[libcxx-commits] [libcxx] [libc++] Fix ABI break introduced by switching to _LIBCPP_COMPRESSED_PAIR (PR #154686)
Nikolas Klauser via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Aug 22 07:45:13 PDT 2025
https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/154686
>From b07285ab918aab36e915a35ee17c6a735291af02 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Thu, 21 Aug 2025 09:26:19 +0200
Subject: [PATCH 1/2] [libc++] Fix ABI break introduced by switching to
_LIBCPP_COMPRESSED_PAIR
---
libcxx/docs/ReleaseNotes/21.rst | 14 +++++
libcxx/include/__memory/compressed_pair.h | 53 +++++++++++++------
libcxx/include/string | 14 ++++-
.../associative/map/abi.compile.pass.cpp | 4 +-
.../associative/set/abi.compile.pass.cpp | 4 +-
.../unord.map/abi.compile.pass.cpp | 6 +--
.../unord.set/abi.compile.pass.cpp | 6 +--
.../sequences/deque/abi.compile.pass.cpp | 6 +--
.../sequences/list/abi.compile.pass.cpp | 4 +-
.../vector.bool/abi.compile.pass.cpp | 4 +-
.../sequences/vector/abi.compile.pass.cpp | 4 +-
.../unique.ptr.ctor/default.pass.cpp | 4 ++
.../unique.ptr.ctor/nullptr.pass.cpp | 4 ++
.../unique.ptr.ctor/pointer.pass.cpp | 4 ++
14 files changed, 87 insertions(+), 44 deletions(-)
diff --git a/libcxx/docs/ReleaseNotes/21.rst b/libcxx/docs/ReleaseNotes/21.rst
index 91123ffa3e34b..1eaba67b18465 100644
--- a/libcxx/docs/ReleaseNotes/21.rst
+++ b/libcxx/docs/ReleaseNotes/21.rst
@@ -148,6 +148,20 @@ ABI Affecting Changes
comparison between shared libraries, since all RTTI has the correct visibility now. There is no behaviour change on
Clang.
+- LLVM 20 contains an ABI break when
+ (1) using empty allocators which derive from the same base class, or
+ (2) having an object of the same type as an empty member in one of the following classes that could be layed out at
+ the beginnig of the object
+
+ (1) applies to ``unordered_map``, ``unordered_set``, ``unordered_multimap``, ``unordered_multiset`` and ``deque``.
+ (2) applies to the containers listed in (1) as well as ``map``, ``set``, ``multimap``, ``multiset``, ``list`` and
+ ``vector``.
+
+ Fixing this causes a regression in ``unique_ptr``. Specifically, constant evaluation fails when the deleter relies on
+ being value-initialized. If the deleter works if it is default initialized or is not trivially default constructible
+ it is not affected.
+
+ For more details see https://llvm.org/PR154146.
Build System Changes
--------------------
diff --git a/libcxx/include/__memory/compressed_pair.h b/libcxx/include/__memory/compressed_pair.h
index fb7b7b7afcc8c..220dafe856716 100644
--- a/libcxx/include/__memory/compressed_pair.h
+++ b/libcxx/include/__memory/compressed_pair.h
@@ -80,21 +80,44 @@ class __compressed_pair_padding {
template <class _ToPad>
class __compressed_pair_padding<_ToPad, true> {};
-# define _LIBCPP_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2) \
- _LIBCPP_NO_UNIQUE_ADDRESS __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>))) T1 Initializer1; \
- _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _); \
- _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2; \
- _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _)
-
-# define _LIBCPP_COMPRESSED_TRIPLE(T1, Initializer1, T2, Initializer2, T3, Initializer3) \
- _LIBCPP_NO_UNIQUE_ADDRESS \
- __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>), \
- __aligned__(::std::__compressed_pair_alignment<T3>))) T1 Initializer1; \
- _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _); \
- _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2; \
- _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _); \
- _LIBCPP_NO_UNIQUE_ADDRESS T3 Initializer3; \
- _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T3> _LIBCPP_CONCAT3(__padding3_, __LINE__, _)
+# ifdef _LIBCPP_COMPILER_GCC
+# define _LIBCPP_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2) \
+ _LIBCPP_NO_UNIQUE_ADDRESS __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>))) T1 Initializer1; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _); \
+ _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _)
+
+# define _LIBCPP_COMPRESSED_TRIPLE(T1, Initializer1, T2, Initializer2, T3, Initializer3) \
