[llvm] [libcxx] [libcxxabi] [clang] [lldb] [libc] [clang-tools-extra] [libc++] Fix the behavior of throwing `operator new` under -fno-exceptions (PR #69498)

Louis Dionne via cfe-commits cfe-commits at lists.llvm.org
Tue Jan 16 12:52:51 PST 2024


https://github.com/ldionne updated https://github.com/llvm/llvm-project/pull/69498

>From 6f89b118ed56ad7a3af1996e19ccd30cc893c51e Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Wed, 14 Jun 2023 17:49:22 -0700
Subject: [PATCH 1/7] [libc++] Fix the behavior of throwing `operator new`
 under -fno-exceptions

In D144319, Clang tried to land a change that would cause some functions
that are not supposed to return nullptr to optimize better. As reported
in https://reviews.llvm.org/D144319#4203982, libc++ started seeing failures
in its CI shortly after this change was landed.

As explained in D146379, the reason for these failures is that libc++'s
throwing `operator new` can in fact return nullptr when compiled with
exceptions disabled. However, this contradicts the Standard, which
clearly says that the throwing version of `operator new(size_t)`
should never return nullptr. This is actually a long standing issue.
I've previously seen a case where LTO would optimize incorrectly based
on the assumption that `operator new` doesn't return nullptr, an
assumption that was violated in that case because libc++.dylib was
compiled with -fno-exceptions.

Unfortunately, fixing this is kind of tricky. The Standard has a few
requirements for the allocation functions, some of which are impossible
to satisfy under -fno-exceptions:
1. `operator new(size_t)` must never return nullptr
2. `operator new(size_t, nothrow_t)` must call the throwing version
     and return nullptr on failure to allocate
3. We can't throw exceptions when compiled with -fno-exceptions

In the case where exceptions are enabled, things work nicely. `new(size_t)`
throws and `new(size_t, nothrow_t)` uses a try-catch to return nullptr.
However, when compiling the library with -fno-exceptions, we can't throw
an exception from `new(size_t)`, and we can't catch anything from
`new(size_t, nothrow_t)`. The only thing we can do from `new(size_t)`
is actually abort the program, which does not make it possible for
`new(size_t, nothrow_t)` to catch something and return nullptr.

This patch makes the following changes:
1. When compiled with -fno-exceptions, the throwing version of
   `operator new` will now abort on failure instead of returning
   nullptr on failure. This resolves the issue that the compiler
   could mis-compile based on the assumption that nullptr is never
   returned. This constitutes an API and ABI breaking change for
   folks compiling the library with -fno-exceptions (which is not
   the general public, who merely uses libc++ headers but use a
   shared library that has already been compiled). This should mostly
   impact vendors and other folks who compile libc++.dylib themselves.

2. When the library is compiled with -fexceptions, the nothrow version
   of `operator new` has no change. When the library is compiled with
   -fno-exceptions, the nothrow version of `operator new` will now check
   whether the throwing version of `operator new` has been overridden.
   If it has not been overridden, then it will use an implementation
   equivalent to that of the throwing `operator new`, except it will
   return nullptr on failure to allocate (instead of terminating).
   However, if the throwing `operator new` has been overridden, it is
   now an error NOT to also override the nothrow `operator new`. Indeed,
   there is no way for us to implement a valid nothrow `operator new`
   without knowing the exact implementation of the throwing version.

rdar://103958777

Differential Revision: https://reviews.llvm.org/D150610
---
 libcxx/docs/ReleaseNotes/18.rst               |  23 +++
 libcxx/include/CMakeLists.txt                 |   1 +
 libcxx/include/__overridable_function         | 119 +++++++++++++
 libcxx/src/new.cpp                            |  99 ++++++++---
 ...new_not_overridden_fno_exceptions.pass.cpp |  59 ++++++
 .../new_dont_return_nullptr.pass.cpp          |  37 ++++
 libcxx/test/support/check_assertion.h         |   6 +
 libcxx/utils/generate_iwyu_mapping.py         |   2 +
 libcxxabi/src/stdlib_new_delete.cpp           | 168 +++++++++++-------
 9 files changed, 426 insertions(+), 88 deletions(-)
 create mode 100644 libcxx/include/__overridable_function
 create mode 100644 libcxx/test/libcxx/language.support/support.dynamic/assert.nothrow_new_not_overridden_fno_exceptions.pass.cpp
 create mode 100644 libcxx/test/libcxx/language.support/support.dynamic/new_dont_return_nullptr.pass.cpp

diff --git a/libcxx/docs/ReleaseNotes/18.rst b/libcxx/docs/ReleaseNotes/18.rst
index 9e509db6359c4aa..405f1e172893bc1 100644
--- a/libcxx/docs/ReleaseNotes/18.rst
+++ b/libcxx/docs/ReleaseNotes/18.rst
@@ -142,6 +142,29 @@ LLVM 20
 ABI Affecting Changes
 ---------------------
 
+- When the shared/static library is built with ``-fno-exceptions``, the behavior of ``operator new`` was changed
+  to make it standards-conforming. In LLVM 17 and before, the throwing versions of ``operator new`` would return
+  ``nullptr`` upon failure to allocate, when the shared/static library was built with exceptions disabled. This
+  was non-conforming, since the throwing versions of ``operator new`` are never expected to return ``nullptr``, and
+  this non-conformance could actually lead to miscompiles in subtle cases.
+
+  Starting in LLVM 18, the throwing versions of ``operator new`` will abort the program when they fail to allocate
+  if the shared/static library has been built with ``-fno-exceptions``. This is consistent with the behavior of all
+  other potentially-throwing functions in the library, which abort the program instead of throwing when ``-fno-exceptions``
+  is used.
+
+  Furthermore, when the shared/static library is built with ``-fno-exceptions``, users who override the throwing
+  version of ``operator new`` will now need to also override the ``std::nothrow_t`` version of ``operator new`` if
+  they want to use it. Indeed, this is because there is no way to implement a conforming ``operator new(nothrow)``
+  from a conforming potentially-throwing ``operator new`` when compiled with ``-fno-exceptions``. In that case, using
+  ``operator new(nothrow)`` without overriding it explicitly but after overriding the throwing ``operator new`` will
+  result in an error.
+
+  Note that this change only impacts vendors/users that build the shared/static library themselves and pass
+  ``-DLIBCXX_ENABLE_EXCEPTIONS=OFF``, which is not the default configuration. If you are using the default
+  configuration of the library, the libc++ shared/static library will be built with exceptions enabled, and
+  there is no change between LLVM 17 and LLVM 18, even for users who build their own code using ``-fno-exceptions``.
+
 - The symbol of a non-visible function part of ``std::system_error`` was removed.
   This is not a breaking change as the private function ``__init`` was never referenced internally outside of the dylib.
 
