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Differential D63154

Make GCC in C++03 Unsupported
ClosedPublic

Authored by EricWF on Jun 11 2019, 11:14 AM.

Details

Reviewers
ldionne
mclow.lists
EricWF
Commits
rG5de7cacf0797: Make GCC in C++03 Unsupported
rL363219: Make GCC in C++03 Unsupported
rCXX363219: Make GCC in C++03 Unsupported
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.

Diff Detail

Event Timeline

EricWF created this revision.Jun 11 2019, 11:14 AM
Herald added subscribers: libcxx-commits, arphaman, dexonsmith, aprantl. · View Herald TranscriptJun 11 2019, 11:14 AM
zoecarver added a subscriber: zoecarver.Jun 12 2019, 10:02 AM

A few things:

  • Can / should we update lit or the lit config?
  • When I run the tests in gnu++03 mode, I don't get error "libc++ does not support using GCC with C++03. Please enable C++11". Maybe I am doing something wrong, though.
  • In all fairness, the reason (most of) the tests fail is not because of g++03 but because they are disabled for c++03 and not gnu++03.
EricWF accepted this revision.Jun 12 2019, 1:26 PM

I plan to commit this shortly.

This is a louder and farther reaching way to announce the GCC C++03 support drop.

This revision is now accepted and ready to land.Jun 12 2019, 1:26 PM
ldionne accepted this revision.Jun 12 2019, 1:30 PM
Closed by commit rL363219: Make GCC in C++03 Unsupported (authored by EricWF). · Explain WhyJun 12 2019, 5:34 PM
This revision was automatically updated to reflect the committed changes.
Herald added a project: Restricted Project. · View Herald TranscriptJun 12 2019, 5:34 PM
Herald added subscribers: llvm-commits, christof. · View Herald Transcript

Revision Contents

PathSize
docs/
4 lines
include/
4 lines

Diff 204119

docs/index.rst

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macOS i386, x86_64 Clang, GCC libc++abi macOS i386, x86_64 Clang, GCC libc++abi
FreeBSD 10+ i386, x86_64, ARM Clang, GCC libcxxrt, libc++abi FreeBSD 10+ i386, x86_64, ARM Clang, GCC libcxxrt, libc++abi
Linux i386, x86_64 Clang, GCC libc++abi Linux i386, x86_64 Clang, GCC libc++abi
============ ==================== ============ ======================== ============ ==================== ============ ========================
The following minimum compiler versions are strongly recommended. The following minimum compiler versions are strongly recommended.
* Clang 3.5 and above * Clang 3.5 and above
* GCC 4.7 and above. * GCC 5.0 and above.
Anything older *may* work. The C++03 dialect is only supported for Clang compilers.
C++ Dialect Support C++ Dialect Support
--------------------- ---------------------
* C++11 - Complete * C++11 - Complete
* `C++14 - Complete <http://libcxx.llvm.org/cxx1y_status.html>`__ * `C++14 - Complete <http://libcxx.llvm.org/cxx1y_status.html>`__
* `C++17 - In Progress <http://libcxx.llvm.org/cxx1z_status.html>`__ * `C++17 - In Progress <http://libcxx.llvm.org/cxx1z_status.html>`__
* `Post C++14 Technical Specifications - In Progress <http://libcxx.llvm.org/ts1z_status.html>`__ * `Post C++14 Technical Specifications - In Progress <http://libcxx.llvm.org/ts1z_status.html>`__
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include/__config

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#elif defined(__IBMCPP__) #elif defined(__IBMCPP__)
# define _LIBCPP_COMPILER_IBM # define _LIBCPP_COMPILER_IBM
#endif #endif
#ifndef _LIBCPP_CLANG_VER #ifndef _LIBCPP_CLANG_VER
#define _LIBCPP_CLANG_VER 0 #define _LIBCPP_CLANG_VER 0
#endif #endif
#if defined(_LIBCPP_COMPILER_GCC) && __cplusplus < 201103L
#error "libc++ does not support using GCC with C++03. Please enable C++11"
#endif
// FIXME: ABI detection should be done via compiler builtin macros. This // FIXME: ABI detection should be done via compiler builtin macros. This
// is just a placeholder until Clang implements such macros. For now assume // is just a placeholder until Clang implements such macros. For now assume
// that Windows compilers pretending to be MSVC++ target the Microsoft ABI, // that Windows compilers pretending to be MSVC++ target the Microsoft ABI,
// and allow the user to explicitly specify the ABI to handle cases where this // and allow the user to explicitly specify the ABI to handle cases where this
// heuristic falls short. // heuristic falls short.
#if defined(_LIBCPP_ABI_FORCE_ITANIUM) && defined(_LIBCPP_ABI_FORCE_MICROSOFT) #if defined(_LIBCPP_ABI_FORCE_ITANIUM) && defined(_LIBCPP_ABI_FORCE_MICROSOFT)
# error "Only one of _LIBCPP_ABI_FORCE_ITANIUM and _LIBCPP_ABI_FORCE_MICROSOFT can be defined" # error "Only one of _LIBCPP_ABI_FORCE_ITANIUM and _LIBCPP_ABI_FORCE_MICROSOFT can be defined"
#elif defined(_LIBCPP_ABI_FORCE_ITANIUM) #elif defined(_LIBCPP_ABI_FORCE_ITANIUM)
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