[libcxx-commits] [libcxx] [openmp] [libc++] basic_ios<wchar_t> cannot store fill character WCHAR_MAX (PR #89305)

Xing Xue via libcxx-commits libcxx-commits at lists.llvm.org
Fri Jul 12 07:30:10 PDT 2024


https://github.com/xingxue-ibm updated https://github.com/llvm/llvm-project/pull/89305

>From 7bd12fb10c31a2ce354a6cfcee63938ad0597013 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Wed, 10 Apr 2024 13:32:16 -0400
Subject: [PATCH 01/20] Use lower max threads to reduce the testing time on
 AIX.

---
 openmp/runtime/test/worksharing/for/collapse_test.inc | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/openmp/runtime/test/worksharing/for/collapse_test.inc b/openmp/runtime/test/worksharing/for/collapse_test.inc
index 3075bd04e958f..74ec83a059d91 100644
--- a/openmp/runtime/test/worksharing/for/collapse_test.inc
+++ b/openmp/runtime/test/worksharing/for/collapse_test.inc
@@ -15,7 +15,12 @@
 #define LOOP_TYPE2 LOOP_TYPES
 #define LOOP_STYPE2 LOOP_TYPES
 
+#if defined _AIX
+// Use lower max threads to reduce the testing time
+#define MAX_THREADS 64
+#else
 #define MAX_THREADS 256
+#endif
 
 #if defined VERBOSE
 #define PRINTF(...) printf(__VA_ARGS__)

>From ecbcaa8f504f86b138202dd853403166df0f60cd Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Thu, 18 Apr 2024 10:59:41 -0400
Subject: [PATCH 02/20] Changes for cases where wint_t and wchar_t have the
 same width. The changes are compatible with the current IBM provided libc++.

---
 libcxx/include/ios                            | 55 +++++++++++++++++--
 .../std.manip/setfill_eof.pass.cpp            | 31 +++++++++++
 2 files changed, 80 insertions(+), 6 deletions(-)
 create mode 100644 libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp

diff --git a/libcxx/include/ios b/libcxx/include/ios
index 00c1d5c2d4bc5..bc31f30f14acf 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -224,6 +224,7 @@ storage-class-specifier const error_category& iostream_category() noexcept;
 #include <__system_error/error_code.h>
 #include <__system_error/error_condition.h>
 #include <__system_error/system_error.h>
+#include <__type_traits/conditional.h>
 #include <__utility/swap.h>
 #include <__verbose_abort>
 #include <version>
@@ -521,6 +522,31 @@ inline _LIBCPP_HIDE_FROM_ABI void ios_base::exceptions(iostate __iostate) {
   clear(__rdstate_);
 }
 
+template <class _Traits>
+// Attribute 'packed' is used to keep the layout compatible with the previous
+// definition of '__set_' in basic_ios on AIX.
+struct _LIBCPP_PACKED _OptionalFill {
+  _OptionalFill() : __set_(false) { }
+  _OptionalFill& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
+  bool __is_set() const { return __set_; }
+  typename _Traits::int_type __fill() const { return __fill_val_; }
+
+private:
+  typename _Traits::int_type __fill_val_;
+  bool __set_;
+};
+
+template <class _Traits>
+struct _LIBCPP_PACKED _SentinelValueFill {
+  _SentinelValueFill() : __fill_val_(_Traits::eof()) { }
+  _SentinelValueFill& operator=(typename _Traits::int_type __x) { __fill_val_ = __x; return *this; }
+  bool __is_set() const { return __fill_val_ != _Traits::eof(); }
+  typename _Traits::int_type __fill() const { return __fill_val_; }
+
+private:
+  typename _Traits::int_type __fill_val_;
+};
+
 template <class _CharT, class _Traits>
 class _LIBCPP_TEMPLATE_VIS basic_ios : public ios_base {
 public:
@@ -590,7 +616,24 @@ protected:
 
 private:
   basic_ostream<char_type, traits_type>* __tie_;
-  mutable int_type __fill_;
+
+#if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
+// AIX and 64-bit MVS must use _OptionalFill for ABI backward compatibility.
+  using _FillType = _OptionalFill<_Traits>;
+#else
+#if defined(_WIN32)
+  static const bool _OptOutForABICompat = true;
+#else
+  static const bool _OptOutForABICompat = false;
+#endif
+
+  using _FillType = _If<
+      sizeof(char_type) >= sizeof(int_type) && !_OptOutForABICompat,
+      _OptionalFill<_Traits>,
+      _SentinelValueFill<_Traits>
+  >;
+#endif
+  mutable _FillType __fill_;
 };
 
 template <class _CharT, class _Traits>
@@ -605,7 +648,7 @@ template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI void basic_ios<_CharT, _Traits>::init(basic_streambuf<char_type, traits_type>* __sb) {
   ios_base::init(__sb);
   __tie_  = nullptr;
-  __fill_ = traits_type::eof();
+  __fill_ = widen(' ');
 }
 
 template <class _CharT, class _Traits>
@@ -655,16 +698,16 @@ inline _LIBCPP_HIDE_FROM_ABI _CharT basic_ios<_CharT, _Traits>::widen(char __c)
 
 template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI _CharT basic_ios<_CharT, _Traits>::fill() const {
-  if (traits_type::eq_int_type(traits_type::eof(), __fill_))
+  if (!__fill_.__is_set())
     __fill_ = widen(' ');
-  return __fill_;
+  return __fill_.__fill();
 }
 
