[libc++] Unconditionally provide the ability for users to enable assertions

[ldionne]

Summary

Previously, assertions could only be enabled if the library was configured
to include support for the Debug mode. This patch unconditionally adds the
very small amount of code needed to support assertions (and assertions
only, not the whole iterator debugging functionality) to the shared library.

This means that users can turn assertions on in their code regardless of
how the library was configured, which makes it a true user-facing knob.
I think this makes sense, just like we allow users to decide whether
assert() is enabled or not.

If they don't enable assertions in their code, nothing changes compared
to before this patch, except the shared library will now be a tiny bit
larger than it used to be. In my opinion, this is insignificant compared
to the benefit of being able to turn assertions on, and it's not even
worth the complexity to provide a knob to turn that part off.

Also note that technically, functions compiled inside the shared library
may or may not have assertions enabled, depending on whether the vendor
configured the library with LIBCXX_ENABLE_ASSERTIONS when building.
This can lead to ODR violations, however I think those are all benign.

Finally, since we're adding symbols to the shared library, this means
that this feature won't be available when linking against older dylibs,
hence the accompanying availability markup.

[ldionne]

Note that we could have interesting design discussions:

1. Is it really necessary to allow setting the "assertion handler" at runtime, i.e. couldn't we get rid of __libcpp_set_debug_function and just always call abort()?
2. Why don't we change the name of __libcpp_debug_function & friends to be more like __libcpp_assertion_handler?

Since the debug mode has been shipping in the LLVM release, my assumption is that people might have been enabling assertions and expecting ABI stability, which would be fair. If we make any of the changes above, we would break that.

As you might guess, my main goal is to eventually be able to enable assertions on Apple platforms, which isn't supported right now (we build our dylib with LIBCXX_ENABLE_DEBUG_MODE_SUPPORT=OFF). Technically, these ABI breaks would not impact us because we haven't been shipping those symbols, however they would impact users on other platforms. That's why I've decided to avoid going down that route so far.

[ldionne]

Add ABI lists to try to fix the CI

[ldionne]

Does anyone want to chime in? Maybe libc++ vendors have an opinion about this? Vendors, have you been shipping the debug mode? With what stability guarantees? Unless told otherwise, my assumption is that folks have been shipping the library as-is, which means with support for the debug mode enabled (which unfortunately ties our hands a bit in this change).

[manojgupta]

We had never defined LIBCXX_ENABLE_ASSERTIONS and _LIBCPP_DEBUG explicitly in our cmake config for Chrome OS.

[manojgupta]

Running readelf shows following in our shipping shared libs.

 231: 00000000000c7db0     8 OBJECT  GLOBAL DEFAULT   23 _ZNSt3__123__libcpp_debug_functionE
 668: 0000000000075160    13 FUNC    GLOBAL DEFAULT   14 _ZNSt3__127__libcpp_set_debug_functionEPFvRKNS_19__libcpp_debug_infoEE
1438: 0000000000075110    79 FUNC    GLOBAL DEFAULT   14 _ZNSt3__129__libcpp_abort_debug_functionERKNS_19__libcpp_debug_infoE
1979: 0000000000074ef0   544 FUNC    GLOBAL DEFAULT   14 _ZNKSt3__119__libcpp_debug_info4whatEv

[ldionne]

In D119969#3349883, @manojgupta wrote:

Running readelf shows following in our shipping shared libs.

 231: 00000000000c7db0     8 OBJECT  GLOBAL DEFAULT   23 _ZNSt3__123__libcpp_debug_functionE
 668: 0000000000075160    13 FUNC    GLOBAL DEFAULT   14 _ZNSt3__127__libcpp_set_debug_functionEPFvRKNS_19__libcpp_debug_infoEE
1438: 0000000000075110    79 FUNC    GLOBAL DEFAULT   14 _ZNSt3__129__libcpp_abort_debug_functionERKNS_19__libcpp_debug_infoE
1979: 0000000000074ef0   544 FUNC    GLOBAL DEFAULT   14 _ZNKSt3__119__libcpp_debug_info4whatEv

Okay, right -- so as expected, you've been shipping support for the debug mode as part of the dylib, and if any user has been enabling assertions with _LIBCPP_DEBUG=0, they are relying on those symbols. So I think my proposed direction stands (it's this patch as-is). It's not the most elegant design since we're reusing old __libcpp_debug_foo for assertions, but I think it's the best decision engineering-wise for the time being.

