[libc++] Remove _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION

[philnik]

Summary

In the stable ABI, the destructor was always defined inline, so all TUs have a weak definition of the symbols, which means that if the strong definition from the dylib is removed, there is still an equivalent weak definition available.
In the unstable ABI, there is still bad_function_call::what() as the key function, so there will be a strong definition of the symbols in the dylib and no weak definitions.

[Mordante]

Do you know why we have two key functions, which both require ABI v2?

libcxx/include/__functional/function.h

52–53

Why is this still needed?

[ldionne]

I don't understand how that's not an ABI break. When _LIBCPP_ABI_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE is not defined (i.e. in the stable ABI), we don't have a key function. https://reviews.llvm.org/D92397 may be interesting to consult for additional context.

[philnik]

In D142842#4119097, @ldionne wrote:

I don't understand how that's not an ABI break. When _LIBCPP_ABI_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE is not defined (i.e. in the stable ABI), we don't have a key function. https://reviews.llvm.org/D92397 may be interesting to consult for additional context.

I never said it's not an ABI break. It's just that we already have bad_function_call::what() when the unstable ABI is enabled. We don't need two key functions.

[ldionne]

So we just went over the patch again. Basically, this undoes the part of D92397 that unconditionally defines ~bad_function_call() in the dylib, but not the part that adds a separate setting for controlling whether bad_function_call::what() is defined in the dylib, and hence whether a key function is provided in the dylib.

In the stable ABI, before this patch:

1. User code will always contain a weak definition of the bad_function_call vtable since we did not provide a key-function-declaration in the headers. Hence, no user code should contain hard references to bad_function_call's vtable, destructor or else. This was checked against the apps in the App Store, so I think our understanding is confirmed here.
2. The dylib always provides a strong vtable that contains ~bad_function_call(), but not bad_function_call::what().

In the stable ABI, after this patch:

1. User code will always contain a weak definition of the vtable too.
2. The dylib won't provide a strong vtable for bad_function_call anymore, but that shouldn't be used today as noted above.

In the unstable ABI, before this patch, there was a strong vtable in the dylib with ~bad_function_call() and bad_function_call::what(), and users are relying on it. After the patch, it's the same thing except that we don't define ~bad_function_call() manually, we let the compiler do it.

After all these considerations, I think this is likely safe to do, and we'll indeed remove a few symbols from the dylib, and clean up our __config a bit. Please update the ABI list and explain the above in your commit message + in the ABI list change log. I'd like to see this one last time to double check before it goes out.

libcxx/src/functional.cpp

27

You will need to grep for Address Sanitizer doesn't instrument weak symbols on Linux. and adjust the UNSUPPORTED there. It should become something like UNSUPPORTED: asan && libcpp-abi-version=2.

[philnik]

Address comments

libcxx/include/__functional/function.h

52–53

It's not.

libcxx/src/functional.cpp

27

IIUC this shouldn't be a problem anymore, since we don't have any weak symbols in ABIv2, and have only weak symbols in ABIv1 again.

[daltenty]

Unfortunately, some of this doesn't really work on AIX linkage model. Specifically, the linker doesn't follow the resolution semantic of trying to resolve weak references locally first that you see on other platforms (we have an option -bweaklocal to enable that), so we prefer the strong def we see in the library. This means this assumption doesn't hold on AIX:

• User code will always contain a weak definition of the bad_function_call vtable since we did not provide a key-function-declaration in the headers. Hence, no user code should contain hard references to bad_function_call's vtable, destructor or else.

So removing the symbol from the dylib will break the ABI for us.

[hubert.reinterpretcast]

@philnik, can you update this to always have the key function on AIX and remove it for other platforms?

libcxx/include/__config

 #    endif
 #  endif
 
 #  if defined(_LIBCPP_BUILDING_LIBRARY) || _LIBCPP_ABI_VERSION >= 2
 // Enable additional explicit instantiations of iostreams components. This
 // reduces the number of weak definitions generated in programs that use
 // iostreams by providing a single strong definition in the shared library.
 #    define _LIBCPP_ABI_ENABLE_ADDITIONAL_IOSTREAM_EXPLICIT_INSTANTIATIONS_1
-
-// Define a key function for `bad_function_call` in the library, to centralize
-// its vtable and typeinfo to libc++ rather than having all other libraries
-// using that class define their own copies.
-#    define _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
 #  endif
 
 #  define _LIBCPP_TOSTRING2(x) #x
 #  define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
 
 #  if __cplusplus < 201103L
 #    define _LIBCPP_CXX03_LANG
 #  endif

libcxx/include/__functional/function.h

 
 // bad_function_call
 
 _LIBCPP_DIAGNOSTIC_PUSH
 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables")
 class _LIBCPP_EXCEPTION_ABI bad_function_call
     : public exception
 {
 public:
-// Note that when a key function is not used, every translation unit that uses
-// bad_function_call will end up containing a weak definition of the vtable and
-// typeinfo.
-#ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
-    ~bad_function_call() _NOEXCEPT override;
-#else
-    ~bad_function_call() _NOEXCEPT override {}
-#endif
-
 #ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE
     const char* what() const _NOEXCEPT override;
 #endif
 };
 _LIBCPP_DIAGNOSTIC_POP
 
 _LIBCPP_NORETURN inline _LIBCPP_INLINE_VISIBILITY
 void __throw_bad_function_call()
 {

libcxx/lib/abi/CHANGELOG.TXT

 `generate-cxx-abilist` target, and look at the diff.
 