 template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI _CharT basic_ios<_CharT, _Traits>::fill(char_type __ch) {
-  if (traits_type::eq_int_type(traits_type::eof(), __fill_))
+  if (!__fill_.__is_set())
     __fill_ = widen(' ');
-  char_type __r = __fill_;
+  char_type __r = __fill_.__fill();
   __fill_       = __ch;
   return __r;
 }
diff --git a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
new file mode 100644
index 0000000000000..042e07cb80bbd
--- /dev/null
+++ b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Test that weof as a wchar_t value can be set as the fill character.
+
+// UNSUPPORTED: no-wide-characters
+// REQUIRES: target=powerpc{{(64)?}}-ibm-aix || target=s390x-ibm-zos
+
+#include <iomanip>
+#include <ostream>
+#include <cassert>
+#include <string>
+
+template <class CharT>
+struct testbuf : public std::basic_streambuf<CharT> {
+  testbuf() {}
+};
+
+int main(int, char**) {
+  testbuf<wchar_t> sb;
+  std::wostream os(&sb);
+  os << std::setfill((wchar_t)std::char_traits<wchar_t>::eof());
+  assert(os.fill() == (wchar_t)std::char_traits<wchar_t>::eof());
+
+  return 0;
+}

>From a54d54a900d6da9d3164b02cb86d5379f0d70c35 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Fri, 19 Apr 2024 11:05:18 -0400
Subject: [PATCH 03/20] Addressed comments. - update a comment

---
 libcxx/include/ios | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/include/ios b/libcxx/include/ios
index bc31f30f14acf..3961504f62b39 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -524,7 +524,7 @@ inline _LIBCPP_HIDE_FROM_ABI void ios_base::exceptions(iostate __iostate) {
 
 template <class _Traits>
 // Attribute 'packed' is used to keep the layout compatible with the previous
-// definition of '__set_' in basic_ios on AIX.
+// definition of the '__fill_' and '_set_' pair in basic_ios on AIX & z/OS.
 struct _LIBCPP_PACKED _OptionalFill {
   _OptionalFill() : __set_(false) { }
   _OptionalFill& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }

>From 8fa650aa6a5bf5caa6d235a7fe8cca93e39ad5fd Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Mon, 3 Jun 2024 13:23:10 -0400
Subject: [PATCH 04/20] Addressed comments. -define and use macros
 _LIBCXX_IOS_MAY_USE_OPTIONAL_FILL and _LIBCXX_IOS_FORCE_OPTIONAL_FILL.

---
 libcxx/include/__configuration/abi.h | 13 +++++++++++++
 libcxx/include/ios                   | 15 +++++----------
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index 17aceb042f524..b83e7bf97a559 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -91,6 +91,15 @@
 #  define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_STRING_VIEW
 // Dont' add an inline namespace for `std::filesystem`
 #    define _LIBCPP_ABI_NO_FILESYSTEM_INLINE_NAMESPACE
+// libcxx std::basic_ios uses WEOF to indicate that the fill value is
+// uninitialized. However, on platforms where the size of char_type is
+// equal to or greater than the size of int_type,
+// std::char_traits<char_type>::eq_int_type() cannot distinguish between WEOF
+// and WCHAR_MAX. Helper class _OptionalFill is used for targets where a
+// variable is needed to indicate whether the fill value has been initialized.
+// Existing targets where this would break ABI compatibility can choose to keep
+// the existing ABI by undefining macro _LIBCXX_IOS_MAY_USE_OPTIONAL_FILL.
+#  define _LIBCXX_IOS_MAY_USE_OPTIONAL_FILL
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support
@@ -108,6 +117,10 @@
 #  if defined(__FreeBSD__) && __FreeBSD__ < 14
 #    define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
 #  endif
+// AIX and 64-bit MVS must use _OptionalFill for ABI backward compatibility.
+#  if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
+#    define _LIBCXX_IOS_FORCE_OPTIONAL_FILL
+#  endif
 #endif
 
 // We had some bugs where we use [[no_unique_address]] together with construct_at,
diff --git a/libcxx/include/ios b/libcxx/include/ios
index 3961504f62b39..d056b1ccb8681 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -617,21 +617,16 @@ protected:
 private:
   basic_ostream<char_type, traits_type>* __tie_;
 
-#if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
-// AIX and 64-bit MVS must use _OptionalFill for ABI backward compatibility.
+#if defined(_LIBCXX_IOS_FORCE_OPTIONAL_FILL)
   using _FillType = _OptionalFill<_Traits>;
-#else
-#if defined(_WIN32)
-  static const bool _OptOutForABICompat = true;
-#else
-  static const bool _OptOutForABICompat = false;
-#endif
-
+#elif defined(_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL)
   using _FillType = _If<
-      sizeof(char_type) >= sizeof(int_type) && !_OptOutForABICompat,
+      sizeof(char_type) >= sizeof(int_type),
       _OptionalFill<_Traits>,
       _SentinelValueFill<_Traits>
   >;
+#else
+  using _FillType = _SentinelValueFill<_Traits>;
 #endif
   mutable _FillType __fill_;
 };

>From c033ea9b0486c5dd8a240cb01aee05848dff4a65 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Tue, 4 Jun 2024 06:51:49 -0400
Subject: [PATCH 05/20] Add _LIBCPP_HIDE_FROM_ABI.

