[LLVMdev] Purpose of LLVM_ENABLE_LIBCXX and LLVM_ENABLE_LIBCXXABI

[Martell Malone]

>
> I think you are more correct though

Meant to say more likely correct. :)

I don't have an ego problem this was a typo

Michael
> In short Michael
>
Also said his name twice,
I need to start reading things twice before I click the send button

On Tue, Jul 28, 2015 at 1:40 PM, Martell Malone <martellmalone at gmail.com>
wrote:

> Hi Renato
>
> I had the impression that Michael actually wanted the latter, like GCC
>> has autoconf options to change the default behaviour of the compiler
>> at build time.
>>
>> I may be wrong, though.
>
>
> yes I thought that was what he wanted because he said
>
> Are they used to indicate that if the C/C++ compiler for building LLVM is
>> clang
>>
>
> That combined with the standard cmake flags cmake_cxx_compiler and
> cmake_c_compiler should result in the host being clang and using libc++ and
> libc++abi
> I was a little thrown off by the wording of the question
> I think you are more correct though I misread the second part of the
> question where he said what he actually wanted the resulting compiler to
> use libc++
> Sorry for the noise
>
> Michael
>
> In short Michael it should not matter if clang was build with libc++ or
> libstdc++ the resulting compiler can use both after the fact
> As Renato described above in more detail LDFLAGS="--stdlib=libc++ -lc++abi
> -lunwind" would be used to tell that resulting compiler to link its
> programs with libc++, libc++abi and libunwind
> Rather than using libstdc++ and libgcc_eh
>
> If you want to know how to change the default to do this without touching
> the LDFLAGS for your target this you must look in the clang repo under
> lib/driver
> If you have any trouble doing this I'd be glad to help you change the
> default behavior if let me know what your target is.
>
> Kind Regards
> Martell
>
> On Tue, Jul 28, 2015 at 1:06 PM, Renato Golin <renato.golin at linaro.org>
> wrote:
>
>> On 28 July 2015 at 12:10, Schlottke-Lakemper, Michael
>> <m.schlottke-lakemper at aia.rwth-aachen.de> wrote:
>> > I am wondering about the meaning of the cmake flags LLVM_ENABLE_LIBCXX
>> and LLVM_ENABLE_LIBCXXABI. Are they used to indicate that if the C/C++
>> compiler for building LLVM is clang, and if this clang has support for
>> libc++ with libc++Abi, that both lib++ and libc++abi should be used
>> (instead of linking to the default, which at least on Linux is libstdc++)?
>> If not, what is the canonical way of building LLVM with libc++? I am asking
>> since I seem to have problems linking libLLVMSupport.a to a program while
>> using clang with libc++, whereas the used clang/LLVM compiler was
>> originally built with clang and libstdc++.
>>
>> Hi Michael,
>>
>> Just cloning the libc++ / libc++abi / libunwind repositories in
>> llvm/projects (or symlinking them there) is enough for CMake to pick
>> it up, build and test on check-all. The libraries will be available on
>> build/lib.
>>
>>
>> > Related to the first question: do the above flags also make clang use
>> libc++ by default, or is this configurable via a different set of flags?
>>
>> No. I believe this is an old flag when that wasn't enable by default
>> (and now it's used to disable it if you have the dirs). But maybe
>> heavier users of libc++ can help you there.
>>
>> You still have to use LDFLAGS="--stdlib=libc++ -lc++abi -lunwind" and,
>> if you're not installing it anywhere (but using the build directory),
>> add LD_LIBRARY_PATH=build/lib to your environment.
>>
>> If you run into inconsistencies because libgcc_s / libgcc_eh are
>> included, or if libstdc++ is included as well, you may have to use
>> -nostdlibc++ to clean up the internal options, and then use --stdlib
>> and friends to create a new set.
>>
>> It is confusing, but we don't want to make those libraries the default
>> on any environment that might still need/use libstdc++/libgcc by
>> default. This is more of a system-wide decision. For testing purposes,
>> though, it works well.
>>
>> cheers,
>> --renato
>>
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>>
>
>
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