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 7d0defa26b0f73c..7d5a07d995e2675 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -570,6 +570,7 @@ set(files
   __numeric/transform_exclusive_scan.h
   __numeric/transform_inclusive_scan.h
   __numeric/transform_reduce.h
+  __overridable_function
   __random/bernoulli_distribution.h
   __random/binomial_distribution.h
   __random/cauchy_distribution.h
diff --git a/libcxx/include/__overridable_function b/libcxx/include/__overridable_function
new file mode 100644
index 000000000000000..81dd81bdc0ae8a2
--- /dev/null
+++ b/libcxx/include/__overridable_function
@@ -0,0 +1,119 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___OVERRIDABLE_FUNCTION
+#define _LIBCPP___OVERRIDABLE_FUNCTION
+
+#include <__config>
+#include <cstdint>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+//
+// This file provides the std::__is_function_overridden utility, which allows checking
+// whether an overridable function (typically a weak symbol) like `operator new`
+// has been overridden by a user or not.
+//
+// This is a low-level utility which does not work on all platforms, since it needs
+// to make assumptions about the object file format in use. Furthermore, it requires
+// the "base definition" of the function (the one we want to check whether it has been
+// overridden) to be annotated with the _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
+//
+// This currently works with Mach-O files (used on Darwin) and with ELF files (used on Linux
+// and others). On platforms where we know how to implement this detection, the macro
+// _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION is defined to 1, and it is defined to 0 on
+// other platforms. The _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro is defined to
+// nothing on unsupported platforms so that it can be used to decorate functions regardless
+// of whether detection is actually supported.
+//
+// How does this work?
+// -------------------
+//
+// Let's say we want to check whether a weak function `f` has been overridden by the user.
+// The general mechanism works by placing `f`'s definition (in the libc++ built library)
+// inside a special section, which we do using the `__section__` attribute via the
+// _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE macro.
+//
+// Then, when comes the time to check whether the function has been overridden, we take
+// the address of the function and we check whether it falls inside the special function
+// we created. This can be done by finding pointers to the start and the end of the section
+// (which is done differently for ELF and Mach-O), and then checking whether `f` falls
+// within those bounds. If it falls within those bounds, then `f` is still inside the
+// special section and so it is the version we defined in the libc++ built library, i.e.
+// it was not overridden. Otherwise, it was overridden by the user because it falls
+// outside of the section.
+//
+// Important note
+// --------------
+//
+// This mechanism should never be used outside of the libc++ built library. In particular,
+// attempting to use this within the libc++ headers will not work at all because we don't
+// want to be defining special sections inside user's executables which use our headers.
+// This is provided inside libc++'s include tree solely to make it easier to share with
+// libc++abi, which needs the same mechanism.
+//
+
+#if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
+
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
+#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE                                                                 \
+    __attribute__((__section__("__TEXT,__lcxx_override,regular,pure_instructions")))
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+template <class _Ret, class... _Args>
+_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
+  // Declare two dummy bytes and give them these special `__asm` values. These values are
+  // defined by the linker, which means that referring to `&__lcxx_override_start` will
+  // effectively refer to the address where the section starts (and same for the end).
+  extern char __lcxx_override_start __asm("section$start$__TEXT$__lcxx_override");
+  extern char __lcxx_override_end __asm("section$end$__TEXT$__lcxx_override");
+
+  // Now get a uintptr_t out of these locations, and out of the function pointer.
+  uintptr_t __start = reinterpret_cast<uintptr_t>(&__lcxx_override_start);
+  uintptr_t __end   = reinterpret_cast<uintptr_t>(&__lcxx_override_end);
+  uintptr_t __ptr   = reinterpret_cast<uintptr_t>(__fptr);
+
+  // Finally, the function was overridden if it falls outside of the section's bounds.
+  return __ptr < __start || __ptr > __end;
+}
+_LIBCPP_END_NAMESPACE_STD
+
+#elif defined(_LIBCPP_OBJECT_FORMAT_ELF)
+
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
+#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override")))
+
+// This is very similar to what we do for Mach-O above. The ELF linker will implicitly define
+// variables with those names corresponding to the start and the end of the section.
+//
+// See https://stackoverflow.com/questions/16552710/how-do-you-get-the-start-and-end-addresses-of-a-custom-elf-section
+extern char __start___lcxx_override;
+extern char __stop___lcxx_override;
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+template <class _Ret, class... _Args>
+_LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) noexcept {
+  uintptr_t __start = reinterpret_cast<uintptr_t>(&__start___lcxx_override);
+  uintptr_t __end   = reinterpret_cast<uintptr_t>(&__stop___lcxx_override);
+  uintptr_t __ptr   = reinterpret_cast<uintptr_t>(__fptr);
+
+  return __ptr < __start || __ptr > __end;
+}
+_LIBCPP_END_NAMESPACE_STD
+
+#else
+
+#  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 0
+#  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE /* nothing */
+
+#endif
+
+#endif // _LIBCPP___OVERRIDABLE_FUNCTION
diff --git a/libcxx/src/new.cpp b/libcxx/src/new.cpp
index 033bba5c1fc95b6..7a3ad4136010611 100644
--- a/libcxx/src/new.cpp
+++ b/libcxx/src/new.cpp
@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include <__memory/aligned_alloc.h>
+#include <__overridable_function>
+#include <cstddef>
 #include <cstdlib>
 #include <new>
 
@@ -15,6 +17,10 @@
 // The code below is copied as-is into libc++abi's libcxxabi/src/stdlib_new_delete.cpp
 // file. The version in this file is the canonical one.
 
+inline void __throw_bad_alloc_shim() { std::__throw_bad_alloc(); }
+
+#  define _LIBCPP_ASSERT_SHIM(expr, str) _LIBCPP_ASSERT(expr, str)
+
 // ------------------ BEGIN COPY ------------------
 // Implement all new and delete operators as weak definitions
 // in this shared library, so that they can be overridden by programs
@@ -36,41 +42,61 @@ static void* operator_new_impl(std::size_t size) noexcept {
   return p;
 }
 
-_LIBCPP_WEAK void* operator new(std::size_t size) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new(std::size_t size) _THROW_BAD_ALLOC {
   void* p = operator_new_impl(size);
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   if (p == nullptr)
-    throw std::bad_alloc();
-#  endif
+    __throw_bad_alloc_shim();
   return p;
 }
 
 _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
+#  ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
+  _LIBCPP_ASSERT_SHIM(
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new)),
+      "libc++ was configured with exceptions disabled and `operator new(size_t)` has been overridden, "
+      "but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because "
+      "`operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case "
+      "it fails to allocate, making it impossible for `operator new(size_t, nothrow_t)` to fulfill its "
+      "contract (since it should return nullptr upon failure).");
+#    endif
+
+  return operator_new_impl(size);
+#  else
   void* p = nullptr;
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   try {
-#  endif // _LIBCPP_HAS_NO_EXCEPTIONS
     p = ::operator new(size);
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
   }
-#  endif // _LIBCPP_HAS_NO_EXCEPTIONS
   return p;
+#  endif
 }
 
-_LIBCPP_WEAK void* operator new[](size_t size) _THROW_BAD_ALLOC { return ::operator new(size); }
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new[](size_t size) _THROW_BAD_ALLOC {
+  return ::operator new(size);
+}
 
 _LIBCPP_WEAK void* operator new[](size_t size, const std::nothrow_t&) noexcept {
+#  ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
+  _LIBCPP_ASSERT_SHIM(
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new[])),
+      "libc++ was configured with exceptions disabled and `operator new[](size_t)` has been overridden, "
+      "but `operator new[](size_t, nothrow_t)` has not been overridden. This is problematic because "
+      "`operator new[](size_t, nothrow_t)` must call `operator new[](size_t)`, which will terminate in case "
+      "it fails to allocate, making it impossible for `operator new[](size_t, nothrow_t)` to fulfill its "
+      "contract (since it should return nullptr upon failure).");
+#    endif
+
+  return operator_new_impl(size);
+#  else
   void* p = nullptr;
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   try {
-#  endif // _LIBCPP_HAS_NO_EXCEPTIONS
     p = ::operator new[](size);
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
   }
-#  endif // _LIBCPP_HAS_NO_EXCEPTIONS
   return p;
+#  endif
 }
 
 _LIBCPP_WEAK void operator delete(void* ptr) noexcept { std::free(ptr); }
@@ -107,43 +133,64 @@ static void* operator_new_aligned_impl(std::size_t size, std::align_val_t alignm
   return p;
 }
 
-_LIBCPP_WEAK void* operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
+operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
   void* p = operator_new_aligned_impl(size, alignment);
-#    ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   if (p == nullptr)
-    throw std::bad_alloc();
-#    endif
+    __throw_bad_alloc_shim();
   return p;
 }
 