---
 libcxx/include/ios | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/libcxx/include/ios b/libcxx/include/ios
index d056b1ccb8681..3099dbe8d9f96 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -526,10 +526,10 @@ template <class _Traits>
 // Attribute 'packed' is used to keep the layout compatible with the previous
 // definition of the '__fill_' and '_set_' pair in basic_ios on AIX & z/OS.
 struct _LIBCPP_PACKED _OptionalFill {
-  _OptionalFill() : __set_(false) { }
-  _OptionalFill& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
-  bool __is_set() const { return __set_; }
-  typename _Traits::int_type __fill() const { return __fill_val_; }
+  _LIBCPP_HIDE_FROM_ABI _OptionalFill() : __set_(false) { }
+  _LIBCPP_HIDE_FROM_ABI _OptionalFill& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
+  _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __set_; }
+  _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
 
 private:
   typename _Traits::int_type __fill_val_;
@@ -538,10 +538,10 @@ private:
 
 template <class _Traits>
 struct _LIBCPP_PACKED _SentinelValueFill {
-  _SentinelValueFill() : __fill_val_(_Traits::eof()) { }
-  _SentinelValueFill& operator=(typename _Traits::int_type __x) { __fill_val_ = __x; return *this; }
-  bool __is_set() const { return __fill_val_ != _Traits::eof(); }
-  typename _Traits::int_type __fill() const { return __fill_val_; }
+  _LIBCPP_HIDE_FROM_ABI _SentinelValueFill() : __fill_val_(_Traits::eof()) { }
+  _LIBCPP_HIDE_FROM_ABI _SentinelValueFill& operator=(typename _Traits::int_type __x) { __fill_val_ = __x; return *this; }
+  _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __fill_val_ != _Traits::eof(); }
+  _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
 
 private:
   typename _Traits::int_type __fill_val_;

>From 5288cb33586bd5ac22efabfde95ff143222c366c Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Tue, 4 Jun 2024 07:08:40 -0400
Subject: [PATCH 06/20] Removed changes in unrelated test case.

---
 openmp/runtime/test/worksharing/for/collapse_test.inc | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/openmp/runtime/test/worksharing/for/collapse_test.inc b/openmp/runtime/test/worksharing/for/collapse_test.inc
index 74ec83a059d91..3075bd04e958f 100644
--- a/openmp/runtime/test/worksharing/for/collapse_test.inc
+++ b/openmp/runtime/test/worksharing/for/collapse_test.inc
@@ -15,12 +15,7 @@
 #define LOOP_TYPE2 LOOP_TYPES
 #define LOOP_STYPE2 LOOP_TYPES
 
-#if defined _AIX
-// Use lower max threads to reduce the testing time
-#define MAX_THREADS 64
-#else
 #define MAX_THREADS 256
-#endif
 
 #if defined VERBOSE
 #define PRINTF(...) printf(__VA_ARGS__)

>From 00f4cd285b2a8281168486377a170246a33e1c34 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Tue, 4 Jun 2024 12:20:39 -0400
Subject: [PATCH 07/20] Add description of ABI change in ReleaseNote.

---
 libcxx/docs/ReleaseNotes/19.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index 0bc343acd281c..935d065a5e0ac 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -146,6 +146,13 @@ ABI Affecting Changes
   ``random_device`` could throw a ``system_error`` with this value. It now
   throws ``ENOMSG``.
 
+- libcxx ``std::basic_ios`` uses ``WEOF`` to indicate that the fill value is uninitialized. However, on platforms
+  where the size of ``char_type`` is equal to or greater than the size of ``int_type``,
+  ``std::char_traits<char_type>::eq_int_type()`` cannot distinguish between ``WEOF`` and ``WCHAR_MAX``. Helper
+  class ``_OptionalFill`` is used for targets where a variable is needed to indicate whether the fill value
+  has been initialized. This is an ABI break on platforms where the size of ``char_type`` is equal to or greater
+  than the size of ``int_type``. Existing targets affected by this change can choose to keep the existing ABI
+  by undefining macro ``_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL``.
 
 Build System Changes
 --------------------

>From e71aafc0a3e39773b4e3c4a5f79bd8aa3a5fcf14 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Wed, 5 Jun 2024 11:07:47 -0400
Subject: [PATCH 08/20] Address comment. - expect the test case to pass except
 windows in abi version 1.

---
 .../input.output/iostream.format/std.manip/setfill_eof.pass.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
index 042e07cb80bbd..6cf27ac2de103 100644
--- a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
+++ b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
@@ -9,7 +9,7 @@
 // Test that weof as a wchar_t value can be set as the fill character.
 
 // UNSUPPORTED: no-wide-characters
-// REQUIRES: target=powerpc{{(64)?}}-ibm-aix || target=s390x-ibm-zos
+// XFAIL: target=={{.+}}-windows-{{.+}} && stdlib=libc++ && libcpp-abi-version=1
 
 #include <iomanip>
 #include <ostream>

>From b2ba84cd16831abe195b812936cca01e9480c0b0 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Fri, 7 Jun 2024 13:10:59 -0400
Subject: [PATCH 09/20] Use helper class _FillHelper by default in ABI version
 2.

---
 libcxx/docs/ReleaseNotes/19.rst                 | 11 +++++------
 libcxx/include/__configuration/abi.h            | 17 +++++++++--------
 libcxx/include/ios                              | 17 +++++------------
 .../std.manip/setfill_eof.pass.cpp              |  2 +-
 4 files changed, 20 insertions(+), 27 deletions(-)

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index 935d065a5e0ac..678f99c60fd81 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -147,12 +147,11 @@ ABI Affecting Changes
   throws ``ENOMSG``.
 