[dim]

In D119969#3351389, @ldionne wrote:

In D119969#3349883, @manojgupta wrote:

Running readelf shows following in our shipping shared libs.

 231: 00000000000c7db0     8 OBJECT  GLOBAL DEFAULT   23 _ZNSt3__123__libcpp_debug_functionE
 668: 0000000000075160    13 FUNC    GLOBAL DEFAULT   14 _ZNSt3__127__libcpp_set_debug_functionEPFvRKNS_19__libcpp_debug_infoEE
1438: 0000000000075110    79 FUNC    GLOBAL DEFAULT   14 _ZNSt3__129__libcpp_abort_debug_functionERKNS_19__libcpp_debug_infoE
1979: 0000000000074ef0   544 FUNC    GLOBAL DEFAULT   14 _ZNKSt3__119__libcpp_debug_info4whatEv

Okay, right -- so as expected, you've been shipping support for the debug mode as part of the dylib, and if any user has been enabling assertions with _LIBCPP_DEBUG=0, they are relying on those symbols. So I think my proposed direction stands (it's this patch as-is). It's not the most elegant design since we're reusing old __libcpp_debug_foo for assertions, but I think it's the best decision engineering-wise for the time being.

We've got something similar on FreeBSD:

% readelf -aW /lib/libc++.so.1
...
Relocation section '.rela.dyn' at offset 0x354f8 contains 2161 entries:
...
00000000000fc6e0  0000012c00000001 R_X86_64_64            0000000000092f60 std::__1::__libcpp_abort_debug_function(std::__1::__libcpp_debug_info const&) + 0
00000000000fb130  0000019b00000006 R_X86_64_GLOB_DAT      00000000000fc6e0 std::__1::__libcpp_debug_function + 0
...
Relocation section '.rela.plt' at offset 0x41f90 contains 572 entries:
...
00000000000fca38  0000085700000007 R_X86_64_JUMP_SLOT     0000000000092d10 std::__1::__libcpp_debug_info::what() const + 0
...
Symbol table '.dynsym' contains 2398 entries:
...
   300: 0000000000092f60    82 FUNC    GLOBAL DEFAULT   14 std::__1::__libcpp_abort_debug_function(std::__1::__libcpp_debug_info const&)
   411: 00000000000fc6e0     8 OBJECT  GLOBAL DEFAULT   25 std::__1::__libcpp_debug_function
  1468: 0000000000092fc0    18 FUNC    GLOBAL DEFAULT   14 std::__1::__libcpp_set_debug_function(void (*)(std::__1::__libcpp_debug_info const&))
  2135: 0000000000092d10   586 FUNC    GLOBAL DEFAULT   14 std::__1::__libcpp_debug_info::what() const

These all originate in debug.cpp. I think we have been shipping it like this since we switched from libstd++ to libc++ years ago.

So this change is fine from our perspective.

[EricWF]

Looks good to me.

[ldionne]

I'm going to abandon this since I think we can do much better by providing an assertion handler that can be customized at compile-time instead of piggy-backing on the existing debug mode infrastructure, see D121123.

libcxx/CMakeLists.txt

 
 #===============================================================================
 # Setup CMake Options
 #===============================================================================
 include(CMakeDependentOption)
 include(HandleCompilerRT)
 
 # Basic options ---------------------------------------------------------------
-option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." OFF)
+option(LIBCXX_ENABLE_ASSERTIONS
+  "Enable assertions inside the compiled library. Note that assertions can be
+   enabled by users in their own code regardless of whether the library has been
+   compiled with assertions enabled." OFF)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
 option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
 set(ENABLE_FILESYSTEM_DEFAULT ON)
 if (WIN32 AND NOT MINGW)
   # Filesystem is buildable for windows, but it requires __int128 helper
   # functions, that currently are provided by libgcc or compiler_rt builtins.
   # These are available in MinGW environments, but not currently in MSVC

libcxx/docs/ReleaseNotes.rst

 - The header ``<experimental/filesystem>`` has been removed. Instead, use
   ``<filesystem>`` header. The associated macro
   ``_LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM`` has also been removed.
 
