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"} } */