 - libcxx ``std::basic_ios`` uses ``WEOF`` to indicate that the fill value is uninitialized. However, on platforms
-  where the size of ``char_type`` is equal to or greater than the size of ``int_type``,
-  ``std::char_traits<char_type>::eq_int_type()`` cannot distinguish between ``WEOF`` and ``WCHAR_MAX``. Helper
-  class ``_OptionalFill`` is used for targets where a variable is needed to indicate whether the fill value
-  has been initialized. This is an ABI break on platforms where the size of ``char_type`` is equal to or greater
-  than the size of ``int_type``. Existing targets affected by this change can choose to keep the existing ABI
-  by undefining macro ``_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL``.
+  where the size of ``char_type`` is equal to or greater than the size of ``int_type`` and ``char_type`` is unsigned,
+  ``std::char_traits<char_type>::eq_int_type()`` cannot distinguish between ``WEOF`` and ``WCHAR_MAX``. New helper
+  class ``_FillHelper`` uses a boolean variable to indicate whether the fill value has been initialized so that a fill
+  value ``WEOF`` set by the user won't be treated as indicating the fill value is uninitialized.
+  Undefining macro ``_LIBCXX_IOS_USE_FILL_HELPER`` to keep the ABI verson 1 behavior if needed.
 
 Build System Changes
 --------------------
diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index b83e7bf97a559..646ec3100a8dd 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -93,13 +93,14 @@
 #    define _LIBCPP_ABI_NO_FILESYSTEM_INLINE_NAMESPACE
 // libcxx std::basic_ios uses WEOF to indicate that the fill value is
 // uninitialized. However, on platforms where the size of char_type is
-// equal to or greater than the size of int_type,
+// equal to or greater than the size of int_type and char_type is unsigned,
 // std::char_traits<char_type>::eq_int_type() cannot distinguish between WEOF
-// and WCHAR_MAX. Helper class _OptionalFill is used for targets where a
-// variable is needed to indicate whether the fill value has been initialized.
-// Existing targets where this would break ABI compatibility can choose to keep
-// the existing ABI by undefining macro _LIBCXX_IOS_MAY_USE_OPTIONAL_FILL.
-#  define _LIBCXX_IOS_MAY_USE_OPTIONAL_FILL
+// and WCHAR_MAX. New helper class _FillHelper uses a boolean variable to indicate
+// whether the fill value has been initialized so that a fill value WEOF set
+// by the user won't be treated as indicating the fill value is uninitialized.
+// Undefining macro _LIBCXX_IOS_USE_FILL_HELPER to keep the ABI verson 1
+// behavior if needed.
+#  define _LIBCXX_IOS_USE_FILL_HELPER
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support
@@ -117,9 +118,9 @@
 #  if defined(__FreeBSD__) && __FreeBSD__ < 14
 #    define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
 #  endif
-// AIX and 64-bit MVS must use _OptionalFill for ABI backward compatibility.
+// AIX and 64-bit MVS must use _FillHelper for ABI backward compatibility.
 #  if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
-#    define _LIBCXX_IOS_FORCE_OPTIONAL_FILL
+#    define _LIBCXX_IOS_USE_FILL_HELPER
 #  endif
 #endif
 
diff --git a/libcxx/include/ios b/libcxx/include/ios
index 3099dbe8d9f96..6ccef00358530 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -525,9 +525,9 @@ inline _LIBCPP_HIDE_FROM_ABI void ios_base::exceptions(iostate __iostate) {
 template <class _Traits>
 // Attribute 'packed' is used to keep the layout compatible with the previous
 // definition of the '__fill_' and '_set_' pair in basic_ios on AIX & z/OS.
-struct _LIBCPP_PACKED _OptionalFill {
-  _LIBCPP_HIDE_FROM_ABI _OptionalFill() : __set_(false) { }
-  _LIBCPP_HIDE_FROM_ABI _OptionalFill& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
+struct _LIBCPP_PACKED _FillHelper {
+  _LIBCPP_HIDE_FROM_ABI _FillHelper() : __set_(false) { }
+  _LIBCPP_HIDE_FROM_ABI _FillHelper& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
   _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __set_; }
   _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
 
@@ -617,14 +617,8 @@ protected:
 private:
   basic_ostream<char_type, traits_type>* __tie_;
 
-#if defined(_LIBCXX_IOS_FORCE_OPTIONAL_FILL)
-  using _FillType = _OptionalFill<_Traits>;
-#elif defined(_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL)
-  using _FillType = _If<
-      sizeof(char_type) >= sizeof(int_type),
-      _OptionalFill<_Traits>,
-      _SentinelValueFill<_Traits>
-  >;
+#if defined(_LIBCXX_IOS_USE_FILL_HELPER)
+  using _FillType = _FillHelper<_Traits>;
 #else
   using _FillType = _SentinelValueFill<_Traits>;
 #endif
@@ -643,7 +637,6 @@ template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI void basic_ios<_CharT, _Traits>::init(basic_streambuf<char_type, traits_type>* __sb) {
   ios_base::init(__sb);
   __tie_  = nullptr;
-  __fill_ = widen(' ');
 }
 
 template <class _CharT, class _Traits>
diff --git a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
index 6cf27ac2de103..66b8d0314799a 100644
--- a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
+++ b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
@@ -9,7 +9,7 @@
 // Test that weof as a wchar_t value can be set as the fill character.
 