+ _LIBCPP_NO_UNIQUE_ADDRESS \
+ __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>), \
+ __aligned__(::std::__compressed_pair_alignment<T3>))) T1 Initializer1; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _); \
+ _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _); \
+ _LIBCPP_NO_UNIQUE_ADDRESS T3 Initializer3; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T3> _LIBCPP_CONCAT3(__padding3_, __LINE__, _)
+# else
+# define _LIBCPP_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2) \
+ struct { \
+ _LIBCPP_NO_UNIQUE_ADDRESS \
+ __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>))) T1 Initializer1; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _); \
+ _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _); \
+ }
+
+# define _LIBCPP_COMPRESSED_TRIPLE(T1, Initializer1, T2, Initializer2, T3, Initializer3) \
+ struct { \
+ _LIBCPP_NO_UNIQUE_ADDRESS \
+ __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>), \
+ __aligned__(::std::__compressed_pair_alignment<T3>))) T1 Initializer1; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _); \
+ _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _); \
+ _LIBCPP_NO_UNIQUE_ADDRESS T3 Initializer3; \
+ _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T3> _LIBCPP_CONCAT3(__padding3_, __LINE__, _); \
+ }
+# endif
#else
# define _LIBCPP_COMPRESSED_PAIR(T1, Name1, T2, Name2) \
diff --git a/libcxx/include/string b/libcxx/include/string
index 3c1523eb5de33..1d197654b9fee 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -974,7 +974,12 @@ public:
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string()
_NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value)
- : __rep_() {
+# if _LIBCPP_STD_VER >= 20 // TODO(LLVM 23): Remove this condition; this is a workaround for https://llvm.org/PR154567
+ : __rep_(__short())
+# else
+ : __rep_()
+# endif
+ {
__annotate_new(0);
}
@@ -984,7 +989,12 @@ public:
# else
_NOEXCEPT
# endif
- : __rep_(), __alloc_(__a) {
+# if _LIBCPP_STD_VER >= 20 // TODO(LLVM 23): Remove this condition; this is a workaround for https://llvm.org/PR154567
+ : __rep_(__short()),
+# else
+ : __rep_(),
+# endif
+ __alloc_(__a) {
__annotate_new(0);
}
diff --git a/libcxx/test/libcxx/containers/associative/map/abi.compile.pass.cpp b/libcxx/test/libcxx/containers/associative/map/abi.compile.pass.cpp
index 04c1802bc84f6..908338631bb51 100644
--- a/libcxx/test/libcxx/containers/associative/map/abi.compile.pass.cpp
+++ b/libcxx/test/libcxx/containers/associative/map/abi.compile.pass.cpp
@@ -8,8 +8,6 @@
// UNSUPPORTED: libcpp-abi-no-compressed-pair-padding
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <cstdint>
#include <map>
@@ -90,7 +88,7 @@ struct user_struct {
};
#if __SIZE_WIDTH__ == 64
-static_assert(sizeof(user_struct) == 32, "");
+static_assert(sizeof(user_struct) == 24, "");
static_assert(TEST_ALIGNOF(user_struct) == 8, "");
static_assert(sizeof(map_alloc<int, std::allocator<std::pair<const int, int> > >) == 24, "");
diff --git a/libcxx/test/libcxx/containers/associative/set/abi.compile.pass.cpp b/libcxx/test/libcxx/containers/associative/set/abi.compile.pass.cpp
index 1b85bb7c93773..626f573967efb 100644
--- a/libcxx/test/libcxx/containers/associative/set/abi.compile.pass.cpp
+++ b/libcxx/test/libcxx/containers/associative/set/abi.compile.pass.cpp
@@ -8,8 +8,6 @@
// UNSUPPORTED: libcpp-abi-no-compressed-pair-padding
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <cstdint>
#include <set>
@@ -90,7 +88,7 @@ struct user_struct {
};
#if __SIZE_WIDTH__ == 64
-static_assert(sizeof(user_struct) == 32, "");
+static_assert(sizeof(user_struct) == 24, "");
static_assert(TEST_ALIGNOF(user_struct) == 8, "");
static_assert(sizeof(set_alloc<int, std::allocator<int> >) == 24, "");
diff --git a/libcxx/test/libcxx/containers/associative/unord.map/abi.compile.pass.cpp b/libcxx/test/libcxx/containers/associative/unord.map/abi.compile.pass.cpp
index 0476872487036..e6d27d245c472 100644
--- a/libcxx/test/libcxx/containers/associative/unord.map/abi.compile.pass.cpp
+++ b/libcxx/test/libcxx/containers/associative/unord.map/abi.compile.pass.cpp
@@ -12,8 +12,6 @@
// unordered containers changes when bounded unique_ptr is enabled.