 - Transitive includes of ``<algorithm>`` have been removed. If you see compiler errors
   related to missing declarations inside namespace ``std`` when updating libc++,
   you are probably missing ``#include <algorithm>`` in a file where you use algorithms.
 
+- Users can now enable assertions inside libc++ by defining ``_LIBCPP_DEBUG=0`` on
+  the command-line, regardless of how the library was built (usually by a vendor).
+  Note that if the library was built with assertions disabled, any function that is
+  compiled into the library won't have assertions enabled, but functions compiled into
+  your own code (usually those defined in the headers) will have assertions enabled.
+
+  This feature requires library support, so it is not enabled when deploying on
+  platforms that have not previously shipped with the ability to turn assertions on.
+
 ABI Changes
 -----------
 
 - The ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro controlling whether we use an
   emulation for ``std::nullptr_t`` in C++03 mode has been removed. After this change,
   ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` will not be honoured anymore and there
   will be no way to opt back into the C++03 emulation of ``std::nullptr_t``.
 
 Build System Changes
 --------------------
 
 - Support for standalone builds have been entirely removed from libc++, libc++abi and
   libunwind. Please use :ref:`these instructions <build instructions>` for building
   libc++, libc++abi and/or libunwind.

libcxx/include/__assert

 // -*- 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___ASSERT
 #define _LIBCPP___ASSERT
 
+#include <__availability>
 #include <__config>
 #include <iosfwd> // for std::string
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #if _LIBCPP_DEBUG_LEVEL >= 1

 struct _LIBCPP_TEMPLATE_VIS __libcpp_debug_info {
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
   __libcpp_debug_info()
       : __file_(nullptr), __line_(-1), __pred_(nullptr), __msg_(nullptr) {}
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
   __libcpp_debug_info(const char* __f, int __l, const char* __p, const char* __m)
     : __file_(__f), __line_(__l), __pred_(__p), __msg_(__m) {}
 
+  _LIBCPP_AVAILABILITY_ASSERTIONS
   _LIBCPP_FUNC_VIS string what() const;
 
   const char* __file_;
   int __line_;
   const char* __pred_;
   const char* __msg_;
 };
 
 /// __libcpp_debug_function_type - The type of the assertion failure handler.
 typedef void(*__libcpp_debug_function_type)(__libcpp_debug_info const&);
 
 /// __libcpp_debug_function - The handler function called when a _LIBCPP_ASSERT
 ///    fails.
+_LIBCPP_AVAILABILITY_ASSERTIONS
 extern _LIBCPP_EXPORTED_FROM_ABI __libcpp_debug_function_type __libcpp_debug_function;
 
 /// __libcpp_abort_debug_function - A debug handler that aborts when called.
-_LIBCPP_NORETURN _LIBCPP_FUNC_VIS
+_LIBCPP_NORETURN _LIBCPP_AVAILABILITY_ASSERTIONS _LIBCPP_FUNC_VIS
 void __libcpp_abort_debug_function(__libcpp_debug_info const&);
 
 /// __libcpp_set_debug_function - Set the debug handler to the specified
 ///    function.
-_LIBCPP_FUNC_VIS
+_LIBCPP_AVAILABILITY_ASSERTIONS _LIBCPP_FUNC_VIS
 bool __libcpp_set_debug_function(__libcpp_debug_function_type __func);
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ASSERT

libcxx/include/__availability

#if defined(_LIBCPP_BUILDING_LIBRARY) ||                                        \
     !__has_extension(pragma_clang_attribute_external_declaration)
 #   if !defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 #       define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
 #   endif
 #endif
 
