Is Clang++ Abi Same as G++

Is clang++ ABI same as g++?

According to the clang libc++ page, they're targeting

ABI compatibility with gcc's libstdc++ for some low-level features such as exception objects, rtti and memory allocation.

which seems to imply that they're not targeting 100% compatibility. For example, on that page they also say:

From years of experience (including having implemented the standard library before), we've learned many things about implementing the standard containers which require ABI breakage and fundamental changes to how they are implemented. For example, it is generally accepted that building std::string using the "short string optimization" instead of using Copy On Write (COW) is a superior approach for multicore machines (particularly in C++'0x, which has rvalue references). Breaking ABI compatibility with old versions of the library was determined to be critical to achieving the performance goals of libc++.

I believe that GCC is still using a reference-counted COW, so it appears that clang isn't worried about ABI compatibility with std::string (either with older clang compiled binaries or with GCC).

If clang++ and g++ are ABI incompatible, what is used for shared libraries in binary?

even for things as core as standard containers

Standard containers are not all that "core". (For typical implementations) they are implemented entirely in valid C++ in headers, and if you compile the same headers with G++ and Clang++ you'll get ABI compatible output. You should only get incompatibilities "even for things as core as standard containers" if you use different versions of the container headers, not just by using Clang instead of GCC.

Both GCC and Clang conform to a cross-vendor, cross-platform C++ ABI (originally developed for the Itanium architecture, but also used for x86, x86_64, SPARC etc.) The really core things such as class layout, name mangling, exception handling, vtables etc. are specified by that ABI and Clang and GCC both follow it.

So in other words, if you compile the same source with GCC and Clang you'll get ABI-compatible binaries.

If you want to understand this stuff better see my What's an ABI and why is it so complicated? slides.

GCC 4.0, 4.2 and LLVM ABI Compatibility

Clang is ABI-compatible with code generated by gcc. Clang also includes experimental support for some newer Objective-C ABIs, but compiling for the newer ABI requires flags, and generated code can be mixed with GCC-generated code anyway.

GCC/Clang x86_64 C++ ABI mismatch when returning a tuple?

As davmac's answer shows, the libstdc++ std::tuple is trivially copy constructible, but not trivially move constructible. The two compilers disagree on whether the move constructor should affect the argument passing conventions.

The C++ ABI thread you linked to seems to explain that disagreement:
http://sourcerytools.com/pipermail/cxx-abi-dev/2016-February/002891.html

In summary, Clang implements exactly what the ABI spec says, but G++ implements what it was supposed to say, but wasn't updated to actually say.

gcc vs clang common library issue

g++ and clang++ are compatible as compilers (because they both follow the Itanium ABI), but they may come with incompatible standard library implementations.

g++ comes with a standard library implementation called libstdc++. You can direct g++ to use a different implementation but this is not exactly trivial.

clang++ sometimes comes without a standard library implementation of its own (and is configured to use implementation provided by g++), and sometimes comes with an implementation called libc++. One can easily switch clang++ to use either libc++ or libstdc++ with a single command line option.

So your question boils down to what standard library implementation(s) your applications use. If they use the same implementation, you need to build Boost with that implementation (and either compiler). If they use different implementations, you need two separate builds of Boost.

Mixing components built against different standard library implementations in the same application can sometimes be done, but is not straightforward, entails a lot of restrictions, and with things like boost is either not feasible or downright impossible.

Are llvm-gcc and clang binary compatible with gcc? - particularly mingw gcc on Windows

Yes, for C code Clang and GCC are compatible (they both use the GNU Toolchain for linking, in fact.) You just have to make sure that you tell clang to create compiled objects and not intermediate bitcode objects. C ABI is well-defined, so the only issue is storage format.

C++ is not portable between compilers in the slightest; different compilers use different virtual table calls, constructors, destruction, name mangling, template implementations, etc. As a rule you should assume objects from one C++ compiler will not work with another.

However yes, at the time of writing Clang++ is able to use GCC/C++ compiled libraries as well; I recently set up a rig to compile C++ programs with clang using G++'s standard runtime library and it compiles+links just fine.

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