// UNSUPPORTED: libcpp-has-abi-bounded-unique_ptr
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <cstdint>
#include <unordered_map>
@@ -94,7 +92,7 @@ struct user_struct {
};
#if __SIZE_WIDTH__ == 64
-static_assert(sizeof(user_struct) == 48, "");
+static_assert(sizeof(user_struct) == 40, "");
static_assert(TEST_ALIGNOF(user_struct) == 8, "");
static_assert(sizeof(unordered_map_alloc<int, std::allocator<std::pair<const int, int> > >) == 40, "");
@@ -126,7 +124,7 @@ struct TEST_ALIGNAS(32) AlignedHash {};
struct UnalignedEqualTo {};
// This part of the ABI has been broken between LLVM 19 and LLVM 20.
-static_assert(sizeof(std::unordered_map<int, int, AlignedHash, UnalignedEqualTo>) == 64, "");
+static_assert(sizeof(std::unordered_map<int, int, AlignedHash, UnalignedEqualTo>) == 96, "");
static_assert(TEST_ALIGNOF(std::unordered_map<int, int, AlignedHash, UnalignedEqualTo>) == 32, "");
#elif __SIZE_WIDTH__ == 32
diff --git a/libcxx/test/libcxx/containers/associative/unord.set/abi.compile.pass.cpp b/libcxx/test/libcxx/containers/associative/unord.set/abi.compile.pass.cpp
index e9d88a126de6c..1658e7e33f850 100644
--- a/libcxx/test/libcxx/containers/associative/unord.set/abi.compile.pass.cpp
+++ b/libcxx/test/libcxx/containers/associative/unord.set/abi.compile.pass.cpp
@@ -12,8 +12,6 @@
// unordered containers changes when bounded unique_ptr is enabled.
// UNSUPPORTED: libcpp-has-abi-bounded-unique_ptr
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <cstdint>
#include <unordered_set>
@@ -94,7 +92,7 @@ struct user_struct {
};
#if __SIZE_WIDTH__ == 64
-static_assert(sizeof(user_struct) == 48, "");
+static_assert(sizeof(user_struct) == 40, "");
static_assert(TEST_ALIGNOF(user_struct) == 8, "");
static_assert(sizeof(unordered_set_alloc<int, std::allocator<int> >) == 40, "");
@@ -125,7 +123,7 @@ struct TEST_ALIGNAS(32) AlignedHash {};
struct UnalignedEqualTo {};
// This part of the ABI has been broken between LLVM 19 and LLVM 20.
-static_assert(sizeof(std::unordered_set<int, AlignedHash, UnalignedEqualTo>) == 64, "");
+static_assert(sizeof(std::unordered_set<int, AlignedHash, UnalignedEqualTo>) == 96, "");
static_assert(TEST_ALIGNOF(std::unordered_set<int, AlignedHash, UnalignedEqualTo>) == 32, "");
#elif __SIZE_WIDTH__ == 32
diff --git a/libcxx/test/libcxx/containers/sequences/deque/abi.compile.pass.cpp b/libcxx/test/libcxx/containers/sequences/deque/abi.compile.pass.cpp
index 38f957f416c87..a19897cbb1588 100644
--- a/libcxx/test/libcxx/containers/sequences/deque/abi.compile.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/deque/abi.compile.pass.cpp
@@ -8,8 +8,6 @@
// UNSUPPORTED: libcpp-abi-no-compressed-pair-padding
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <cstdint>
#include <deque>
@@ -88,14 +86,14 @@ static_assert(sizeof(std::deque<int, min_allocator<int> >) == 48, "");
static_assert(sizeof(std::deque<int, test_allocator<int> >) == 80, "");
static_assert(sizeof(std::deque<int, small_iter_allocator<int> >) == 12, "");
static_assert(sizeof(std::deque<int, final_small_iter_allocator<int> >) == 16, "");
-static_assert(sizeof(std::deque<int, common_base_allocator<int> >) == 56, "");
+static_assert(sizeof(std::deque<int, common_base_allocator<int> >) == 48, "");
static_assert(sizeof(std::deque<char>) == 48, "");
static_assert(sizeof(std::deque<char, min_allocator<char> >) == 48, "");
static_assert(sizeof(std::deque<char, test_allocator<char> >) == 80, "");
static_assert(sizeof(std::deque<char, small_iter_allocator<char> >) == 12, "");
static_assert(sizeof(std::deque<char, final_small_iter_allocator<char> >) == 16, "");
-static_assert(sizeof(std::deque<char, common_base_allocator<char> >) == 56, "");
+static_assert(sizeof(std::deque<char, common_base_allocator<char> >) == 48, "");
static_assert(TEST_ALIGNOF(std::deque<int>) == 8, "");
static_assert(TEST_ALIGNOF(std::deque<int, min_allocator<int> >) == 8, "");
diff --git a/libcxx/test/libcxx/containers/sequences/list/abi.compile.pass.cpp b/libcxx/test/libcxx/containers/sequences/list/abi.compile.pass.cpp
