mirror of
https://github.com/darlinghq/darling-libcxx.git
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f68c6d52a1
Summary:
This patch make G++03 explicitly unsupported with libc++, as discussed on the mailing lists.
Below is the rational for this decision.
----------------------------------------------------------------------------------------------------
libc++ claims to support GCC with C++03 ("G++03"), and this is a problem for our users.
Our C++03 users are all using Clang. They must be. Less than 9% of the C++03 tests pass with GCC [1][2]. No non-trivial C++ program could work.
Attempting to support G++03 impacts our QoI considerably. Unlike Clang, G++03 offers almost no C++11 extensions. If we could remove all the fallbacks for G++03, it would mean libc++ could::
* Improve Correctness:
Every `#ifdef _LIBCPP_HAS_NO_<C++11-feature>` is a bug manifest. It exists to admit for deviant semantics.
* Achieve ABI stability between C++03 and C++11
Differences between our C++03 and C++Rest branches contain ABI bugs. For example `std::nullptr_t` and `std::function::operator()(...)` are currently incompatible between C++11 and C++03, but could be fixed.
* Decrease Compile Times and Memory Usage:
Writing efficient SFINAE requires C++11. Using alias templates, libc++ could reduce the number of instantiations it produces substantially.
* Decrease Binary Size
Similar to the last point, G++03 forces metaprogramming techniques that emit more debug information [3] [4]. Compared to libstdc++, debug information size increases of +10% are not uncommon.
Reviewers: ldionne, mclow.lists, EricWF
Reviewed By: ldionne, EricWF
Subscribers: zoecarver, aprantl, dexonsmith, arphaman, libcxx-commits, #libc
Differential Revision: https://reviews.llvm.org/D63154
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363219 91177308-0d34-0410-b5e6-96231b3b80d8
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.. _index:
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=============================
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"libc++" C++ Standard Library
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=============================
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Overview
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========
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libc++ is a new implementation of the C++ standard library, targeting C++11 and
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above.
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* Features and Goals
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* Correctness as defined by the C++11 standard.
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* Fast execution.
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* Minimal memory use.
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* Fast compile times.
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* ABI compatibility with gcc's libstdc++ for some low-level features
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such as exception objects, rtti and memory allocation.
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* Extensive unit tests.
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* Design and Implementation:
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* Extensive unit tests
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* Internal linker model can be dumped/read to textual format
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* Additional linking features can be plugged in as "passes"
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* OS specific and CPU specific code factored out
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Getting Started with libc++
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---------------------------
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.. toctree::
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:maxdepth: 2
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ReleaseNotes
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UsingLibcxx
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BuildingLibcxx
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TestingLibcxx
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.. toctree::
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:hidden:
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FeatureTestMacroTable
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Current Status
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--------------
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After its initial introduction, many people have asked "why start a new
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library instead of contributing to an existing library?" (like Apache's
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libstdcxx, GNU's libstdc++, STLport, etc). There are many contributing
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reasons, but some of the major ones are:
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* From years of experience (including having implemented the standard
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library before), we've learned many things about implementing
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the standard containers which require ABI breakage and fundamental changes
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to how they are implemented. For example, it is generally accepted that
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building std::string using the "short string optimization" instead of
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using Copy On Write (COW) is a superior approach for multicore
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machines (particularly in C++11, which has rvalue references). Breaking
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ABI compatibility with old versions of the library was
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determined to be critical to achieving the performance goals of
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libc++.
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* Mainline libstdc++ has switched to GPL3, a license which the developers
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of libc++ cannot use. libstdc++ 4.2 (the last GPL2 version) could be
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independently extended to support C++11, but this would be a fork of the
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codebase (which is often seen as worse for a project than starting a new
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independent one). Another problem with libstdc++ is that it is tightly
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integrated with G++ development, tending to be tied fairly closely to the
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matching version of G++.
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* STLport and the Apache libstdcxx library are two other popular
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candidates, but both lack C++11 support. Our experience (and the
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experience of libstdc++ developers) is that adding support for C++11 (in
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particular rvalue references and move-only types) requires changes to
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almost every class and function, essentially amounting to a rewrite.
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Faced with a rewrite, we decided to start from scratch and evaluate every
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design decision from first principles based on experience.
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Further, both projects are apparently abandoned: STLport 5.2.1 was
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released in Oct'08, and STDCXX 4.2.1 in May'08.