 _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
+#    ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#      if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
+  _LIBCPP_ASSERT_SHIM(
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new)),
+      "libc++ was configured with exceptions disabled and `operator new(size_t, align_val_t)` has been overridden, "
+      "but `operator new(size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
+      "`operator new(size_t, align_val_t, nothrow_t)` must call `operator new(size_t, align_val_t)`, which will "
+      "terminate in case it fails to allocate, making it impossible for `operator new(size_t, align_val_t, nothrow_t)` "
+      "to fulfill its contract (since it should return nullptr upon failure).");
+#      endif
+
+  return operator_new_aligned_impl(size, alignment);
+#    else
   void* p = nullptr;
-#    ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   try {
-#    endif // _LIBCPP_HAS_NO_EXCEPTIONS
     p = ::operator new(size, alignment);
-#    ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
   }
-#    endif // _LIBCPP_HAS_NO_EXCEPTIONS
   return p;
+#    endif
 }
 
-_LIBCPP_WEAK void* operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
+operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
   return ::operator new(size, alignment);
 }
 
 _LIBCPP_WEAK void* operator new[](size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
+#    ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#      if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
+  _LIBCPP_ASSERT_SHIM(
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new[])),
+      "libc++ was configured with exceptions disabled and `operator new[](size_t, align_val_t)` has been overridden, "
+      "but `operator new[](size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
+      "`operator new[](size_t, align_val_t, nothrow_t)` must call `operator new[](size_t, align_val_t)`, which will "
+      "terminate in case it fails to allocate, making it impossible for `operator new[](size_t, align_val_t, "
+      "nothrow_t)` "
+      "to fulfill its contract (since it should return nullptr upon failure).");
+#      endif
+
+  return operator_new_aligned_impl(size, alignment);
+#    else
   void* p = nullptr;
-#    ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   try {
-#    endif // _LIBCPP_HAS_NO_EXCEPTIONS
     p = ::operator new[](size, alignment);
-#    ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
   }
-#    endif // _LIBCPP_HAS_NO_EXCEPTIONS
   return p;
+#    endif
 }
 
 _LIBCPP_WEAK void operator delete(void* ptr, std::align_val_t) noexcept { std::__libcpp_aligned_free(ptr); }
diff --git a/libcxx/test/libcxx/language.support/support.dynamic/assert.nothrow_new_not_overridden_fno_exceptions.pass.cpp b/libcxx/test/libcxx/language.support/support.dynamic/assert.nothrow_new_not_overridden_fno_exceptions.pass.cpp
new file mode 100644
index 000000000000000..7aa51b365f74eba
--- /dev/null
+++ b/libcxx/test/libcxx/language.support/support.dynamic/assert.nothrow_new_not_overridden_fno_exceptions.pass.cpp
@@ -0,0 +1,59 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// void* operator new(std::size_t, const std::nothrow_t&);
+// void* operator new(std::size_t, std::align_val_t, const std::nothrow_t&);
+// void* operator new[](std::size_t, const std::nothrow_t&);
+// void* operator new[](std::size_t, std::align_val_t, const std::nothrow_t&);
+
+// This test ensures that we catch the case where `new` has been overridden but `new(nothrow)`
+// has not been overridden, and the library is compiled with -fno-exceptions.
+//
+// In that case, it is impossible for libc++ to provide a Standards conforming implementation
+// of `new(nothrow)`, so the only viable option is to terminate the program.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03
+// XFAIL: availability-verbose_abort-missing
+
+// We only know how to diagnose this on platforms that use the ELF or Mach-O object file formats.
+// XFAIL: target={{.+}}-windows-{{.+}}
+
+// TODO: We currently don't have a way to express that the built library was
+//       compiled with -fno-exceptions, so if the library was built with support
+//       for exceptions but we run the test suite without exceptions, this will
+//       spuriously fail.
+// REQUIRES: no-exceptions
+
+#include <cstddef>
+#include <new>
+
+#include "check_assertion.h"
+
+// Override the throwing versions of operator new, but not the nothrow versions.
+alignas(32) char DummyData[32 * 3];
+void* operator new(std::size_t) { return DummyData; }
+void* operator new(std::size_t, std::align_val_t) { return DummyData; }
+void* operator new[](std::size_t) { return DummyData; }
+void* operator new[](std::size_t, std::align_val_t) { return DummyData; }
+
+void operator delete(void*) noexcept {}
+void operator delete(void*, std::align_val_t) noexcept {}
+void operator delete[](void*) noexcept {}
+void operator delete[](void*, std::align_val_t) noexcept {}
+
+int main(int, char**) {
+  std::size_t size       = 3;
+  std::align_val_t align = static_cast<std::align_val_t>(32);
+  EXPECT_DEATH((void)operator new(size, std::nothrow));
+  EXPECT_DEATH((void)operator new(size, align, std::nothrow));
+  EXPECT_DEATH((void)operator new[](size, std::nothrow));
+  EXPECT_DEATH((void)operator new[](size, align, std::nothrow));
+
+  return 0;
+}
diff --git a/libcxx/test/libcxx/language.support/support.dynamic/new_dont_return_nullptr.pass.cpp b/libcxx/test/libcxx/language.support/support.dynamic/new_dont_return_nullptr.pass.cpp
new file mode 100644
index 000000000000000..548046b0ab43d28
--- /dev/null
+++ b/libcxx/test/libcxx/language.support/support.dynamic/new_dont_return_nullptr.pass.cpp
@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// void* operator new(std::size_t);
+// void* operator new(std::size_t, std::align_val_t);
+// void* operator new[](std::size_t);
+// void* operator new[](std::size_t, std::align_val_t);
+
+// This test ensures that we abort the program instead of returning nullptr
+// when we fail to satisfy the allocation request. The throwing versions of
+// `operator new` must never return nullptr on failure to allocate (per the
+// Standard) and the compiler actually relies on that for optimizations.
+// Returning nullptr from the throwing `operator new` can basically result
+// in miscompiles.
+
+// REQUIRES: has-unix-headers
+// REQUIRES: no-exceptions
+// UNSUPPORTED: c++03, c++11, c++14
+
+#include <cstddef>
+#include <limits>
+#include <new>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  EXPECT_DEATH((void)operator new(std::numeric_limits<std::size_t>::max()));
+  EXPECT_DEATH((void)operator new(std::numeric_limits<std::size_t>::max(), static_cast<std::align_val_t>(32)));
+  EXPECT_DEATH((void)operator new[](std::numeric_limits<std::size_t>::max()));
+  EXPECT_DEATH((void)operator new[](std::numeric_limits<std::size_t>::max(), static_cast<std::align_val_t>(32)));
+  return 0;
+}
diff --git a/libcxx/test/support/check_assertion.h b/libcxx/test/support/check_assertion.h
index 98dd95b11556e6c..34e41e8f0d8eaf8 100644
--- a/libcxx/test/support/check_assertion.h
+++ b/libcxx/test/support/check_assertion.h
@@ -10,6 +10,7 @@
 #define TEST_SUPPORT_CHECK_ASSERTION_H
 
 #include <cassert>
+#include <csignal>
 #include <cstdarg>
 #include <cstddef>
 #include <cstdio>
@@ -257,9 +258,14 @@ void std::__libcpp_verbose_abort(char const* format, ...) {
   std::exit(DeathTest::RK_Terminate);
 }
 