index 1737497ab159e..ad10051e514fe 100644
--- a/libcxx/test/libcxx/containers/sequences/list/abi.compile.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/list/abi.compile.pass.cpp
@@ -6,8 +6,6 @@
//
//===----------------------------------------------------------------------===//
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <cstdint>
#include <list>
@@ -65,7 +63,7 @@ struct user_struct {
};
#if __SIZE_WIDTH__ == 64
-static_assert(sizeof(user_struct) == 32, "");
+static_assert(sizeof(user_struct) == 24, "");
static_assert(TEST_ALIGNOF(user_struct) == 8, "");
static_assert(sizeof(std::list<int>) == 24, "");
diff --git a/libcxx/test/libcxx/containers/sequences/vector.bool/abi.compile.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector.bool/abi.compile.pass.cpp
index 97e09ab4ca868..109e97a3fe8be 100644
--- a/libcxx/test/libcxx/containers/sequences/vector.bool/abi.compile.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector.bool/abi.compile.pass.cpp
@@ -8,8 +8,6 @@
// UNSUPPORTED: libcpp-abi-no-compressed-pair-padding
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <cstdint>
#include <vector>
@@ -67,7 +65,7 @@ struct user_struct {
};
#if __SIZE_WIDTH__ == 64
-static_assert(sizeof(user_struct) == 32, "");
+static_assert(sizeof(user_struct) == 24, "");
static_assert(TEST_ALIGNOF(user_struct) == 8, "");
static_assert(sizeof(std::vector<bool>) == 24, "");
diff --git a/libcxx/test/libcxx/containers/sequences/vector/abi.compile.pass.cpp b/libcxx/test/libcxx/containers/sequences/vector/abi.compile.pass.cpp
index 879e143510752..1a55d1359eeb0 100644
--- a/libcxx/test/libcxx/containers/sequences/vector/abi.compile.pass.cpp
+++ b/libcxx/test/libcxx/containers/sequences/vector/abi.compile.pass.cpp
@@ -6,8 +6,6 @@
//
//===----------------------------------------------------------------------===//
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <cstdint>
#include <vector>
@@ -73,7 +71,7 @@ struct user_struct {
};
#if __SIZE_WIDTH__ == 64
-static_assert(sizeof(user_struct) == 32, "");
+static_assert(sizeof(user_struct) == 24, "");
static_assert(TEST_ALIGNOF(user_struct) == 8, "");
static_assert(sizeof(std::vector<int>) == 24, "");
diff --git a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/default.pass.cpp b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/default.pass.cpp
index 7f4c90922d6c3..e2abfd17d27c3 100644
--- a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/default.pass.cpp
+++ b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/default.pass.cpp
@@ -82,6 +82,10 @@ TEST_CONSTEXPR_CXX23 bool test_basic() {
p.get_deleter().set_state(5);
assert(p.get_deleter().state() == 5);
}
+// TODO: Remove this check once https://llvm.org/PR154567 is fixed
+#if TEST_STD_VER >= 23 && defined(TEST_COMPILER_CLANG)
+ if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
{
std::unique_ptr<ElemType, DefaultCtorDeleter<ElemType> > p;
assert(p.get() == 0);
diff --git a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/nullptr.pass.cpp b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/nullptr.pass.cpp
index 45017a03b95dd..f5e0541684d13 100644
--- a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/nullptr.pass.cpp
+++ b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/nullptr.pass.cpp
@@ -47,6 +47,10 @@ TEST_CONSTEXPR_CXX23 void test_basic() {
assert(p.get() == 0);
assert(p.get_deleter().state() == 0);
}
+// TODO: Remove this check once https://llvm.org/PR154567 is fixed
+#if TEST_STD_VER >= 23 && defined(TEST_COMPILER_CLANG)
+ if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
{
std::unique_ptr<VT, DefaultCtorDeleter<VT> > p(nullptr);
assert(p.get() == 0);
diff --git a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/pointer.pass.cpp b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/pointer.pass.cpp
index cbce5c9c74c5a..e9912d4574af6 100644
--- a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/pointer.pass.cpp
+++ b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.ctor/pointer.pass.cpp
@@ -75,6 +75,10 @@ TEST_CONSTEXPR_CXX23 void test_pointer() {
