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[libc++] Rework the whole availability markup implementation
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Currently, vendor-specific availability markup is enabled by default.
This means that even when building against trunk libc++, the headers
will by default prevent you from using some features that were not
released in the dylib on your target platform. This is a source of
frustration since people building libc++ from sources are usually not
trying to use some vendor's released dylib.

For that reason, I've been thinking for a long time that availability
annotations should be off by default, which is the primary change that
this commit enables.

In addition, it reworks the implementation to make it easier for new
vendors to add availability annotations for their platform, and it
refreshes the documentation to reflect the current state of the codebase.

Finally, a CMake configuration option is added to control whether
availability annotations should be turned on for the flavor of libc++
being created. The intent is for vendors like Apple to turn it on, and
for the upstream libc++ to leave it off (the default).

Differential Revision: https://reviews.llvm.org/D90843
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@ldionne
ldionne committed Nov 5, 2020
1 parent afe9264 commit 2eadbc8
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Showing 29 changed files with 261 additions and 196 deletions.
8 changes: 8 additions & 0 deletions libcxx/CMakeLists.txt
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Expand Up @@ -121,6 +121,13 @@ option(LIBCXX_ENABLE_LOCALIZATION
the C locale API (e.g. embedded). When localization is not supported,
several parts of the library will be disabled: <iostream>, <regex>, <locale>
will be completely unusable, and other parts may be only partly available." ON)
option(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS
"Whether to turn on vendor availability annotations on declarations that depend
on definitions in a shared library. By default, we assume that we're not building
libc++ for any specific vendor, and we disable those annotations. Vendors wishing
to provide compile-time errors when using features unavailable on some version of
the shared library they shipped should turn this on and see `include/__availability`
for more details." OFF)
option(LIBCXX_TEST_GDB_PRETTY_PRINTERS "Test gdb pretty printers." OFF)
set(LIBCXX_TEST_CONFIG "${CMAKE_CURRENT_SOURCE_DIR}/test/configs/legacy.cfg.in" CACHE STRING
"The Lit testing configuration to use when running the tests.")
Expand Down Expand Up @@ -844,6 +851,7 @@ config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
config_define_if(LIBCXX_ENABLE_PARALLEL_ALGORITHMS _LIBCPP_HAS_PARALLEL_ALGORITHMS)
config_define_if_not(LIBCXX_ENABLE_RANDOM_DEVICE _LIBCPP_HAS_NO_RANDOM_DEVICE)
config_define_if_not(LIBCXX_ENABLE_LOCALIZATION _LIBCPP_HAS_NO_LOCALIZATION)
config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)

if (LIBCXX_ABI_DEFINES)
set(abi_defines)
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1 change: 1 addition & 0 deletions libcxx/cmake/caches/Apple.cmake
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Expand Up @@ -11,6 +11,7 @@ set(LIBCXX_TYPEINFO_COMPARISON_IMPLEMENTATION "1" CACHE STRING "")
set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
set(LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT ON CACHE BOOL "")
set(LIBCXX_ENABLE_DEBUG_MODE_SUPPORT OFF CACHE BOOL "")
set(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS ON CACHE BOOL "")

set(LIBCXXABI_ENABLE_PIC OFF CACHE BOOL "")
set(LIBCXXABI_ENABLE_ASSERTIONS OFF CACHE BOOL "")
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96 changes: 0 additions & 96 deletions libcxx/docs/DesignDocs/AvailabilityMarkup.rst
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This file was deleted.