+[[noreturn]] inline void abort_handler(int) {
+  std::exit(DeathTest::RK_Terminate);
+}
+
 template <class Func>
 inline bool ExpectDeath(const char* stmt, Func&& func, AssertionInfoMatcher Matcher) {
   std::set_terminate(terminate_handler);
+  std::signal(SIGABRT, abort_handler);
   DeathTest DT(Matcher);
   DeathTest::ResultKind RK = DT.Run(func);
   auto OnFailure = [&](const char* msg) {
diff --git a/libcxx/utils/generate_iwyu_mapping.py b/libcxx/utils/generate_iwyu_mapping.py
index 343538a6cae4819..88840eff8f6e293 100644
--- a/libcxx/utils/generate_iwyu_mapping.py
+++ b/libcxx/utils/generate_iwyu_mapping.py
@@ -65,6 +65,8 @@ def generate_map(include):
             continue
         elif i == "__node_handle":
             public = ["map", "set", "unordered_map", "unordered_set"]
+        elif i == "__overridable_function":
+            continue
         elif i == "__pstl_algorithm":
             continue
         elif i == "__pstl_config_site.in":
diff --git a/libcxxabi/src/stdlib_new_delete.cpp b/libcxxabi/src/stdlib_new_delete.cpp
index 6c9990f063dde66..5bd9b5e58b36c6a 100644
--- a/libcxxabi/src/stdlib_new_delete.cpp
+++ b/libcxxabi/src/stdlib_new_delete.cpp
@@ -7,7 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 #include "__cxxabi_config.h"
+#include "abort_message.h"
 #include <__memory/aligned_alloc.h>
+#include <__overridable_function>
+#include <cstddef>
 #include <cstdlib>
 #include <new>
 
@@ -25,6 +28,20 @@
 #  error libc++ and libc++abi seem to disagree on whether exceptions are enabled
 #endif
 
+inline void __throw_bad_alloc_shim() {
+#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
+  throw std::bad_alloc();
+#else
+  abort_message("bad_alloc was thrown in -fno-exceptions mode");
+#endif
+}
+
+#define _LIBCPP_ASSERT_SHIM(expr, str)                                                                                 \
+  do {                                                                                                                 \
+    if (!expr)                                                                                                         \
+      abort_message(str);                                                                                              \
+  } while (false)
+
 // ------------------ BEGIN COPY ------------------
 // Implement all new and delete operators as weak definitions
 // in this shared library, so that they can be overridden by programs
@@ -46,64 +63,74 @@ static void* operator_new_impl(std::size_t size) noexcept {
   return p;
 }
 
-_LIBCPP_WEAK
-void* operator new(std::size_t size) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new(std::size_t size) _THROW_BAD_ALLOC {
   void* p = operator_new_impl(size);
-#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   if (p == nullptr)
-    throw std::bad_alloc();
-#endif
+    __throw_bad_alloc_shim();
   return p;
 }
 
-_LIBCPP_WEAK
-void* operator new(size_t size, const std::nothrow_t&) noexcept {
+_LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
+#ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#  if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
+  _LIBCPP_ASSERT_SHIM(
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new)),
+      "libc++ was configured with exceptions disabled and `operator new(size_t)` has been overridden, "
+      "but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because "
+      "`operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case "
+      "it fails to allocate, making it impossible for `operator new(size_t, nothrow_t)` to fulfill its "
+      "contract (since it should return nullptr upon failure).");
+#  endif
+
+  return operator_new_impl(size);
+#else
   void* p = nullptr;
-#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   try {
-#endif // _LIBCPP_HAS_NO_EXCEPTIONS
     p = ::operator new(size);
-#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
   }
-#endif // _LIBCPP_HAS_NO_EXCEPTIONS
   return p;
+#endif
 }
 
-_LIBCPP_WEAK
-void* operator new[](size_t size) _THROW_BAD_ALLOC { return ::operator new(size); }
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void* operator new[](size_t size) _THROW_BAD_ALLOC {
+  return ::operator new(size);
+}
 
-_LIBCPP_WEAK
-void* operator new[](size_t size, const std::nothrow_t&) noexcept {
+_LIBCPP_WEAK void* operator new[](size_t size, const std::nothrow_t&) noexcept {
+#ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#  if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
+  _LIBCPP_ASSERT_SHIM(
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new[])),
+      "libc++ was configured with exceptions disabled and `operator new[](size_t)` has been overridden, "
+      "but `operator new[](size_t, nothrow_t)` has not been overridden. This is problematic because "
+      "`operator new[](size_t, nothrow_t)` must call `operator new[](size_t)`, which will terminate in case "
+      "it fails to allocate, making it impossible for `operator new[](size_t, nothrow_t)` to fulfill its "
+      "contract (since it should return nullptr upon failure).");
+#  endif
+
+  return operator_new_impl(size);
+#else
   void* p = nullptr;
-#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   try {
-#endif // _LIBCPP_HAS_NO_EXCEPTIONS
     p = ::operator new[](size);
-#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
   }
-#endif // _LIBCPP_HAS_NO_EXCEPTIONS
   return p;
+#endif
 }
 
-_LIBCPP_WEAK
-void operator delete(void* ptr) noexcept { std::free(ptr); }
+_LIBCPP_WEAK void operator delete(void* ptr) noexcept { std::free(ptr); }
 
-_LIBCPP_WEAK
-void operator delete(void* ptr, const std::nothrow_t&) noexcept { ::operator delete(ptr); }
+_LIBCPP_WEAK void operator delete(void* ptr, const std::nothrow_t&) noexcept { ::operator delete(ptr); }
 
-_LIBCPP_WEAK
-void operator delete(void* ptr, size_t) noexcept { ::operator delete(ptr); }
+_LIBCPP_WEAK void operator delete(void* ptr, size_t) noexcept { ::operator delete(ptr); }
 
-_LIBCPP_WEAK
-void operator delete[](void* ptr) noexcept { ::operator delete(ptr); }
+_LIBCPP_WEAK void operator delete[](void* ptr) noexcept { ::operator delete(ptr); }
 
-_LIBCPP_WEAK
-void operator delete[](void* ptr, const std::nothrow_t&) noexcept { ::operator delete[](ptr); }
+_LIBCPP_WEAK void operator delete[](void* ptr, const std::nothrow_t&) noexcept { ::operator delete[](ptr); }
 
-_LIBCPP_WEAK
-void operator delete[](void* ptr, size_t) noexcept { ::operator delete[](ptr); }
+_LIBCPP_WEAK void operator delete[](void* ptr, size_t) noexcept { ::operator delete[](ptr); }
 
 #if !defined(_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION)
 
@@ -127,70 +154,87 @@ static void* operator_new_aligned_impl(std::size_t size, std::align_val_t alignm
   return p;
 }
 
-_LIBCPP_WEAK
-void* operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
+operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
   void* p = operator_new_aligned_impl(size, alignment);
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   if (p == nullptr)
-    throw std::bad_alloc();
-#  endif
+    __throw_bad_alloc_shim();
   return p;
 }
 
-_LIBCPP_WEAK
-void* operator new(size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
+_LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
+#  ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
+  _LIBCPP_ASSERT_SHIM(
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new)),
+      "libc++ was configured with exceptions disabled and `operator new(size_t, align_val_t)` has been overridden, "
+      "but `operator new(size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
+      "`operator new(size_t, align_val_t, nothrow_t)` must call `operator new(size_t, align_val_t)`, which will "
+      "terminate in case it fails to allocate, making it impossible for `operator new(size_t, align_val_t, nothrow_t)` "
+      "to fulfill its contract (since it should return nullptr upon failure).");
+#    endif
+
+  return operator_new_aligned_impl(size, alignment);
+#  else
   void* p = nullptr;
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   try {
-#  endif // _LIBCPP_HAS_NO_EXCEPTIONS
     p = ::operator new(size, alignment);
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
   }
-#  endif // _LIBCPP_HAS_NO_EXCEPTIONS
   return p;
+#  endif
 }
 
-_LIBCPP_WEAK
-void* operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
+_LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE _LIBCPP_WEAK void*
+operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC {
   return ::operator new(size, alignment);
 }
 