 // UNSUPPORTED: no-wide-characters
-// XFAIL: target=={{.+}}-windows-{{.+}} && stdlib=libc++ && libcpp-abi-version=1
+// REQUIRES: target=powerpc{{(64)?}}-ibm-aix || target=s390x-ibm-zos || libcpp-abi-version=2
 
 #include <iomanip>
 #include <ostream>

>From 33e58edb18cd607db0b7754caddd5ff1620fc826 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Fri, 7 Jun 2024 14:16:39 -0400
Subject: [PATCH 10/20] Remove unnecessary include of
 __type_traits/conditional.h.

---
 libcxx/include/ios | 1 -
 1 file changed, 1 deletion(-)

diff --git a/libcxx/include/ios b/libcxx/include/ios
index 6ccef00358530..4ffba6bda93e1 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -224,7 +224,6 @@ storage-class-specifier const error_category& iostream_category() noexcept;
 #include <__system_error/error_code.h>
 #include <__system_error/error_condition.h>
 #include <__system_error/system_error.h>
-#include <__type_traits/conditional.h>
 #include <__utility/swap.h>
 #include <__verbose_abort>
 #include <version>

>From 933f8880ec8bf2925a12bf4f82ae427becc3c87b Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Tue, 11 Jun 2024 12:13:54 -0400
Subject: [PATCH 11/20] Addressed comments. - add note that the change only
 affect ABI version 2 - remove "undefining _LIBCXX_IOS_USE_FILL_HELPER to keep
 the ABI verson 1 behavior" - add init() functions for class _FillHelper and
 _SentinelValueFill

---
 libcxx/docs/ReleaseNotes/19.rst      | 15 +++++++++------
 libcxx/include/__configuration/abi.h |  2 --
 libcxx/include/ios                   |  5 +++--
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index 678f99c60fd81..1684551cbe4a8 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -146,12 +146,15 @@ ABI Affecting Changes
   ``random_device`` could throw a ``system_error`` with this value. It now
   throws ``ENOMSG``.
 
-- libcxx ``std::basic_ios`` uses ``WEOF`` to indicate that the fill value is uninitialized. However, on platforms
-  where the size of ``char_type`` is equal to or greater than the size of ``int_type`` and ``char_type`` is unsigned,
-  ``std::char_traits<char_type>::eq_int_type()`` cannot distinguish between ``WEOF`` and ``WCHAR_MAX``. New helper
-  class ``_FillHelper`` uses a boolean variable to indicate whether the fill value has been initialized so that a fill
-  value ``WEOF`` set by the user won't be treated as indicating the fill value is uninitialized.
-  Undefining macro ``_LIBCXX_IOS_USE_FILL_HELPER`` to keep the ABI verson 1 behavior if needed.
+- libcxx ``std::basic_ios`` uses ``WEOF`` to indicate that the fill value is
+  uninitialized. However, on platforms where the size of ``char_type`` is
+  equal to or greater than the size of ``int_type`` and ``char_type`` is
+  unsigned, ``std::char_traits<char_type>::eq_int_type()`` cannot distinguish
+  between ``WEOF`` and ``WCHAR_MAX``. Helper class ``_FillHelper`` is
+  added where a boolean variable is used to indicate whether the fill value
+  has been initialized so that a fill value ``WEOF`` set by the user won't be
+  treated as indicating the fill value is uninitialized. ``_FillHelper`` is
+  used in ABI version 2 and does not affect ABI version 1.
 
 Build System Changes
 --------------------
diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index 646ec3100a8dd..051425f78a9a6 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -98,8 +98,6 @@
 // and WCHAR_MAX. New helper class _FillHelper uses a boolean variable to indicate
 // whether the fill value has been initialized so that a fill value WEOF set
 // by the user won't be treated as indicating the fill value is uninitialized.
-// Undefining macro _LIBCXX_IOS_USE_FILL_HELPER to keep the ABI verson 1
-// behavior if needed.
 #  define _LIBCXX_IOS_USE_FILL_HELPER
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
diff --git a/libcxx/include/ios b/libcxx/include/ios
index 4ffba6bda93e1..b7d1f2750c75b 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -525,7 +525,7 @@ template <class _Traits>
 // Attribute 'packed' is used to keep the layout compatible with the previous
 // definition of the '__fill_' and '_set_' pair in basic_ios on AIX & z/OS.
 struct _LIBCPP_PACKED _FillHelper {
-  _LIBCPP_HIDE_FROM_ABI _FillHelper() : __set_(false) { }
+  _LIBCPP_HIDE_FROM_ABI void init() { __set_ = false; }
   _LIBCPP_HIDE_FROM_ABI _FillHelper& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
   _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __set_; }
   _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
@@ -537,7 +537,7 @@ private:
 
 template <class _Traits>
 struct _LIBCPP_PACKED _SentinelValueFill {
-  _LIBCPP_HIDE_FROM_ABI _SentinelValueFill() : __fill_val_(_Traits::eof()) { }
+  _LIBCPP_HIDE_FROM_ABI void init() { __fill_val_ = _Traits::eof(); }
   _LIBCPP_HIDE_FROM_ABI _SentinelValueFill& operator=(typename _Traits::int_type __x) { __fill_val_ = __x; return *this; }
   _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __fill_val_ != _Traits::eof(); }
   _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
@@ -636,6 +636,7 @@ template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI void basic_ios<_CharT, _Traits>::init(basic_streambuf<char_type, traits_type>* __sb) {
   ios_base::init(__sb);
   __tie_  = nullptr;
+  __fill_.init();
 }
 
 template <class _CharT, class _Traits>

>From 7ef7b825b1de1f11179ace3298469a13c39299ac Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Tue, 11 Jun 2024 13:26:49 -0400
Subject: [PATCH 12/20] Rename init() to __init().

