Re: [PATCH] c++, v4: Retry the aliasing of base/complete cdtor optimization at import_export_decl time [PR113208]

[Jakub Jelinek]

On Thu, Apr 25, 2024 at 02:02:32PM +0200, Jakub Jelinek wrote:
> I've tried the following patch, but unfortunately that lead to large
> number of regressions:
> +FAIL: g++.dg/cpp0x/initlist25.C  -std=c++17 (test for excess errors)

So the reduced testcase for this is
template <typename T, typename U> struct A {
  T a1;
  U a2;
  template <typename V, typename W, bool = true>
  constexpr A(V &&x, W &&y) : a1(x), a2(y) {}
};
template <typename> struct B;
namespace std {
template <class> struct initializer_list {
  int *_M_array;
  decltype (sizeof 0) _M_len;
};
}
template <typename T, typename U> struct C {
  void foo (std::initializer_list<A<const T, U>>);
};
template <class> struct D;
template <typename T, typename = D<T>, typename = B<T>>
struct E { E (const char *); ~E (); };
int
main ()
{
  C<E<char>, E<char>> m;
  m.foo ({{"t", "t"}, {"y", "y"}});
}
Without the patch I've just posted or even with the earlier version
of the patch the
_ZN1AIK1EIc1DIcE1BIcEES5_EC[12]IRA2_KcSB_Lb1EEEOT_OT0_
ctors were emitted, but with this patch they are unresolved externals.

The reason is that the code actually uses (calls) the
_ZN1AIK1EIc1DIcE1BIcEES5_EC1IRA2_KcSB_Lb1EEEOT_OT0_
__ct_comp constructor, that one has TREE_USED, while the
_ZN1AIK1EIc1DIcE1BIcEES5_EC2IRA2_KcSB_Lb1EEEOT_OT0_
__ct_base constructor is not TREE_USED.

But the c_parse_final_cleanups loop over
FOR_EACH_VEC_SAFE_ELT (deferred_fns, i, decl)
will ignore the TREE_USED __ct_comp because it is an alias
and so has !DECL_SAVED_TREE:
5273              if (!DECL_SAVED_TREE (decl))
5274                continue;

With the following incremental patch the tests in make check-g++
(haven't tried the coroutine one) which failed with the earlier patch
now pass.

--- gcc/cp/decl2.cc.jj  2024-04-25 10:52:21.057535959 +0200
+++ gcc/cp/decl2.cc     2024-04-25 16:19:17.385547357 +0200
@@ -5271,7 +5271,19 @@ c_parse_final_cleanups (void)
            generate_tls_wrapper (decl);
 
          if (!DECL_SAVED_TREE (decl))
-           continue;
+           {
+             cgraph_node *node;
+             tree tgt;
+             /* Even when maybe_clone_body created same body alias
+                has no DECL_SAVED_TREE, if its alias target does,
+                don't skip it.  */
+             if (!DECL_CLONED_FUNCTION (decl)
+                 || !(node = cgraph_node::get (decl))
+                 || !node->cpp_implicit_alias
+                 || !(tgt = node->get_alias_target_tree ())
+                 || !DECL_SAVED_TREE (tgt))
+               continue;
+           }
 
          cgraph_node *node = cgraph_node::get_create (decl);
 
@@ -5299,7 +5311,7 @@ c_parse_final_cleanups (void)
                node = node->get_alias_target ();
 
              node->call_for_symbol_thunks_and_aliases (clear_decl_external,
-                                                     NULL, true);
+                                                       NULL, true);
              /* If we mark !DECL_EXTERNAL one of the symbols in some comdat
                 group, we need to mark all symbols in the same comdat group
                 that way.  */
@@ -5309,7 +5321,7 @@ c_parse_final_cleanups (void)
                     next != node;
                     next = dyn_cast<cgraph_node *> (next->same_comdat_group))
                  next->call_for_symbol_thunks_and_aliases (clear_decl_external,
-                                                         NULL, true);
+                                                           NULL, true);
            }
 
          /* If we're going to need to write this function out, and


        Jakub