On Wed, Apr 24, 2024 at 08:43:46PM -0400, Jason Merrill wrote:
> > Then can_alias_cdtor would return false, because it ends with:
> >    /* Don't use aliases for weak/linkonce definitions unless we can put both
> >       symbols in the same COMDAT group.  */
> >    return (DECL_INTERFACE_KNOWN (fn)
> >            && (SUPPORTS_ONE_ONLY || !DECL_WEAK (fn))
> >            && (!DECL_ONE_ONLY (fn)
> >                || (HAVE_COMDAT_GROUP && DECL_WEAK (fn))));
> > Should we change that DECL_INTERFACE_KNOWN (fn) in there to
> > (DECL_INTERFACE_KNOWN (fn) || something) then and what that
> > something should be?  HAVE_COMDAT_GROUP && DECL_ONE_ONLY (fn)?
> 
> Yes, I think reorganize to
> 
> ((DECL_INTERFACE_KNOWN (fn) && !DECL_WEAK (fn) && !DECL_ONE_ONLY (fn))
>  || (HAVE_COMDAT_GROUP && DECL_ONE_ONLY (fn))

I've tried the following patch, but unfortunately that lead to large
number of regressions:
+FAIL: g++.dg/coroutines/torture/co-yield-04-complex-local-state.C (test for 
excess errors)
+FAIL: g++.dg/coroutines/torture/func-params-08.C (test for excess errors)
+FAIL: g++.dg/coroutines/torture/func-params-09-awaitable-parms.C (test for 
excess errors)
+FAIL: g++.dg/cpp0x/constexpr-initlist.C  -std=c++11 (test for excess errors)
+FAIL: g++.dg/cpp0x/constexpr-initlist.C  -std=c++14 (test for excess errors)
+FAIL: g++.dg/cpp0x/constexpr-initlist.C  -std=c++17 (test for excess errors)
+FAIL: g++.dg/cpp0x/constexpr-initlist.C  -std=c++20 (test for excess errors)
+FAIL: g++.dg/cpp0x/constexpr-initlist.C  -std=c++23 (test for excess errors)
+FAIL: g++.dg/cpp0x/constexpr-initlist.C  -std=c++26 (test for excess errors)
+FAIL: g++.dg/cpp0x/initlist25.C  -std=c++11 (test for excess errors)
+FAIL: g++.dg/cpp0x/initlist25.C  -std=c++14 (test for excess errors)
+FAIL: g++.dg/cpp0x/initlist25.C  -std=c++17 (test for excess errors)
+FAIL: g++.dg/cpp0x/initlist25.C  -std=c++20 (test for excess errors)
+FAIL: g++.dg/cpp0x/initlist25.C  -std=c++23 (test for excess errors)
+FAIL: g++.dg/cpp0x/initlist25.C  -std=c++26 (test for excess errors)
+FAIL: g++.dg/cpp1y/pr95226.C  -std=c++20 (test for excess errors)
+FAIL: g++.dg/cpp1y/pr95226.C  -std=c++23 (test for excess errors)
+FAIL: g++.dg/cpp1y/pr95226.C  -std=c++26 (test for excess errors)
+FAIL: g++.dg/cpp1z/decomp12.C  -std=c++23 (test for excess errors)
+FAIL: g++.dg/cpp1z/decomp12.C  -std=c++26 (test for excess errors)
+FAIL: g++.dg/cpp1z/eval-order2.C  -std=c++20 (test for excess errors)
+FAIL: g++.dg/cpp1z/eval-order2.C  -std=c++23 (test for excess errors)
+FAIL: g++.dg/cpp1z/eval-order2.C  -std=c++26 (test for excess errors)
+FAIL: g++.dg/cpp2a/srcloc17.C  -std=c++20 (test for excess errors)
+FAIL: g++.dg/cpp2a/srcloc17.C  -std=c++23 (test for excess errors)
+FAIL: g++.dg/cpp2a/srcloc17.C  -std=c++26 (test for excess errors)
+FAIL: g++.old-deja/g++.jason/template31.C  -std=c++20 (test for excess errors)
+FAIL: g++.old-deja/g++.jason/template31.C  -std=c++23 (test for excess errors)
+FAIL: g++.old-deja/g++.jason/template31.C  -std=c++26 (test for excess errors)
+FAIL: 20_util/unique_ptr/creation/for_overwrite.cc  -std=gnu++26 (test for 
excess errors)
+FAIL: 23_containers/span/cons_1_assert_neg.cc  -std=gnu++20 (test for excess 
errors)
+FAIL: 23_containers/span/cons_1_assert_neg.cc  -std=gnu++26 (test for excess 
errors)
+FAIL: 23_containers/span/cons_2_assert_neg.cc  -std=gnu++20 (test for excess 
errors)
+FAIL: 23_containers/span/cons_2_assert_neg.cc  -std=gnu++26 (test for excess 
errors)
+FAIL: std/ranges/repeat/1.cc  -std=gnu++23 (test for excess errors)
+FAIL: std/ranges/repeat/1.cc  -std=gnu++26 (test for excess errors)