1 change: 1 addition & 0 deletions libcxx/include/CMakeLists.txt
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@@ -1,4 +1,5 @@
set(files
__availability
__bit_reference
__bsd_locale_defaults.h
__bsd_locale_fallbacks.h
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206 changes: 206 additions & 0 deletions libcxx/include/__availability
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@@ -0,0 +1,206 @@
// -*- 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___AVAILABILITY
#define _LIBCPP___AVAILABILITY

#include <__config>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif

// Libc++ is shipped by various vendors. In particular, it is used as a system
// library on macOS, iOS and other Apple platforms. In order for users to be
// able to compile a binary that is intended to be deployed to an older version
// of a platform, Clang provides availability attributes [1]. These attributes
// can be placed on declarations and are used to describe the life cycle of a
// symbol in the library.
//
// The main goal is to ensure a compile-time error if a symbol that hasn't been
// introduced in a previously released library is used in a program that targets
// that previously released library. Normally, this would be a load-time error
// when one tries to launch the program against the older library.
//
// For example, the filesystem library was introduced in the dylib in macOS 10.15.
// If a user compiles on a macOS 10.15 host but targets macOS 10.13 with their
// program, the compiler would normally not complain (because the required
// declarations are in the headers), but the dynamic loader would fail to find
// the symbols when actually trying to launch the program on macOS 10.13. To
// turn this into a compile-time issue instead, declarations are annotated with
// when they were introduced, and the compiler can produce a diagnostic if the
// program references something that isn't available on the deployment target.
//
// This mechanism is general in nature, and any vendor can add their markup to
// the library (see below). Whenever a new feature is added that requires support
// in the shared library, a macro should be added below to mark this feature
// as unavailable. When vendors decide to ship the feature as part of their
// shared library, they can update the markup appropriately.
//
// Note that this mechanism is disabled by default in the "upstream" libc++.
// Availability annotations are only meaningful when shipping libc++ inside
// a platform (i.e. as a system library), and so vendors that want them should
// turn those annotations on at CMake configuration time.
//
// [1]: https://clang.llvm.org/docs/AttributeReference.html#availability


// For backwards compatibility, allow users to define _LIBCPP_DISABLE_AVAILABILITY
// for a while.
#if defined(_LIBCPP_DISABLE_AVAILABILITY)
# if !defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
# define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
# endif
#endif

// Availability markup is disabled when building the library, or when the compiler
// doesn't support the proper attributes.
#if defined(_LIBCPP_BUILDING_LIBRARY) || \
defined(_LIBCXXABI_BUILDING_LIBRARY) || \
!__has_feature(attribute_availability_with_strict) || \
!__has_feature(attribute_availability_in_templates) || \
!__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 the availability of std::shared_mutex and std::shared_timed_mutex,
// which were added to the dylib later.
# define _LIBCPP_AVAILABILITY_SHARED_MUTEX

// These macros control the availability of std::bad_optional_access and
// other exception types. These were put in the shared library to prevent
// code bloat from every user program defining the vtable for these exception
// types.
# define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
# define _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS
# define _LIBCPP_AVAILABILITY_BAD_ANY_CAST

// This controls the availability of std::uncaught_exceptions().
# define _LIBCPP_AVAILABILITY_UNCAUGHT_EXCEPTIONS

// This controls the availability of the sized version of ::operator delete,
// which was added to the dylib later.
# define _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE

// This controls the availability of the std::future_error exception.
# define _LIBCPP_AVAILABILITY_FUTURE_ERROR

// This controls the availability of std::type_info's vtable.
// I can't imagine how using std::type_info can work at all if
// this isn't supported.
# define _LIBCPP_AVAILABILITY_TYPEINFO_VTABLE

// This controls the availability of std::locale::category members
// (e.g. std::locale::collate), which are defined in the dylib.
# define _LIBCPP_AVAILABILITY_LOCALE_CATEGORY

// This controls the availability of atomic operations on std::shared_ptr
// (e.g. `std::atomic_store(std::shared_ptr)`), which require a shared
// lock table located in the dylib.
# define _LIBCPP_AVAILABILITY_ATOMIC_SHARED_PTR

// These macros control the availability of all parts of <filesystem> that
// depend on something in the dylib.
# define _LIBCPP_AVAILABILITY_FILESYSTEM
# define _LIBCPP_AVAILABILITY_FILESYSTEM_PUSH
# define _LIBCPP_AVAILABILITY_FILESYSTEM_POP

// This controls the availability of std::to_chars.
# define _LIBCPP_AVAILABILITY_TO_CHARS