-_LIBCPP_WEAK
-void* operator new[](size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
+_LIBCPP_WEAK void* operator new[](size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept {
+#  ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#    if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
+  _LIBCPP_ASSERT_SHIM(
+      !std::__is_function_overridden(static_cast<void* (*)(std::size_t, std::align_val_t)>(&operator new[])),
+      "libc++ was configured with exceptions disabled and `operator new[](size_t, align_val_t)` has been overridden, "
+      "but `operator new[](size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
+      "`operator new[](size_t, align_val_t, nothrow_t)` must call `operator new[](size_t, align_val_t)`, which will "
+      "terminate in case it fails to allocate, making it impossible for `operator new[](size_t, align_val_t, "
+      "nothrow_t)` "
+      "to fulfill its contract (since it should return nullptr upon failure).");
+#    endif
+
+  return operator_new_aligned_impl(size, alignment);
+#  else
   void* p = nullptr;
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   try {
-#  endif // _LIBCPP_HAS_NO_EXCEPTIONS
     p = ::operator new[](size, alignment);
-#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
   }
-#  endif // _LIBCPP_HAS_NO_EXCEPTIONS
   return p;
+#  endif
 }
 
-_LIBCPP_WEAK
-void operator delete(void* ptr, std::align_val_t) noexcept { std::__libcpp_aligned_free(ptr); }
+_LIBCPP_WEAK void operator delete(void* ptr, std::align_val_t) noexcept { std::__libcpp_aligned_free(ptr); }
 
-_LIBCPP_WEAK
-void operator delete(void* ptr, std::align_val_t alignment, const std::nothrow_t&) noexcept {
+_LIBCPP_WEAK void operator delete(void* ptr, std::align_val_t alignment, const std::nothrow_t&) noexcept {
   ::operator delete(ptr, alignment);
 }
 
-_LIBCPP_WEAK
-void operator delete(void* ptr, size_t, std::align_val_t alignment) noexcept { ::operator delete(ptr, alignment); }
+_LIBCPP_WEAK void operator delete(void* ptr, size_t, std::align_val_t alignment) noexcept {
+  ::operator delete(ptr, alignment);
+}
 
-_LIBCPP_WEAK
-void operator delete[](void* ptr, std::align_val_t alignment) noexcept { ::operator delete(ptr, alignment); }
+_LIBCPP_WEAK void operator delete[](void* ptr, std::align_val_t alignment) noexcept {
+  ::operator delete(ptr, alignment);
+}
 
-_LIBCPP_WEAK
-void operator delete[](void* ptr, std::align_val_t alignment, const std::nothrow_t&) noexcept {
+_LIBCPP_WEAK void operator delete[](void* ptr, std::align_val_t alignment, const std::nothrow_t&) noexcept {
   ::operator delete[](ptr, alignment);
 }
 
-_LIBCPP_WEAK
-void operator delete[](void* ptr, size_t, std::align_val_t alignment) noexcept { ::operator delete[](ptr, alignment); }
+_LIBCPP_WEAK void operator delete[](void* ptr, size_t, std::align_val_t alignment) noexcept {
+  ::operator delete[](ptr, alignment);
+}
 
 #endif // !_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
 // ------------------ END COPY ------------------

>From 1b5144f9f8d18580b6ceca264ce033e7ede0907b Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Tue, 14 Nov 2023 22:55:06 -0500
Subject: [PATCH 2/7] Improve diagnostic

---
 libcxx/src/new.cpp                  | 14 +++++++++-----
 libcxxabi/src/stdlib_new_delete.cpp | 14 +++++++++-----
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/libcxx/src/new.cpp b/libcxx/src/new.cpp
index 7a3ad4136010611..5ca9d6b43da443f 100644
--- a/libcxx/src/new.cpp
+++ b/libcxx/src/new.cpp
@@ -58,7 +58,8 @@ _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
       "but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case "
       "it fails to allocate, making it impossible for `operator new(size_t, nothrow_t)` to fulfill its "
-      "contract (since it should return nullptr upon failure).");
+      "contract (since it should return nullptr upon failure). Please make sure you override "
+      "`operator new(size_t, nothrow_t)` as well.");
 #    endif
 
   return operator_new_impl(size);
@@ -85,7 +86,8 @@ _LIBCPP_WEAK void* operator new[](size_t size, const std::nothrow_t&) noexcept {
       "but `operator new[](size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, nothrow_t)` must call `operator new[](size_t)`, which will terminate in case "
       "it fails to allocate, making it impossible for `operator new[](size_t, nothrow_t)` to fulfill its "
-      "contract (since it should return nullptr upon failure).");
+      "contract (since it should return nullptr upon failure). Please make sure you override "
+      "`operator new[](size_t, nothrow_t)` as well.");
 #    endif
 
   return operator_new_impl(size);
@@ -150,7 +152,8 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
       "but `operator new(size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, align_val_t, nothrow_t)` must call `operator new(size_t, align_val_t)`, which will "
       "terminate in case it fails to allocate, making it impossible for `operator new(size_t, align_val_t, nothrow_t)` "
-      "to fulfill its contract (since it should return nullptr upon failure).");
+      "to fulfill its contract (since it should return nullptr upon failure). Please make sure you override "
+      "`operator new(size_t, align_val_t, nothrow_t)` as well.");
 #      endif
 
   return operator_new_aligned_impl(size, alignment);
@@ -178,8 +181,9 @@ _LIBCPP_WEAK void* operator new[](size_t size, std::align_val_t alignment, const
       "but `operator new[](size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, align_val_t, nothrow_t)` must call `operator new[](size_t, align_val_t)`, which will "
       "terminate in case it fails to allocate, making it impossible for `operator new[](size_t, align_val_t, "
-      "nothrow_t)` "
-      "to fulfill its contract (since it should return nullptr upon failure).");
+      "nothrow_t)` to fulfill its contract (since it should return nullptr upon failure). Please make sure you "
+      "override "
+      "`operator new[](size_t, align_val_t, nothrow_t)` as well.");
 #      endif
 
   return operator_new_aligned_impl(size, alignment);
diff --git a/libcxxabi/src/stdlib_new_delete.cpp b/libcxxabi/src/stdlib_new_delete.cpp
index 5bd9b5e58b36c6a..cba6c3968775ab2 100644
--- a/libcxxabi/src/stdlib_new_delete.cpp
+++ b/libcxxabi/src/stdlib_new_delete.cpp
@@ -79,7 +79,8 @@ _LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
       "but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case "
       "it fails to allocate, making it impossible for `operator new(size_t, nothrow_t)` to fulfill its "
-      "contract (since it should return nullptr upon failure).");
+      "contract (since it should return nullptr upon failure). Please make sure you override "
+      "`operator new(size_t, nothrow_t)` as well.");
 #  endif
 
   return operator_new_impl(size);
@@ -106,7 +107,8 @@ _LIBCPP_WEAK void* operator new[](size_t size, const std::nothrow_t&) noexcept {
       "but `operator new[](size_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, nothrow_t)` must call `operator new[](size_t)`, which will terminate in case "
       "it fails to allocate, making it impossible for `operator new[](size_t, nothrow_t)` to fulfill its "
-      "contract (since it should return nullptr upon failure).");
+      "contract (since it should return nullptr upon failure). Please make sure you override "
+      "`operator new[](size_t, nothrow_t)` as well.");
 #  endif
 
   return operator_new_impl(size);
@@ -171,7 +173,8 @@ _LIBCPP_WEAK void* operator new(size_t size, std::align_val_t alignment, const s
       "but `operator new(size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new(size_t, align_val_t, nothrow_t)` must call `operator new(size_t, align_val_t)`, which will "
       "terminate in case it fails to allocate, making it impossible for `operator new(size_t, align_val_t, nothrow_t)` "
-      "to fulfill its contract (since it should return nullptr upon failure).");
+      "to fulfill its contract (since it should return nullptr upon failure). Please make sure you override "
+      "`operator new(size_t, align_val_t, nothrow_t)` as well.");
 #    endif
 
   return operator_new_aligned_impl(size, alignment);
@@ -199,8 +202,9 @@ _LIBCPP_WEAK void* operator new[](size_t size, std::align_val_t alignment, const
       "but `operator new[](size_t, align_val_t, nothrow_t)` has not been overridden. This is problematic because "
       "`operator new[](size_t, align_val_t, nothrow_t)` must call `operator new[](size_t, align_val_t)`, which will "
       "terminate in case it fails to allocate, making it impossible for `operator new[](size_t, align_val_t, "
-      "nothrow_t)` "
-      "to fulfill its contract (since it should return nullptr upon failure).");
+      "nothrow_t)` to fulfill its contract (since it should return nullptr upon failure). Please make sure you "
+      "override "
+      "`operator new[](size_t, align_val_t, nothrow_t)` as well.");
 #    endif
 
   return operator_new_aligned_impl(size, alignment);