Errors are like:
func-params-08.C:(.text._ZNSt12_Vector_baseIiSaIiEEC2Ev[_ZNSt12_Vector_baseIiSaIiEEC5Ev]+0x14):
 undefined reference to 
`_ZNSt12_Vector_baseIiSaIiEE12_Vector_implC1EvQ26is_default_constructible_vIN9__gnu_cxx14__alloc_traitsIT0_NS5_10value_typeEE6rebindIT_E5otherEE'
Though, libstdc++.so.6 abilist is the same.
Trying to debug it now.

2024-04-24  Jakub Jelinek  <ja...@redhat.com>
            Jason Merrill  <ja...@redhat.com>

        PR lto/113208
        * decl2.cc (tentative_decl_linkage): Call maybe_make_one_only
        for implicit instantiations of maybe in charge ctors/dtors
        declared inline.
        * optimize.cc (can_alias_cdtor): Adjust condition, for
        HAVE_COMDAT_GROUP && DECL_ONE_ONLY && DECL_WEAK return true even
        if not DECL_INTERFACE_KNOWN.
        * decl.cc (cxx_comdat_group): For DECL_CLONED_FUNCTION_P
        functions if SUPPORTS_ONE_ONLY return DECL_COMDAT_GROUP if already
        set.

        * g++.dg/abi/comdat2.C: New test.
        * g++.dg/abi/comdat3.C: New test.
        * g++.dg/abi/comdat4.C: New test.
        * g++.dg/abi/comdat5.C: New test.
        * g++.dg/lto/pr113208_0.C: New test.
        * g++.dg/lto/pr113208_1.C: New file.
        * g++.dg/lto/pr113208.h: New file.

--- gcc/cp/decl2.cc.jj  2024-04-24 18:28:22.299513620 +0200
+++ gcc/cp/decl2.cc     2024-04-25 10:04:18.049476567 +0200
@@ -3312,16 +3312,23 @@ tentative_decl_linkage (tree decl)
             linkage of all functions, and as that causes writes to
             the data mapped in from the PCH file, it's advantageous
             to mark the functions at this point.  */
-         if (DECL_DECLARED_INLINE_P (decl)
-             && (!DECL_IMPLICIT_INSTANTIATION (decl)
-                 || DECL_DEFAULTED_FN (decl)))
+         if (DECL_DECLARED_INLINE_P (decl))
            {
-             /* This function must have external linkage, as
-                otherwise DECL_INTERFACE_KNOWN would have been
-                set.  */
-             gcc_assert (TREE_PUBLIC (decl));
-             comdat_linkage (decl);
-             DECL_INTERFACE_KNOWN (decl) = 1;
+             if (!DECL_IMPLICIT_INSTANTIATION (decl)
+                 || DECL_DEFAULTED_FN (decl))
+               {
+                 /* This function must have external linkage, as
+                    otherwise DECL_INTERFACE_KNOWN would have been
+                    set.  */
+                 gcc_assert (TREE_PUBLIC (decl));
+                 comdat_linkage (decl);
+                 DECL_INTERFACE_KNOWN (decl) = 1;
+               }
+             else if (DECL_MAYBE_IN_CHARGE_CDTOR_P (decl))
+               /* For implicit instantiations of cdtors try to make
+                  it comdat, so that maybe_clone_body can use aliases.
+                  See PR113208.  */
+               maybe_make_one_only (decl);
            }
        }
       else if (VAR_P (decl))
--- gcc/cp/optimize.cc.jj       2024-04-25 09:47:24.936736833 +0200
+++ gcc/cp/optimize.cc  2024-04-25 10:19:25.338332564 +0200
@@ -220,10 +220,8 @@ can_alias_cdtor (tree fn)
   gcc_assert (DECL_MAYBE_IN_CHARGE_CDTOR_P (fn));
   /* Don't use aliases for weak/linkonce definitions unless we can put both
      symbols in the same COMDAT group.  */
-  return (DECL_INTERFACE_KNOWN (fn)
-         && (SUPPORTS_ONE_ONLY || !DECL_WEAK (fn))
-         && (!DECL_ONE_ONLY (fn)
-             || (HAVE_COMDAT_GROUP && DECL_WEAK (fn))));
+  return (DECL_WEAK (fn) ? (HAVE_COMDAT_GROUP && DECL_ONE_ONLY (fn))
+                        : (DECL_INTERFACE_KNOWN (fn) && !DECL_ONE_ONLY (fn)));
 }
 