---
 libcxx/include/ios | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libcxx/include/ios b/libcxx/include/ios
index b7d1f2750c75b..6db8711139606 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -525,7 +525,7 @@ template <class _Traits>
 // Attribute 'packed' is used to keep the layout compatible with the previous
 // definition of the '__fill_' and '_set_' pair in basic_ios on AIX & z/OS.
 struct _LIBCPP_PACKED _FillHelper {
-  _LIBCPP_HIDE_FROM_ABI void init() { __set_ = false; }
+  _LIBCPP_HIDE_FROM_ABI void __init() { __set_ = false; }
   _LIBCPP_HIDE_FROM_ABI _FillHelper& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
   _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __set_; }
   _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
@@ -537,7 +537,7 @@ private:
 
 template <class _Traits>
 struct _LIBCPP_PACKED _SentinelValueFill {
-  _LIBCPP_HIDE_FROM_ABI void init() { __fill_val_ = _Traits::eof(); }
+  _LIBCPP_HIDE_FROM_ABI void __init() { __fill_val_ = _Traits::eof(); }
   _LIBCPP_HIDE_FROM_ABI _SentinelValueFill& operator=(typename _Traits::int_type __x) { __fill_val_ = __x; return *this; }
   _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __fill_val_ != _Traits::eof(); }
   _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
@@ -636,7 +636,7 @@ template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI void basic_ios<_CharT, _Traits>::init(basic_streambuf<char_type, traits_type>* __sb) {
   ios_base::init(__sb);
   __tie_  = nullptr;
-  __fill_.init();
+  __fill_.__init();
 }
 
 template <class _CharT, class _Traits>

>From 9c493f34b72c94343d109e5d0ef473d7f443fbad Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Fri, 14 Jun 2024 11:33:05 -0400
Subject: [PATCH 13/20] Addressed comments. - Use traits_type instead of
 _Traits to be consistent.

---
 libcxx/include/ios | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libcxx/include/ios b/libcxx/include/ios
index 6db8711139606..6535216fcfb19 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -617,9 +617,9 @@ private:
   basic_ostream<char_type, traits_type>* __tie_;
 
 #if defined(_LIBCXX_IOS_USE_FILL_HELPER)
-  using _FillType = _FillHelper<_Traits>;
+  using _FillType = _FillHelper<traits_type>;
 #else
-  using _FillType = _SentinelValueFill<_Traits>;
+  using _FillType = _SentinelValueFill<traits_type>;
 #endif
   mutable _FillType __fill_;
 };

>From a9280c0db10950246d3fde5b2876aa25dd993cc7 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Thu, 4 Jul 2024 15:33:38 -0400
Subject: [PATCH 14/20] Update libcxx/include/__configuration/abi.h

Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>
---
 libcxx/include/__configuration/abi.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index 051425f78a9a6..6155013f3bcfa 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -91,7 +91,7 @@
 #  define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_STRING_VIEW
 // Dont' add an inline namespace for `std::filesystem`
 #    define _LIBCPP_ABI_NO_FILESYSTEM_INLINE_NAMESPACE
-// libcxx std::basic_ios uses WEOF to indicate that the fill value is
+// std::basic_ios uses WEOF to indicate that the fill value is
 // uninitialized. However, on platforms where the size of char_type is
 // equal to or greater than the size of int_type and char_type is unsigned,
 // std::char_traits<char_type>::eq_int_type() cannot distinguish between WEOF

>From 68edd80a02eab9f2d71a0e1e88f65c70d642f1b9 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Thu, 4 Jul 2024 15:35:20 -0400
Subject: [PATCH 15/20] Update libcxx/include/__configuration/abi.h

Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>
---
 libcxx/include/__configuration/abi.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index 6155013f3bcfa..e071b9fe7e13e 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -95,9 +95,8 @@
 // uninitialized. However, on platforms where the size of char_type is
 // equal to or greater than the size of int_type and char_type is unsigned,
 // std::char_traits<char_type>::eq_int_type() cannot distinguish between WEOF
-// and WCHAR_MAX. New helper class _FillHelper uses a boolean variable to indicate
-// whether the fill value has been initialized so that a fill value WEOF set
-// by the user won't be treated as indicating the fill value is uninitialized.
+// and WCHAR_MAX. This ABI setting determines whether we should instead track whether the fill
+// value has been initialized using a separate boolean, which changes the ABI.
 #  define _LIBCXX_IOS_USE_FILL_HELPER
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))

>From c0f97c32fc33559d11404c1d45fa98d5f4a2e719 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Thu, 4 Jul 2024 15:36:26 -0400
Subject: [PATCH 16/20] Update libcxx/include/__configuration/abi.h

Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>
---
 libcxx/include/__configuration/abi.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index e071b9fe7e13e..213ff2d366268 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -97,7 +97,7 @@
 // std::char_traits<char_type>::eq_int_type() cannot distinguish between WEOF
 // and WCHAR_MAX. This ABI setting determines whether we should instead track whether the fill
 // value has been initialized using a separate boolean, which changes the ABI.
-#  define _LIBCXX_IOS_USE_FILL_HELPER
+#  define _LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support

>From 49f202f4d84fb7517765db7c2fdb12fd01e3a5fd Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Thu, 4 Jul 2024 15:36:57 -0400
Subject: [PATCH 17/20] Update libcxx/include/ios

Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>
---
 libcxx/include/ios | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/include/ios b/libcxx/include/ios
index 6535216fcfb19..6996a103e6958 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -528,7 +528,7 @@ struct _LIBCPP_PACKED _FillHelper {
   _LIBCPP_HIDE_FROM_ABI void __init() { __set_ = false; }
   _LIBCPP_HIDE_FROM_ABI _FillHelper& operator=(typename _Traits::int_type __x) { __set_ = true; __fill_val_ = __x; return *this; }
   _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __set_; }
-  _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
+  _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __get() const { return __fill_val_; }
 
 private:
   typename _Traits::int_type __fill_val_;

>From 29a463543abf3b800f817c386aa1c3b0d5ff7703 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Thu, 4 Jul 2024 18:26:31 -0400
Subject: [PATCH 18/20] Addressed comments. - set cmake variable
 LIBCXX_ABI_DEFINES with the ABI macro for AIX and zOS 64-bit - remove the
 note for this change from the release note. - rename function name __fill()
 to __get() - remove REQUIRES from the test case.

---
 libcxx/cmake/caches/AIX.cmake                          |  1 +
 libcxx/cmake/caches/s390x-ibm-zos-ascii.cmake          |  1 +
 libcxx/cmake/caches/s390x-ibm-zos.cmake                |  1 +
 libcxx/docs/ReleaseNotes/19.rst                        | 10 ----------
 libcxx/include/ios                                     |  8 ++++----
 .../iostream.format/std.manip/setfill_eof.pass.cpp     |  1 -
 6 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/libcxx/cmake/caches/AIX.cmake b/libcxx/cmake/caches/AIX.cmake
index c01aa5b14df06..4ec78f9bbd592 100644
--- a/libcxx/cmake/caches/AIX.cmake
+++ b/libcxx/cmake/caches/AIX.cmake
@@ -15,3 +15,4 @@ set(LIBCXXABI_ENABLE_STATIC OFF CACHE BOOL "")
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
 set(LIBUNWIND_ENABLE_SHARED ON CACHE BOOL "")
 set(LIBUNWIND_ENABLE_STATIC OFF CACHE BOOL "")
+set(LIBCXX_ABI_DEFINES "_LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE" CACHE STRING "")
diff --git a/libcxx/cmake/caches/s390x-ibm-zos-ascii.cmake b/libcxx/cmake/caches/s390x-ibm-zos-ascii.cmake
index 95b7cbe776e05..68b1cc8eff9b0 100644
--- a/libcxx/cmake/caches/s390x-ibm-zos-ascii.cmake
+++ b/libcxx/cmake/caches/s390x-ibm-zos-ascii.cmake
@@ -20,3 +20,4 @@ set(LIBCXX_CXX_ABI system-libcxxabi CACHE STRING "")
 
 set(LIBCXX_ADDITIONAL_COMPILE_FLAGS "-fzos-le-char-mode=ascii" CACHE STRING "")
 set(LIBCXX_ADDITIONAL_LIBRARIES "-L../s390x-ibm-zos/lib -Wl,../s390x-ibm-zos/lib/libunwind.x" CACHE STRING "")
+set(LIBCXX_ABI_DEFINES "_LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE" CACHE STRING "")
diff --git a/libcxx/cmake/caches/s390x-ibm-zos.cmake b/libcxx/cmake/caches/s390x-ibm-zos.cmake
index 3eaed3e67fc16..e51d5ff31ebfe 100644
--- a/libcxx/cmake/caches/s390x-ibm-zos.cmake
+++ b/libcxx/cmake/caches/s390x-ibm-zos.cmake
@@ -15,3 +15,4 @@ set(LIBCXX_DLL_NAME CRTEQCXE CACHE STRING "")
 
 set(LIBCXXABI_DLL_NAME CRTEQCXA CACHE STRING "")
 set(LIBCXXABI_ADDITIONAL_LIBRARIES "-Wl,lib/libunwind.x" CACHE STRING "")
+set(LIBCXX_ABI_DEFINES "_LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE" CACHE STRING "")
diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index 1684551cbe4a8..73dfebe8c1a51 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -146,16 +146,6 @@ ABI Affecting Changes
   ``random_device`` could throw a ``system_error`` with this value. It now
   throws ``ENOMSG``.
 