>From 0f2b77ffa9074b750189e99f8f9cbfd5159a9be1 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Tue, 14 Nov 2023 23:10:35 -0500
Subject: [PATCH 3/7] Make sure to override operator new(nothrow) count_new
 tests

---
 libcxx/test/support/count_new.h | 105 +++++++++++++++++++++-----------
 1 file changed, 68 insertions(+), 37 deletions(-)

diff --git a/libcxx/test/support/count_new.h b/libcxx/test/support/count_new.h
index b6424850101625b..f89cdc0c01487d9 100644
--- a/libcxx/test/support/count_new.h
+++ b/libcxx/test/support/count_new.h
@@ -374,78 +374,109 @@ TEST_DIAGNOSTIC_POP
 MemCounter &globalMemCounter = *getGlobalMemCounter();
 
 #ifndef DISABLE_NEW_COUNT
-void* operator new(std::size_t s) TEST_THROW_SPEC(std::bad_alloc)
-{
-    getGlobalMemCounter()->newCalled(s);
-    void* ret = std::malloc(s);
-    if (ret == nullptr)
-        detail::throw_bad_alloc_helper();
-    return ret;
+// operator new(size_t[, nothrow_t]) and operator delete(size_t[, nothrow_t])
+void* operator new(std::size_t s) TEST_THROW_SPEC(std::bad_alloc) {
+  void* ret = operator new(s, std::nothrow);
+  if (ret == nullptr)
+    detail::throw_bad_alloc_helper();
+  return ret;
 }
 
-void  operator delete(void* p) TEST_NOEXCEPT
-{
-    getGlobalMemCounter()->deleteCalled(p);
-    std::free(p);
+void* operator new(std::size_t s, std::nothrow_t const&) TEST_NOEXCEPT {
+  getGlobalMemCounter()->newCalled(s);
+  void* ret = std::malloc(s);
+  return ret;
 }
 
-void* operator new[](std::size_t s) TEST_THROW_SPEC(std::bad_alloc)
-{
-    getGlobalMemCounter()->newArrayCalled(s);
-    return operator new(s);
+void operator delete(void* p) TEST_NOEXCEPT { operator delete(p, std::nothrow); }
+
+void operator delete(void* p, std::nothrow_t const&) TEST_NOEXCEPT {
+  getGlobalMemCounter()->deleteCalled(p);
+  std::free(p);
 }
 
-void operator delete[](void* p) TEST_NOEXCEPT
-{
-    getGlobalMemCounter()->deleteArrayCalled(p);
-    operator delete(p);
+// operator new[](size_t[, nothrow_t]) and operator delete[](size_t[, nothrow_t])
+void* operator new[](std::size_t s) TEST_THROW_SPEC(std::bad_alloc) {
+  void* ret = operator new[](s, std::nothrow);
+  if (ret == nullptr)
+    detail::throw_bad_alloc_helper();
+  return ret;
 }
 
-#ifndef TEST_HAS_NO_ALIGNED_ALLOCATION
-#if defined(_LIBCPP_MSVCRT_LIKE) || \
-  (!defined(_LIBCPP_VERSION) && defined(_WIN32))
-#define USE_ALIGNED_ALLOC
-#endif
+void* operator new[](std::size_t s, std::nothrow_t const&) TEST_NOEXCEPT {
+  getGlobalMemCounter()->newArrayCalled(s);
+  return operator new(s);
+}
+
+void operator delete[](void* p) TEST_NOEXCEPT { operator delete[](p, std::nothrow); }
+
+void operator delete[](void* p, std::nothrow_t const&) TEST_NOEXCEPT {
+  getGlobalMemCounter()->deleteArrayCalled(p);
+  operator delete(p);
+}
 
+#  ifndef TEST_HAS_NO_ALIGNED_ALLOCATION
+#    if defined(_LIBCPP_MSVCRT_LIKE) || (!defined(_LIBCPP_VERSION) && defined(_WIN32))
+#      define USE_ALIGNED_ALLOC
+#    endif
+
+// operator new(size_t, align_val_t[, nothrow_t]) and operator delete(size_t, align_val_t[, nothrow_t])
 void* operator new(std::size_t s, std::align_val_t av) TEST_THROW_SPEC(std::bad_alloc) {
+  void* p = operator new(s, av, std::nothrow);
+  if (p == nullptr)
+    detail::throw_bad_alloc_helper();
+  return p;
+}
+
+void* operator new(std::size_t s, std::align_val_t av, std::nothrow_t const&) TEST_NOEXCEPT {
   const std::size_t a = static_cast<std::size_t>(av);
   getGlobalMemCounter()->alignedNewCalled(s, a);
-  void *ret = nullptr;
-#ifdef USE_ALIGNED_ALLOC
+  void* ret = nullptr;
+#    ifdef USE_ALIGNED_ALLOC
   ret = _aligned_malloc(s, a);
-#else
+#    else
   assert(posix_memalign(&ret, std::max(a, sizeof(void*)), s) != EINVAL);
-#endif
-  if (ret == nullptr)
-    detail::throw_bad_alloc_helper();
+#    endif
   return ret;
 }
 
-void operator delete(void *p, std::align_val_t av) TEST_NOEXCEPT {
+void operator delete(void* p, std::align_val_t av) TEST_NOEXCEPT { operator delete(p, av, std::nothrow); }
+
+void operator delete(void* p, std::align_val_t av, std::nothrow_t const&) TEST_NOEXCEPT {
   const std::size_t a = static_cast<std::size_t>(av);
   getGlobalMemCounter()->alignedDeleteCalled(p, a);
   if (p) {
-#ifdef USE_ALIGNED_ALLOC
+#    ifdef USE_ALIGNED_ALLOC
     ::_aligned_free(p);
-#else
+#    else
     ::free(p);
-#endif
+#    endif
   }
 }
 
+// operator new[](size_t, align_val_t[, nothrow_t]) and operator delete[](size_t, align_val_t[, nothrow_t])
 void* operator new[](std::size_t s, std::align_val_t av) TEST_THROW_SPEC(std::bad_alloc) {
+  void* ret = operator new[](s, av, std::nothrow);
+  if (ret == nullptr)
+    detail::throw_bad_alloc_helper();
+  return ret;
+}
+
+void* operator new[](std::size_t s, std::align_val_t av, std::nothrow_t const&) TEST_NOEXCEPT {
   const std::size_t a = static_cast<std::size_t>(av);
   getGlobalMemCounter()->alignedNewArrayCalled(s, a);
-  return operator new(s, av);
+  return operator new(s, av, std::nothrow);
 }
 