 /* FN is a [cd]tor, fns is a pointer to an array of length 3.  Fill fns
--- gcc/cp/decl.cc.jj   2024-04-24 18:28:22.231514532 +0200
+++ gcc/cp/decl.cc      2024-04-25 09:47:06.855991224 +0200
@@ -19254,6 +19254,14 @@ cxx_comdat_group (tree decl)
          else
            break;
        }
+      /* If a ctor/dtor has already set the comdat group by
+        maybe_clone_body, don't override it.  */
+      if (SUPPORTS_ONE_ONLY
+         && TREE_CODE (decl) == FUNCTION_DECL
+         && DECL_CLONED_FUNCTION_P (decl)
+         && SUPPORTS_ONE_ONLY)
+       if (tree comdat = DECL_COMDAT_GROUP (decl))
+         return comdat;
     }
 
   return decl;
--- gcc/testsuite/g++.dg/abi/comdat2.C.jj       2024-04-25 09:47:06.855991224 
+0200
+++ gcc/testsuite/g++.dg/abi/comdat2.C  2024-04-25 09:47:06.855991224 +0200
@@ -0,0 +1,26 @@
+// PR lto/113208
+// { dg-do compile { target { c++11 && { *-*-*gnu* } } } }
+// { dg-additional-options "-O2 -fkeep-inline-functions" }
+// { dg-final { scan-assembler "_ZN1BI1CEC5ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC1ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC2ERKS1_,comdat" } }
+
+template <typename T>
+struct A {
+  int foo () const;
+  A (int, int);
+};
+template <typename T>
+struct B : A<T> {
+  constexpr B (const B &x) : A<T> (1, x.foo ()) {}
+  B () : A<T> (1, 2) {}
+};
+struct C;
+struct D : B<C> {};
+void bar (D);
+
+void
+baz (D x)
+{
+  bar (x);
+}
--- gcc/testsuite/g++.dg/abi/comdat3.C.jj       2024-04-25 09:47:06.855991224 
+0200
+++ gcc/testsuite/g++.dg/abi/comdat3.C  2024-04-25 09:47:06.855991224 +0200
@@ -0,0 +1,22 @@
+// PR lto/113208
+// { dg-do compile { target { c++11 && { *-*-*gnu* } } } }
+// { dg-additional-options "-O2" }
+// { dg-final { scan-assembler "_ZN1M1SINS_1P1TELN1N1LE2EEC5Ev,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1M1SINS_1P1TELN1N1LE2EEC1Ev,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1M1SINS_1P1TELN1N1LE2EEC2Ev,comdat" } }
+
+namespace N { enum L { L1, L2, L3 } const O = L3; }
+namespace M {
+  using N::O;
+  using N::L;
+  template <typename, L = O>
+  struct S { constexpr S () {} };
+  namespace P {
+    struct T;
+    struct U { S<T> u; };
+    void foo () { U (); }
+  }
+  extern template class S<P::T>;
+}
+namespace p = M::P;
+template class M::S<p::T>;
--- gcc/testsuite/g++.dg/abi/comdat4.C.jj       2024-04-25 10:27:54.141172257 
+0200
+++ gcc/testsuite/g++.dg/abi/comdat4.C  2024-04-25 10:28:22.493773334 +0200
@@ -0,0 +1,28 @@
+// PR lto/113208
+// { dg-do compile { target { c++11 && { *-*-*gnu* } } } }
+// { dg-additional-options "-O2" }