-- libcxx ``std::basic_ios`` uses ``WEOF`` to indicate that the fill value is
-  uninitialized. However, on platforms where the size of ``char_type`` is
-  equal to or greater than the size of ``int_type`` and ``char_type`` is
-  unsigned, ``std::char_traits<char_type>::eq_int_type()`` cannot distinguish
-  between ``WEOF`` and ``WCHAR_MAX``. Helper class ``_FillHelper`` is
-  added where a boolean variable is used to indicate whether the fill value
-  has been initialized so that a fill value ``WEOF`` set by the user won't be
-  treated as indicating the fill value is uninitialized. ``_FillHelper`` is
-  used in ABI version 2 and does not affect ABI version 1.
-
 Build System Changes
 --------------------
 
diff --git a/libcxx/include/ios b/libcxx/include/ios
index 6996a103e6958..4af04fbd10e02 100644
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -540,7 +540,7 @@ struct _LIBCPP_PACKED _SentinelValueFill {
   _LIBCPP_HIDE_FROM_ABI void __init() { __fill_val_ = _Traits::eof(); }
   _LIBCPP_HIDE_FROM_ABI _SentinelValueFill& operator=(typename _Traits::int_type __x) { __fill_val_ = __x; return *this; }
   _LIBCPP_HIDE_FROM_ABI bool __is_set() const { return __fill_val_ != _Traits::eof(); }
-  _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __fill() const { return __fill_val_; }
+  _LIBCPP_HIDE_FROM_ABI typename _Traits::int_type __get() const { return __fill_val_; }
 
 private:
   typename _Traits::int_type __fill_val_;
@@ -616,7 +616,7 @@ protected:
 private:
   basic_ostream<char_type, traits_type>* __tie_;
 
-#if defined(_LIBCXX_IOS_USE_FILL_HELPER)
+#if defined(_LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE)
   using _FillType = _FillHelper<traits_type>;
 #else
   using _FillType = _SentinelValueFill<traits_type>;
@@ -688,14 +688,14 @@ template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI _CharT basic_ios<_CharT, _Traits>::fill() const {
   if (!__fill_.__is_set())
     __fill_ = widen(' ');
-  return __fill_.__fill();
+  return __fill_.__get();
 }
 
 template <class _CharT, class _Traits>
 inline _LIBCPP_HIDE_FROM_ABI _CharT basic_ios<_CharT, _Traits>::fill(char_type __ch) {
   if (!__fill_.__is_set())
     __fill_ = widen(' ');
-  char_type __r = __fill_.__fill();
+  char_type __r = __fill_.__get();
   __fill_       = __ch;
   return __r;
 }
diff --git a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
index 66b8d0314799a..00d070ac77ca6 100644
--- a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
+++ b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
@@ -9,7 +9,6 @@
 // Test that weof as a wchar_t value can be set as the fill character.
 
 // UNSUPPORTED: no-wide-characters
-// REQUIRES: target=powerpc{{(64)?}}-ibm-aix || target=s390x-ibm-zos || libcpp-abi-version=2
 
 #include <iomanip>
 #include <ostream>

>From 88fc9eb67efa469aeb5a4b370cfe69b9a7832a77 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Thu, 4 Jul 2024 18:35:47 -0400
Subject: [PATCH 19/20] Remove the unused macro definition for AIX/zOS in
 abi.h.

---
 libcxx/docs/ReleaseNotes/19.rst      | 1 +
 libcxx/include/__configuration/abi.h | 4 ----
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index 73dfebe8c1a51..0bc343acd281c 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -146,6 +146,7 @@ ABI Affecting Changes
   ``random_device`` could throw a ``system_error`` with this value. It now
   throws ``ENOMSG``.
 
+
 Build System Changes
 --------------------
 
diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index 213ff2d366268..1a28839884b27 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -115,10 +115,6 @@
 #  if defined(__FreeBSD__) && __FreeBSD__ < 14
 #    define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
 #  endif
-// AIX and 64-bit MVS must use _FillHelper for ABI backward compatibility.
-#  if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
-#    define _LIBCXX_IOS_USE_FILL_HELPER
-#  endif
 #endif
 
 // We had some bugs where we use [[no_unique_address]] together with construct_at,

>From 9ea946ee9e42305f5fbde72d19e08e31cc4f9467 Mon Sep 17 00:00:00 2001
From: Xing Xue <xingxue at outlook.com>
Date: Fri, 12 Jul 2024 14:35:51 +0000
Subject: [PATCH 20/20] setfill() WCHAR_MAX instead of WEOF for the test.

---
 .../{setfill_eof.pass.cpp => setfill_wchar_max.pass.cpp}    | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
 rename libcxx/test/std/input.output/iostream.format/std.manip/{setfill_eof.pass.cpp => setfill_wchar_max.pass.cpp} (76%)

diff --git a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_wchar_max.pass.cpp
similarity index 76%
rename from libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
rename to libcxx/test/std/input.output/iostream.format/std.manip/setfill_wchar_max.pass.cpp
index 00d070ac77ca6..40033c8d28dc6 100644
--- a/libcxx/test/std/input.output/iostream.format/std.manip/setfill_eof.pass.cpp
+++ b/libcxx/test/std/input.output/iostream.format/std.manip/setfill_wchar_max.pass.cpp
@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// Test that weof as a wchar_t value can be set as the fill character.
+// Test that WCHAR_MAX as a wchar_t value can be set as the fill character.
 
 // UNSUPPORTED: no-wide-characters
 
@@ -23,8 +23,8 @@ struct testbuf : public std::basic_streambuf<CharT> {
 int main(int, char**) {
   testbuf<wchar_t> sb;
   std::wostream os(&sb);
-  os << std::setfill((wchar_t)std::char_traits<wchar_t>::eof());
-  assert(os.fill() == (wchar_t)std::char_traits<wchar_t>::eof());
+  os << std::setfill((wchar_t)WCHAR_MAX);
+  assert(os.fill() == (wchar_t)WCHAR_MAX);
 
   return 0;
 }



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