-void operator delete[](void *p, std::align_val_t av) TEST_NOEXCEPT {
+void operator delete[](void* p, std::align_val_t av) TEST_NOEXCEPT { operator delete[](p, av, std::nothrow); }
+
+void operator delete[](void* p, std::align_val_t av, std::nothrow_t const&) TEST_NOEXCEPT {
   const std::size_t a = static_cast<std::size_t>(av);
   getGlobalMemCounter()->alignedDeleteArrayCalled(p, a);
   return operator delete(p, av);
 }
 
-#endif // TEST_HAS_NO_ALIGNED_ALLOCATION
+#  endif // TEST_HAS_NO_ALIGNED_ALLOCATION
 
 #endif // DISABLE_NEW_COUNT
 

>From 82d3e73b83ef0273ce6067ff803ae08cd5e3e406 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Mon, 18 Dec 2023 14:25:18 -0500
Subject: [PATCH 4/7] Add __overridable_function to modulemap

---
 libcxx/include/module.modulemap.in | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index 5f57a8a2b1bf71e..4422598d15dcbf1 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -608,6 +608,10 @@ module std_private_node_handle       [system] {
   header "__node_handle"
   export *
 }
+module std_private_overridable_function [system] {
+  header "__overridable_function"
+  export *
+}
 module std_private_split_buffer      [system] {
   header "__split_buffer"
   export *

>From 398091447a8377f243fa2f6c960089af8301eb54 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Mon, 18 Dec 2023 15:11:48 -0500
Subject: [PATCH 5/7] Fix implementation of count_new.h operators

---
 libcxx/test/support/count_new.h | 132 ++++++++++++++++++++++----------
 1 file changed, 92 insertions(+), 40 deletions(-)

diff --git a/libcxx/test/support/count_new.h b/libcxx/test/support/count_new.h
index f89cdc0c01487d9..93142fc9ad4dede 100644
--- a/libcxx/test/support/count_new.h
+++ b/libcxx/test/support/count_new.h
@@ -376,19 +376,30 @@ MemCounter &globalMemCounter = *getGlobalMemCounter();
 #ifndef DISABLE_NEW_COUNT
 // operator new(size_t[, nothrow_t]) and operator delete(size_t[, nothrow_t])
 void* operator new(std::size_t s) TEST_THROW_SPEC(std::bad_alloc) {
-  void* ret = operator new(s, std::nothrow);
-  if (ret == nullptr)
+  getGlobalMemCounter()->newCalled(s);
+  void* p = std::malloc(s);
+  if (p == nullptr)
     detail::throw_bad_alloc_helper();
-  return ret;
+  return p;
 }
 
 void* operator new(std::size_t s, std::nothrow_t const&) TEST_NOEXCEPT {
+#  ifdef TEST_HAS_NO_EXCEPTIONS
   getGlobalMemCounter()->newCalled(s);
-  void* ret = std::malloc(s);
-  return ret;
+#  else
+  try {
+    getGlobalMemCounter()->newCalled(s);
+  } catch (std::bad_alloc const&) {
+    return nullptr;
+  }
+#  endif
+  return std::malloc(s);
 }
 
-void operator delete(void* p) TEST_NOEXCEPT { operator delete(p, std::nothrow); }
+void operator delete(void* p) TEST_NOEXCEPT {
+  getGlobalMemCounter()->deleteCalled(p);
+  std::free(p);
+}
 
 void operator delete(void* p, std::nothrow_t const&) TEST_NOEXCEPT {
   getGlobalMemCounter()->deleteCalled(p);
@@ -397,22 +408,34 @@ void operator delete(void* p, std::nothrow_t const&) TEST_NOEXCEPT {
 
 // operator new[](size_t[, nothrow_t]) and operator delete[](size_t[, nothrow_t])
 void* operator new[](std::size_t s) TEST_THROW_SPEC(std::bad_alloc) {
-  void* ret = operator new[](s, std::nothrow);
-  if (ret == nullptr)
+  getGlobalMemCounter()->newArrayCalled(s);
+  void* p = std::malloc(s);
+  if (p == nullptr)
     detail::throw_bad_alloc_helper();
-  return ret;
+  return p;
 }
 
 void* operator new[](std::size_t s, std::nothrow_t const&) TEST_NOEXCEPT {
+#  ifdef TEST_HAS_NO_EXCEPTIONS
   getGlobalMemCounter()->newArrayCalled(s);
-  return operator new(s);
+#  else
+  try {
+    getGlobalMemCounter()->newArrayCalled(s);
+  } catch (std::bad_alloc const&) {
+    return nullptr;
+  }
+#  endif
+  return std::malloc(s);
 }
 
-void operator delete[](void* p) TEST_NOEXCEPT { operator delete[](p, std::nothrow); }
+void operator delete[](void* p) TEST_NOEXCEPT {
+  getGlobalMemCounter()->deleteArrayCalled(p);
+  std::free(p);
+}
 
 void operator delete[](void* p, std::nothrow_t const&) TEST_NOEXCEPT {
   getGlobalMemCounter()->deleteArrayCalled(p);
-  operator delete(p);
+  std::free(p);
 }
 
 #  ifndef TEST_HAS_NO_ALIGNED_ALLOCATION
@@ -420,60 +443,89 @@ void operator delete[](void* p, std::nothrow_t const&) TEST_NOEXCEPT {
 #      define USE_ALIGNED_ALLOC
 #    endif
 
+inline void* alocate_aligned_impl(std::size_t size, std::align_val_t align) {
+  const std::size_t alignment = static_cast<std::size_t>(align);
+  void* ret                   = nullptr;
+#    ifdef USE_ALIGNED_ALLOC
+  ret = _aligned_malloc(size, alignment);
+#    else
+  assert(posix_memalign(&ret, std::max(alignment, sizeof(void*)), size) != EINVAL);
+#    endif
+  return ret;
+}
+
+inline void free_aligned_impl(void* ptr, std::align_val_t) {
+  if (ptr) {
+#    ifdef USE_ALIGNED_ALLOC
+    ::_aligned_free(ptr);
+#    else
+    ::free(ptr);
+#    endif
+  }
+}
+
 // operator new(size_t, align_val_t[, nothrow_t]) and operator delete(size_t, align_val_t[, nothrow_t])
 void* operator new(std::size_t s, std::align_val_t av) TEST_THROW_SPEC(std::bad_alloc) {
-  void* p = operator new(s, av, std::nothrow);
+  getGlobalMemCounter()->alignedNewCalled(s, static_cast<std::size_t>(av));
+  void* p = alocate_aligned_impl(s, av);
   if (p == nullptr)
     detail::throw_bad_alloc_helper();
   return p;
 }
 
 void* operator new(std::size_t s, std::align_val_t av, std::nothrow_t const&) TEST_NOEXCEPT {
-  const std::size_t a = static_cast<std::size_t>(av);
-  getGlobalMemCounter()->alignedNewCalled(s, a);
-  void* ret = nullptr;
-#    ifdef USE_ALIGNED_ALLOC
-  ret = _aligned_malloc(s, a);
+#    ifdef TEST_HAS_NO_EXCEPTIONS
+  getGlobalMemCounter()->alignedNewCalled(s, static_cast<std::size_t>(av));
 #    else
-  assert(posix_memalign(&ret, std::max(a, sizeof(void*)), s) != EINVAL);
+  try {
+    getGlobalMemCounter()->alignedNewCalled(s, static_cast<std::size_t>(av));
+  } catch (std::bad_alloc const&) {
+    return nullptr;
+  }
 #    endif
-  return ret;
+  return alocate_aligned_impl(s, av);
 }
 