+// { dg-final { scan-assembler "_ZN1BI1CEC5ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC1ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC2ERKS1_,comdat" } }
+
+template <typename T>
+struct A {
+  int foo () const;
+  A (int, int);
+};
+template <typename T>
+struct B : A<T> {
+  constexpr B (const B &x) : A<T> (1, x.foo ()) {}
+  B () : A<T> (1, 2) {}
+};
+struct C;
+struct D : B<C> {};
+void bar (D);
+
+void
+baz (D x)
+{
+  bar (x);
+}
+
+template struct B<C>;
--- gcc/testsuite/g++.dg/abi/comdat5.C.jj       2024-04-25 10:28:35.842585513 
+0200
+++ gcc/testsuite/g++.dg/abi/comdat5.C  2024-04-25 10:28:52.413352362 +0200
@@ -0,0 +1,28 @@
+// PR lto/113208
+// { dg-do compile { target { c++11 && { *-*-*gnu* } } } }
+// { dg-additional-options "-O2" }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC5ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC1ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC2ERKS1_,comdat" } }
+
+template <typename T>
+struct A {
+  int foo () const;
+  A (int, int);
+};
+template <typename T>
+struct B : A<T> {
+  constexpr B (const B &x) : A<T> (1, x.foo ()) {}
+  B () : A<T> (1, 2) {}
+};
+struct C;
+struct D : B<C> {};
+void bar (D);
+
+void
+baz (D x)
+{
+  bar (x);
+}
+
+extern template struct B<C>;
--- gcc/testsuite/g++.dg/lto/pr113208_0.C.jj    2024-04-25 09:47:06.856991210 
+0200
+++ gcc/testsuite/g++.dg/lto/pr113208_0.C       2024-04-25 09:47:06.855991224 
+0200
@@ -0,0 +1,13 @@
+// { dg-lto-do link }
+// { dg-lto-options { {-O1 -std=c++20 -flto}} }
+// { dg-extra-ld-options "-r -nostdlib -flinker-output=nolto-rel" }
+// { dg-require-linker-plugin "" }
+
+#define CONSTEXPR constexpr
+#include "pr113208.h"
+
+struct QualityValue;
+struct k : vector<QualityValue> {};
+
+void m(k);
+void n(k i) { m(i); }
--- gcc/testsuite/g++.dg/lto/pr113208_1.C.jj    2024-04-25 09:47:06.856991210 
+0200
+++ gcc/testsuite/g++.dg/lto/pr113208_1.C       2024-04-25 09:47:06.856991210 
+0200
@@ -0,0 +1,6 @@
+#define CONSTEXPR 
+#include "pr113208.h"
+
+struct QualityValue;
+vector<QualityValue> values1;
+vector<QualityValue> values{values1};
--- gcc/testsuite/g++.dg/lto/pr113208.h.jj      2024-04-25 09:47:06.856991210 
+0200
+++ gcc/testsuite/g++.dg/lto/pr113208.h 2024-04-25 09:47:06.856991210 +0200
@@ -0,0 +1,10 @@
+template <typename _Tp> struct _Vector_base {
+  int g() const;
+  _Vector_base(int, int);
+};
+template <typename _Tp>
+struct vector : _Vector_base<_Tp> {
+  CONSTEXPR vector(const vector &__x)
+      : _Vector_base<_Tp>(1, __x.g()) {}
+ vector() : _Vector_base<_Tp>(1, 2) {}
+};


        Jakub

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