}
if (!TEST_IS_CONSTANT_EVALUATED)
assert(A::count == 0);
+// TODO: Remove this check once https://llvm.org/PR154567 is fixed
+#if TEST_STD_VER >= 23 && defined(TEST_COMPILER_CLANG)
+ if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
{
A* p = newValue<ValueT>(expect_alive);
if (!TEST_IS_CONSTANT_EVALUATED)
>From ece477e153126c160f15f26e21724aada7c37ffa Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Fri, 22 Aug 2025 16:45:04 +0200
Subject: [PATCH 2/2] Update libcxx/docs/ReleaseNotes/21.rst
Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>
---
libcxx/docs/ReleaseNotes/21.rst | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)
diff --git a/libcxx/docs/ReleaseNotes/21.rst b/libcxx/docs/ReleaseNotes/21.rst
index 1eaba67b18465..9f09da4f267f4 100644
--- a/libcxx/docs/ReleaseNotes/21.rst
+++ b/libcxx/docs/ReleaseNotes/21.rst
@@ -148,18 +148,18 @@ ABI Affecting Changes
comparison between shared libraries, since all RTTI has the correct visibility now. There is no behaviour change on
Clang.
-- LLVM 20 contains an ABI break when
- (1) using empty allocators which derive from the same base class, or
- (2) having an object of the same type as an empty member in one of the following classes that could be layed out at
- the beginnig of the object
-
- (1) applies to ``unordered_map``, ``unordered_set``, ``unordered_multimap``, ``unordered_multiset`` and ``deque``.
- (2) applies to the containers listed in (1) as well as ``map``, ``set``, ``multimap``, ``multiset``, ``list`` and
- ``vector``.
-
- Fixing this causes a regression in ``unique_ptr``. Specifically, constant evaluation fails when the deleter relies on
- being value-initialized. If the deleter works if it is default initialized or is not trivially default constructible
- it is not affected.
+- LLVM 20 contained an ABI break that can result in the size of ``std::unordered_{map,set,multimap,multiset}`` and ``std::deque`` changing when used with an allocator type that is empty and contains a base class that is the same across rebound allocator instantiations (e.g. ``Allocator<int>`` and ``Allocator<char>`` are both empty and contain the same base class).
+ In addition, the layout of a user-defined type that:
+ - contains one of the following containers: ``std::unordered_{map,set,multimap,multiset}``, ``std::deque``, ``std::map``, ``std::set``, ``std::multimap``, ``std::multiset``, ``std::list`` or ``std::vector``, and
+ - passes an empty allocator, comparator or hasher type to that container, and
+ - has a member of that same empty allocator, comparator or hasher type inside the enclosing struct, and
+ - that member is either marked with ``[[no_unique_address]]` or optimized out via the EBO (empty base optimization) technique
+
+ saw its size increase from LLVM 19 to LLVM 20. This was caused by the usage of ``[[no_unique_address]]`` within some of libc++'s containers in a way that allowed subtle interactions with enclosing objects. This is fixed in LLVM 21 when using the Clang compiler (returning to the LLVM 19 ABI), however that implies an ABI break from LLVM 20 to LLVM 21.
+
+ Furthermore, fixing this causes a slight regression to constant evaluation support in ``std::unique_ptr``. Specifically, constant evaluation will now fail when the deleter relies on being value-initialized for constant-evaluation admissibility. If a default-initialized deleter can be used during constant evaluation, or if the default constructor is non-trivial, the ``unique_ptr`` is not affected by this regression. In particular, this regression does not impact any ``unique_ptr`` using the default deleter.
+
+ Note that there is currently no way to realistically fix this ABI break on GCC, therefore GCC will remain on the ABI introduced in LLVM 19. That also means that Clang and GCC will have a slightly different ABI for the small subset of types listed above until we are able to apply the same fix we did with Clang on GCC.
For more details see https://llvm.org/PR154146.
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