-void operator delete(void* p, std::align_val_t av) TEST_NOEXCEPT { operator delete(p, av, std::nothrow); }
+void operator delete(void* p, std::align_val_t av) TEST_NOEXCEPT {
+  getGlobalMemCounter()->alignedDeleteCalled(p, static_cast<std::size_t>(av));
+  free_aligned_impl(p, av);
+}
 
 void operator delete(void* p, std::align_val_t av, std::nothrow_t const&) TEST_NOEXCEPT {
-  const std::size_t a = static_cast<std::size_t>(av);
-  getGlobalMemCounter()->alignedDeleteCalled(p, a);
-  if (p) {
-#    ifdef USE_ALIGNED_ALLOC
-    ::_aligned_free(p);
-#    else
-    ::free(p);
-#    endif
-  }
+  getGlobalMemCounter()->alignedDeleteCalled(p, static_cast<std::size_t>(av));
+  free_aligned_impl(p, av);
 }
 
 // operator new[](size_t, align_val_t[, nothrow_t]) and operator delete[](size_t, align_val_t[, nothrow_t])
 void* operator new[](std::size_t s, std::align_val_t av) TEST_THROW_SPEC(std::bad_alloc) {
-  void* ret = operator new[](s, av, std::nothrow);
-  if (ret == nullptr)
+  getGlobalMemCounter()->alignedNewArrayCalled(s, static_cast<std::size_t>(av));
+  void* p = alocate_aligned_impl(s, av);
+  if (p == nullptr)
     detail::throw_bad_alloc_helper();
-  return ret;
+  return p;
 }
 
 void* operator new[](std::size_t s, std::align_val_t av, std::nothrow_t const&) TEST_NOEXCEPT {
-  const std::size_t a = static_cast<std::size_t>(av);
-  getGlobalMemCounter()->alignedNewArrayCalled(s, a);
-  return operator new(s, av, std::nothrow);
+#    ifdef TEST_HAS_NO_EXCEPTIONS
+  getGlobalMemCounter()->alignedNewArrayCalled(s, static_cast<std::size_t>(av));
+#    else
+  try {
+    getGlobalMemCounter()->alignedNewArrayCalled(s, static_cast<std::size_t>(av));
+  } catch (std::bad_alloc const&) {
+    return nullptr;
+  }
+#    endif
+  return alocate_aligned_impl(s, av);
 }
 
-void operator delete[](void* p, std::align_val_t av) TEST_NOEXCEPT { operator delete[](p, av, std::nothrow); }
+void operator delete[](void* p, std::align_val_t av) TEST_NOEXCEPT {
+  getGlobalMemCounter()->alignedDeleteArrayCalled(p, static_cast<std::size_t>(av));
+  free_aligned_impl(p, av);
+}
 
 void operator delete[](void* p, std::align_val_t av, std::nothrow_t const&) TEST_NOEXCEPT {
-  const std::size_t a = static_cast<std::size_t>(av);
-  getGlobalMemCounter()->alignedDeleteArrayCalled(p, a);
-  return operator delete(p, av);
+  getGlobalMemCounter()->alignedDeleteArrayCalled(p, static_cast<std::size_t>(av));
+  free_aligned_impl(p, av);
 }
 
 #  endif // TEST_HAS_NO_ALIGNED_ALLOCATION

>From e9863c390c900aff3f18b4c24e0aa4a8d8a01e08 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Mon, 18 Dec 2023 15:23:00 -0500
Subject: [PATCH 6/7] Fix formatting

---
 libcxx/test/support/check_assertion.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/libcxx/test/support/check_assertion.h b/libcxx/test/support/check_assertion.h
index 34e41e8f0d8eaf8..9a092d75267db80 100644
--- a/libcxx/test/support/check_assertion.h
+++ b/libcxx/test/support/check_assertion.h
@@ -258,9 +258,7 @@ void std::__libcpp_verbose_abort(char const* format, ...) {
   std::exit(DeathTest::RK_Terminate);
 }
 
-[[noreturn]] inline void abort_handler(int) {
-  std::exit(DeathTest::RK_Terminate);
-}
+[[noreturn]] inline void abort_handler(int) { std::exit(DeathTest::RK_Terminate); }
 
 template <class Func>
 inline bool ExpectDeath(const char* stmt, Func&& func, AssertionInfoMatcher Matcher) {

>From 12e32398362272562cdc8377f7cfb52416773600 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Tue, 16 Jan 2024 15:48:47 -0500
Subject: [PATCH 7/7] Add XFAILs for a few tests that can't work anymore

---
 .../new.size_align_nothrow.replace.indirect.pass.cpp          | 4 ++++
 .../new.size_nothrow.replace.indirect.pass.cpp                | 4 ++++
 .../new.size_align_nothrow.replace.indirect.pass.cpp          | 4 ++++
 .../new.size_nothrow.replace.indirect.pass.cpp                | 4 ++++
 4 files changed, 16 insertions(+)

diff --git a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/new.size_align_nothrow.replace.indirect.pass.cpp b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/new.size_align_nothrow.replace.indirect.pass.cpp
index df8a651932cef1b..f6959172ea24ef9 100644
--- a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/new.size_align_nothrow.replace.indirect.pass.cpp
+++ b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/new.size_align_nothrow.replace.indirect.pass.cpp
@@ -11,6 +11,10 @@
 // Test that we can replace the operator by replacing `operator new[](std::size_t, std::align_val_t)`
 // (the throwing version).
 
+// This doesn't work when the shared library was built with exceptions disabled, because
+// we can't implement the non-throwing new from the throwing new in that case.
+// XFAIL: no-exceptions
+
 // UNSUPPORTED: c++03, c++11, c++14
 // UNSUPPORTED: sanitizer-new-delete
 
diff --git a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/new.size_nothrow.replace.indirect.pass.cpp b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/new.size_nothrow.replace.indirect.pass.cpp
index 70d891b2a82c0a3..84bfe8205b6006c 100644
--- a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/new.size_nothrow.replace.indirect.pass.cpp
+++ b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.array/new.size_nothrow.replace.indirect.pass.cpp
@@ -10,6 +10,10 @@
 
 // Test that we can replace the operator by replacing `operator new[](std::size_t)` (the throwing version).
 
+// This doesn't work when the shared library was built with exceptions disabled, because
+// we can't implement the non-throwing new from the throwing new in that case.
+// XFAIL: no-exceptions
+
 // UNSUPPORTED: sanitizer-new-delete
 // XFAIL: libcpp-no-vcruntime
 // XFAIL: LIBCXX-AIX-FIXME
diff --git a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/new.size_align_nothrow.replace.indirect.pass.cpp b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/new.size_align_nothrow.replace.indirect.pass.cpp
index a68cdab54528c27..2e7fa132890b808 100644
--- a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/new.size_align_nothrow.replace.indirect.pass.cpp
+++ b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/new.size_align_nothrow.replace.indirect.pass.cpp
@@ -10,6 +10,10 @@
 
 // Test that we can replace the operator by replacing `operator new(std::size_t, std::align_val_t)` (the throwing version).
 
+// This doesn't work when the shared library was built with exceptions disabled, because
+// we can't implement the non-throwing new from the throwing new in that case.
+// XFAIL: no-exceptions
+
 // UNSUPPORTED: c++03, c++11, c++14
 // UNSUPPORTED: sanitizer-new-delete
 
diff --git a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/new.size_nothrow.replace.indirect.pass.cpp b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/new.size_nothrow.replace.indirect.pass.cpp
index 64edbfd7e9af993..8b5019cf7eb63ea 100644
--- a/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/new.size_nothrow.replace.indirect.pass.cpp
+++ b/libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.single/new.size_nothrow.replace.indirect.pass.cpp
@@ -10,6 +10,10 @@
 
 // Test that we can replace the operator by replacing `operator new(std::size_t)` (the throwing version).
 
+// This doesn't work when the shared library was built with exceptions disabled, because
+// we can't implement the non-throwing new from the throwing new in that case.
+// XFAIL: no-exceptions
+
 // UNSUPPORTED: sanitizer-new-delete
 // XFAIL: libcpp-no-vcruntime
 // XFAIL: LIBCXX-AIX-FIXME



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