (tree target)
+{
+  return target && TREE_CODE (TREE_TYPE (target)) != METHOD_TYPE
+	 && DECL_NAME (target)
+	 && !strcmp (IDENTIFIER_POINTER (DECL_NAME (target)),
+		     "__cxa_pure_virtual");
+}
+
 /* If TARGET has associated node, record it in the NODES array.
    CAN_REFER specify if program can refer to the target directly.
    if TARGET is unknown (NULL) or it can not be inserted (for example because
@@ -2341,11 +2352,12 @@ maybe_record_node (vec <cgraph_node *> &
 {
   struct cgraph_node *target_node, *alias_target;
   enum availability avail;
+  bool pure_virtual = is_cxa_pure_virtual_p (target);
 
-  /* cxa_pure_virtual and __builtin_unreachable do not need to be added into
+  /* __builtin_unreachable do not need to be added into
      list of targets; the runtime effect of calling them is undefined.
      Only "real" virtual methods should be accounted.  */
-  if (target && TREE_CODE (TREE_TYPE (target)) != METHOD_TYPE)
+  if (target && TREE_CODE (TREE_TYPE (target)) != METHOD_TYPE && !pure_virtual)
     return;
 
   if (!can_refer)
@@ -2388,6 +2400,7 @@ maybe_record_node (vec <cgraph_node *> &
      ??? Maybe it would make sense to be more aggressive for LTO even
      elsewhere.  */
   if (!flag_ltrans
+      && !pure_virtual
       && type_in_anonymous_namespace_p (DECL_CONTEXT (target))
       && (!target_node
           || !referenced_from_vtable_p (target_node)))
@@ -2401,6 +2414,20 @@ maybe_record_node (vec <cgraph_node *> &
     {
       gcc_assert (!target_node->global.inlined_to);
       gcc_assert (target_node->real_symbol_p ());
+      /* Only add pure virtual if it is the only possible target.  This way
+	 we will preserve the diagnostics about pure virtual called in many
+	 cases without disabling optimization in other.  */
+      if (pure_virtual)
+	{
+	  if (nodes.length ())
+	    return;
+	}
+      /* If we found a real target, take away cxa_pure_virtual.  */
+      else if (!pure_virtual && nodes.length () == 1
+	       && is_cxa_pure_virtual_p (nodes[0]->decl))
+	nodes.pop ();
+      if (pure_virtual && nodes.length ())
+	return;
       if (!inserted->add (target))
 	{
 	  cached_polymorphic_call_targets->add (target_node);
@@ -3328,6 +3355,9 @@ possible_polymorphic_call_target_p (tree
           || fcode == BUILT_IN_TRAP))
     return true;
 
+  if (is_cxa_pure_virtual_p (n->decl))
+    return true;
+
   if (!odr_hash)
     return true;
   targets = possible_polymorphic_call_targets (otr_type, otr_token, ctx, &final);
Index: testsuite/g++.dg/ipa/devirt-50.C
===================================================================
--- testsuite/g++.dg/ipa/devirt-50.C	(revision 0)
+++ testsuite/g++.dg/ipa/devirt-50.C	(revision 0)
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -fdump-tree-optimized"  } */
+struct B {
+        B* self;
+        B() : self( this ) { self->f(); }
+        virtual void f() = 0;
+    };
+
+    struct D : B
+    {
+        void f() {}
+    };
+
+    int main()
+    {
+        D d;
+    }
+
+/* { dg-final { scan-tree-dump "cxa_pure_virtual" "optimized"} } */