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[Renato Golin <1488254@xxxxxxxxxxxxxxxxxx>]

(In reply to comment #38)
> I said *tagged* return types, i.e. only those which need to break the ABI
> anyway.

Good point. Thanks for the distinction.


> The new ABI requires the full gcc abi-tag support to be compatible with
> gcc. If you compile everything with clang and disable dual ABI support
> you can get away with less, but what is the point? Just use the clang
> libc++ instead, which requires no changes.

I'm afraid it's not that simple. GNU libraries are the default on all
Linux (and other) distributions, and mixing libc++ with libstdc++ is
bound to create at least as many problems as it solves.

Moreover, FreeBSD had to take many short-cuts to get rid of libstdc++ on
the base system, including making symlinks named after GNU libraries and
tools to LLVM libraries and tools, and the mapping is NOT 1-to-1. See
the libc++ + compiler-rt + libunwind vs. libgcc + libgcc_s + libgcc_eh
connundrum.

We need libc++ to work on the supported architectures with and without
GNU libraries. Getting rid of the GNU counterparts is out of the
question for any practical purpose.

-- 
You received this bug notification because you are a member of Desktop
Packages, which is subscribed to llvm-toolchain-3.6 in Ubuntu.
https://bugs.launchpad.net/bugs/1488254

Title:
  clang++ no longer ABI-compatible with g++

Status in LLVM:
  Confirmed
Status in llvm-toolchain-3.6 package in Ubuntu:
  Confirmed

Bug description:
  $ cat foo.cc
  #include <string>
  std::string hello = "Hello, world!\n";
  $ cat bar.cc
  #include <string>
  #include <iostream>
  extern std::string hello;
  int main() {
      std::cout << hello;
      return 0;
  }
  $ g++ -c foo.cc && g++ foo.o bar.cc && ./a.out
  Hello, world!
  $ clang++ -c foo.cc && clang++ foo.o bar.cc && ./a.out
  Hello, world!
  $ g++ -c foo.cc && clang++ foo.o bar.cc && ./a.out
  /tmp/bar-34fb23.o: In function `main':
  bar.cc:(.text+0x14): undefined reference to `hello'
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  $ clang++ -c foo.cc && g++ foo.o bar.cc && ./a.out
  /tmp/ccqU38Mh.o: In function `main':
  bar.cc:(.text+0x5): undefined reference to `hello[abi:cxx11]'
  collect2: error: ld returned 1 exit status

  In practice, this means that many programs using C++ libraries other
  than libstdc++ fail to compile with clang++. For example, mosh fails
  with undefined references to
  google::protobuf::internal::empty_string_,
  google::protobuf::MessageLite::InitializationErrorString() const, and
  google::protobuf::MessageLite::SerializeAsString() const.

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