 #if defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 
+    // This controls whether users are allowed to turn on assertions
+    // when using the library.
+#   define _LIBCPP_AVAILABILITY_ASSERTIONS
+
     // This controls the availability of std::shared_mutex and std::shared_timed_mutex,
     // which were added to the dylib later.
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_shared_mutex
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_shared_timed_mutex
 
     // These macros control the availability of std::bad_optional_access and
     // other exception types. These were put in the shared library to prevent
// #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_semaphore
     // The library is in development and not ABI stable yet. P2216 is
     // retroactively accepted in C++20. This paper contains ABI breaking
     // changes.
 #   define _LIBCPP_AVAILABILITY_FORMAT
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format
 
 #elif defined(__APPLE__)
 
+#   define _LIBCPP_AVAILABILITY_ASSERTIONS                                      \
+        __attribute__((unavailable))
+
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                    \
         __attribute__((availability(macosx,strict,introduced=10.12)))           \
         __attribute__((availability(ios,strict,introduced=10.0)))               \
         __attribute__((availability(tvos,strict,introduced=10.0)))              \
         __attribute__((availability(watchos,strict,introduced=3.0)))
 #   if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101200) ||    \
         (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ < 100000) || \
         (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ < 100000) ||         \

libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist

 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE14do_curr_symbolEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE14do_frac_digitsEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE16do_decimal_pointEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE16do_negative_signEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE16do_positive_signEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE16do_thousands_sepEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__118__time_get_storageIcE15__do_date_orderEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__118__time_get_storageIwE15__do_date_orderEv', 'type': 'FUNC'}
+{'is_defined': True, 'name': '__ZNKSt3__119__libcpp_debug_info4whatEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__119__shared_weak_count13__get_deleterERKSt9type_info', 'type': 'FUNC'}
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 {'is_defined': True, 'name': '__ZNKSt3__120__codecvt_utf8_utf16IDiE9do_lengthER11__mbstate_tPKcS5_m', 'type': 'FUNC'}

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 {'is_defined': True, 'name': '__ZNSt3__123__libcpp_atomic_monitorEPVKv', 'type': 'FUNC'}
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 {'is_defined': True, 'name': '__ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIyyEEPyEEbT0_S5_T_', 'type': 'FUNC'}
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 {'is_defined': True, 'name': '___cxa_vec_delete2', 'type': 'I'}
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 {'is_defined': True, 'name': '___cxa_vec_new', 'type': 'I'}
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 No newline at end of file

libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist

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 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE16do_negative_signEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE16do_positive_signEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNKSt3__117moneypunct_bynameIwLb1EE16do_thousands_sepEv', 'type': 'FUNC'}
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 No newline at end of file

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

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 No newline at end of file

libcxx/src/CMakeLists.txt

 set(LIBCXX_LIB_CMAKEFILES_DIR "${CMAKE_CURRENT_BINARY_DIR}${CMAKE_FILES_DIRECTORY}"  PARENT_SCOPE)
 
 # Get sources
 set(LIBCXX_SOURCES
   algorithm.cpp
   any.cpp
+  assert.cpp
   atomic.cpp
   barrier.cpp
   bind.cpp
   charconv.cpp
   chrono.cpp
   condition_variable.cpp
   condition_variable_destructor.cpp
   exception.cpp
set(LIBCXX_SOURCES
   utility.cpp
   valarray.cpp
   variant.cpp
   vector.cpp
   )
 
 if (LIBCXX_ENABLE_DEBUG_MODE_SUPPORT)
   list(APPEND LIBCXX_SOURCES
-    assert.cpp
     debug.cpp
     )
 endif()
 
 if (LIBCXX_ENABLE_RANDOM_DEVICE)
   list(APPEND LIBCXX_SOURCES
     random.cpp
     )