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Platform and Compiler Support
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-----------------------------
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libc++ is known to work on the following platforms, using gcc and
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clang.
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Note that functionality provided by ``<atomic>`` is only functional with clang
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and GCC.
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============ ==================== ============ ========================
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OS Arch Compilers ABI Library
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============ ==================== ============ ========================
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macOS i386, x86_64 Clang, GCC libc++abi
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FreeBSD 10+ i386, x86_64, ARM Clang, GCC libcxxrt, libc++abi
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Linux i386, x86_64 Clang, GCC libc++abi
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============ ==================== ============ ========================
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The following minimum compiler versions are strongly recommended.
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* Clang 3.5 and above
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* GCC 5.0 and above.
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The C++03 dialect is only supported for Clang compilers.
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C++ Dialect Support
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---------------------
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* C++11 - Complete
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* `C++14 - Complete <http://libcxx.llvm.org/cxx1y_status.html>`__
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* `C++17 - In Progress <http://libcxx.llvm.org/cxx1z_status.html>`__
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* `Post C++14 Technical Specifications - In Progress <http://libcxx.llvm.org/ts1z_status.html>`__
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* :ref:`C++ Feature Test Macro Status <feature-status>`
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Notes and Known Issues
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----------------------
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This list contains known issues with libc++
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* Building libc++ with ``-fno-rtti`` is not supported. However
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linking against it with ``-fno-rtti`` is supported.
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A full list of currently open libc++ bugs can be `found here`__.
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.. __: https://bugs.llvm.org/buglist.cgi?component=All%20Bugs&product=libc%2B%2B&query_format=advanced&resolution=---&order=changeddate%20DESC%2Cassigned_to%20DESC%2Cbug_status%2Cpriority%2Cbug_id&list_id=74184
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Design Documents
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----------------
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.. toctree::
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:maxdepth: 1
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DesignDocs/AvailabilityMarkup
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DesignDocs/DebugMode
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DesignDocs/CapturingConfigInfo
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DesignDocs/ABIVersioning
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DesignDocs/ExperimentalFeatures
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DesignDocs/VisibilityMacros
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DesignDocs/ThreadingSupportAPI
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DesignDocs/FileTimeType
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DesignDocs/FeatureTestMacros
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DesignDocs/ExtendedCXX03Support
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* `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_
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* `<type_traits> design <http://libcxx.llvm.org/type_traits_design.html>`_
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* `Notes by Marshall Clow`__
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.. __: https://cplusplusmusings.wordpress.com/2012/07/05/clang-and-standard-libraries-on-mac-os-x/
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Build Bots and Test Coverage
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----------------------------
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* `LLVM Buildbot Builders <http://lab.llvm.org:8011/console>`_
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* `Apple Jenkins Builders <http://lab.llvm.org:8080/green/view/Libcxx/>`_
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* `Windows Appveyor Builders <https://ci.appveyor.com/project/llvm-mirror/libcxx>`_
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* `Code Coverage Results <http://efcs.ca/libcxx-coverage>`_
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Getting Involved
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================
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First please review our `Developer's Policy <http://llvm.org/docs/DeveloperPolicy.html>`__
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and `Getting started with LLVM <http://llvm.org/docs/GettingStarted.html>`__.
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**Bug Reports**
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If you think you've found a bug in libc++, please report it using
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the `LLVM Bugzilla`_. If you're not sure, you
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can post a message to the `libcxx-dev mailing list`_ or on IRC.
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**Patches**
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If you want to contribute a patch to libc++, the best place for that is
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`Phabricator <http://llvm.org/docs/Phabricator.html>`_. Please add `libcxx-commits` as a subscriber.
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Also make sure you are subscribed to the `libcxx-commits mailing list <http://lists.llvm.org/mailman/listinfo/libcxx-commits>`_.
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**Discussion and Questions**
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Send discussions and questions to the
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`libcxx-dev mailing list <http://lists.llvm.org/mailman/listinfo/libcxx-dev>`_.
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Quick Links
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===========
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* `LLVM Homepage <http://llvm.org/>`_
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* `libc++abi Homepage <http://libcxxabi.llvm.org/>`_
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* `LLVM Bugzilla <https://bugs.llvm.org/>`_
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* `libcxx-commits Mailing List`_
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* `libcxx-dev Mailing List`_
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* `Browse libc++ Sources <https://github.com/llvm/llvm-project/tree/master/libcxx/>`_
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