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

[Jason Merrill]

On 4/25/24 07:22, Jakub Jelinek wrote:

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  struct A {
   T a1;
   U a2;
   template 
   constexpr A(V &&x, W &&y) : a1(x), a2(y) {}
};
template  struct B;
namespace std {
template  struct initializer_list {
   int *_M_array;
   decltype (sizeof 0) _M_len;
};
}
template  struct C {
   void foo (std::initializer_list>);
};
template  struct D;
template , typename = B>
struct E { E (const char *); ~E (); };
int
main ()
{
   C, E> 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))
5274continue;


Hmm, maybe maybe_clone_body shouldn't clear DECL_SAVED_TREE for aliases, 
but rather set it to some stub like void_node?


Though with all these changes, it's probably better to go with your 
first patch for GCC 14 and delay this approach to 15.  Your v1 patch is 
OK for 14.


Jason

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  struct A {
  T a1;
  U a2;
  template 
  constexpr A(V &&x, W &&y) : a1(x), a2(y) {}
};
template  struct B;
namespace std {
template  struct initializer_list {
  int *_M_array;
  decltype (sizeof 0) _M_len;
};
}
template  struct C {
  void foo (std::initializer_list>);
};
template  struct D;
template , typename = B>
struct E { E (const char *); ~E (); };
int
main ()
{
  C, E> 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))
5274continue;

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 (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