// This controls the availability of the C++20 synchronization library,
// which requires shared library support for various operations
// (see libcxx/src/atomic.cpp).
# define _LIBCPP_AVAILABILITY_SYNC

#elif defined(__APPLE__)

# 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)))
# define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS \
__attribute__((availability(macosx,strict,introduced=10.13))) \
__attribute__((availability(ios,strict,introduced=11.0))) \
__attribute__((availability(tvos,strict,introduced=11.0))) \
__attribute__((availability(watchos,strict,introduced=4.0)))
# define _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS \
_LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
# define _LIBCPP_AVAILABILITY_BAD_ANY_CAST \
_LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
# define _LIBCPP_AVAILABILITY_UNCAUGHT_EXCEPTIONS \
__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)))
# define _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE \
__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)))
# define _LIBCPP_AVAILABILITY_FUTURE_ERROR \
__attribute__((availability(ios,strict,introduced=6.0)))
# define _LIBCPP_AVAILABILITY_TYPEINFO_VTABLE \
__attribute__((availability(macosx,strict,introduced=10.9))) \
__attribute__((availability(ios,strict,introduced=7.0)))
# define _LIBCPP_AVAILABILITY_LOCALE_CATEGORY \
__attribute__((availability(macosx,strict,introduced=10.9))) \
__attribute__((availability(ios,strict,introduced=7.0)))
# define _LIBCPP_AVAILABILITY_ATOMIC_SHARED_PTR \
__attribute__((availability(macosx,strict,introduced=10.9))) \
__attribute__((availability(ios,strict,introduced=7.0)))
# define _LIBCPP_AVAILABILITY_FILESYSTEM \
__attribute__((availability(macosx,strict,introduced=10.15))) \
__attribute__((availability(ios,strict,introduced=13.0))) \
__attribute__((availability(tvos,strict,introduced=13.0))) \
__attribute__((availability(watchos,strict,introduced=6.0)))
# define _LIBCPP_AVAILABILITY_FILESYSTEM_PUSH \
_Pragma("clang attribute push(__attribute__((availability(macosx,strict,introduced=10.15))), apply_to=any(function,record))") \
_Pragma("clang attribute push(__attribute__((availability(ios,strict,introduced=13.0))), apply_to=any(function,record))") \
_Pragma("clang attribute push(__attribute__((availability(tvos,strict,introduced=13.0))), apply_to=any(function,record))") \
_Pragma("clang attribute push(__attribute__((availability(watchos,strict,introduced=6.0))), apply_to=any(function,record))")
# define _LIBCPP_AVAILABILITY_FILESYSTEM_POP \
_Pragma("clang attribute pop") \
_Pragma("clang attribute pop") \
_Pragma("clang attribute pop") \
_Pragma("clang attribute pop")
# define _LIBCPP_AVAILABILITY_TO_CHARS \
_LIBCPP_AVAILABILITY_FILESYSTEM
# define _LIBCPP_AVAILABILITY_SYNC \
__attribute__((unavailable))

#else

// ...New vendors can add availability markup here...

# error "It looks like you're trying to enable vendor availability markup, but you haven't defined the corresponding macros yet!"

#endif

// Define availability attributes that depend on _LIBCPP_NO_EXCEPTIONS.
// Those are defined in terms of the availability attributes above, and
// should not be vendor-specific.
#if defined(_LIBCPP_NO_EXCEPTIONS)
# define _LIBCPP_AVAILABILITY_FUTURE
# define _LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST
# define _LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS
# define _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
#else
# define _LIBCPP_AVAILABILITY_FUTURE _LIBCPP_AVAILABILITY_FUTURE_ERROR
# define _LIBCPP_AVAILABILITY_THROW_BAD_ANY_CAST _LIBCPP_AVAILABILITY_BAD_ANY_CAST
# define _LIBCPP_AVAILABILITY_THROW_BAD_OPTIONAL_ACCESS _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
# define _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS _LIBCPP_AVAILABILITY_BAD_VARIANT_ACCESS
#endif

#endif // _LIBCPP___AVAILABILITY

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