 New entries should be added directly below the "Version" header.
 
 ------------
 Version 17.0
 ------------
 
+* [libc++] Remove _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
+
+  The destructor was always defined inline in the stable ABI, so all TUs have a weak definition of the symbols, which
+  means that if the strong definition from the dylib is removed, there is still an equivalent weak definition available.
+
+  All platforms
+  -------------
+  Symbol removed: _ZNKSt9exception4whatEv
+  Symbol removed: _ZNSt3__117bad_function_callD0Ev
+  Symbol removed: _ZNSt3__117bad_function_callD1Ev
+  Symbol removed: _ZNSt3__117bad_function_callD2Ev
+  Symbol removed: _ZTINSt3__117bad_function_callE
+  Symbol removed: _ZTSNSt3__117bad_function_callE
+  Symbol removed: _ZTVNSt3__117bad_function_callE
+
 * [libc++] Remove explicit instantiations of __insertion_sort_incomplete and __sort5 from the dylib
 
   These instantiations were never visible, because they are only used in `__sort`, which is also explicitly
   instantiated in the dylib.
 
   All platforms
   -------------
   Symbol removed: _ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIaaEEPaEEbT0_S5_T_

libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.abilist

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 No newline at end of file

libcxx/src/functional.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #include <functional>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
-bad_function_call::~bad_function_call() noexcept
-{
-}
-#endif
-
 #ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE
 const char*
 bad_function_call::what() const noexcept
 {
     return "std::bad_function_call";
 }
 #endif
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.bind/copy.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
-// Address Sanitizer doesn't instrument weak symbols on Linux. When a key
-// function is defined for bad_function_call's vtable, its typeinfo and vtable
-// will be defined as strong symbols in the library and weak symbols in other
-// translation units. Only the strong symbol will be instrumented, increasing
-// its size (due to the redzone) and leading to a serious ODR violation
-// resulting in a crash.
-// Some relevant bugs:
-// https://github.com/google/sanitizers/issues/1017
-// https://github.com/google/sanitizers/issues/619
-// https://github.com/google/sanitizers/issues/398
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68016
-// UNSUPPORTED: c++03, asan
+// UNSUPPORTED: c++03
 
 // <functional>
 
 // template<CopyConstructible Fn, CopyConstructible... Types>
 //   unspecified bind(Fn, Types...);
 // template<Returnable R, CopyConstructible Fn, CopyConstructible... Types>
 //   unspecified bind(Fn, Types...);
 

libcxx/test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.inv/invoke.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
 // <functional>
 
 // class function<R(ArgTypes...)>
 
 // R operator()(ArgTypes... args) const
 
-// This test runs in C++03, but we have deprecated using std::function in C++03.
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS -D_LIBCPP_ENABLE_CXX03_FUNCTION
-
-// Address Sanitizer doesn't instrument weak symbols on Linux. When a key
-// function is defined for bad_function_call's vtable, its typeinfo and vtable
-// will be defined as strong symbols in the library and weak symbols in other
-// translation units. Only the strong symbol will be instrumented, increasing
-// its size (due to the redzone) and leading to a serious ODR violation
-// resulting in a crash.
-// Some relevant bugs:
-// https://github.com/google/sanitizers/issues/1017
-// https://github.com/google/sanitizers/issues/619
-// https://github.com/google/sanitizers/issues/398
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68016
-// UNSUPPORTED: asan
-
 #include <functional>
 #include <cassert>
 
 #include "test_macros.h"
 
 
 int count = 0;
 

libcxx/test/std/utilities/function.objects/func.wrap/func.wrap.func/robust_against_adl.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
-// Address Sanitizer doesn't instrument weak symbols on Linux. When a key
-// function is defined for bad_function_call's vtable, its typeinfo and vtable
-// will be defined as strong symbols in the library and weak symbols in other
-// translation units. Only the strong symbol will be instrumented, increasing
-// its size (due to the redzone) and leading to a serious ODR violation
-// resulting in a crash.
-// Some relevant bugs:
-// https://github.com/google/sanitizers/issues/1017
-// https://github.com/google/sanitizers/issues/619
-// https://github.com/google/sanitizers/issues/398
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68016
-// UNSUPPORTED: c++03, asan
+// UNSUPPORTED: c++03
 
 // <functional>
 
 #include <functional>
 
 #include "test_macros.h"
 
 struct Incomplete;