[cfe-users] Using llvm-ar & llvm-ranlib vs ar & ranlib for creating static libraries when thinLTO is enabled.

[Teresa Johnson via cfe-users]

I believe if you install LLVMgold.so into /usr/lib/bfd-plugins, the default
ar and ranlib should pick it up without --plugin. See the info under Usage
at https://llvm.org/docs/GoldPlugin.html.
Teresa

On Mon, Aug 13, 2018 at 8:46 AM Teresa Johnson <tejohnson at google.com> wrote:

> That's correct, we recommend use of the llvm tools for working with
> bitcode. Note, however, that the GNU ar and ranlib do accept a plugin via
> --plugin, so you could try using those with the LLVM gold plugin to work
> with bitcode. I personally use the llvm-ar/llvm-ranlib however.
> Teresa
>
> On Mon, Aug 13, 2018 at 8:38 AM David Blaikie <dblaikie at gmail.com> wrote:
>
>> (Teresa, perhaps you can correct me if I'm wrong here)
>>
>> On Thu, Aug 9, 2018 at 2:21 PM Mateusz Zych via cfe-users <
>> cfe-users at lists.llvm.org> wrote:
>>
>>> Hi :)
>>>
>>> I am trying to compile simple project consisting of an executable "app"
>>> and static library "bar".
>>> Note that I'm enabling thinLTO for both "app" and "bar".
>>>
>>> ////////////////////////////////////////
>>> //               bar.h                //
>>> ////////////////////////////////////////
>>> #ifndef BAR
>>> #define BAR
>>>
>>> void bar ();
>>>
>>> #endif
>>> ////////////////////////////////////////
>>>
>>> ////////////////////////////////////////
>>> //              bar.cpp               //
>>> ////////////////////////////////////////
>>> #include "bar.h"
>>> #include <iostream>
>>>
>>> void bar ()
>>> {
>>>     std::cout << "Bar" << std::endl;
>>> }
>>> ////////////////////////////////////////
>>>
>>> ////////////////////////////////////////
>>> //              app.cpp               //
>>> ////////////////////////////////////////
>>> #include "bar.h"
>>> #include <iostream>
>>>
>>> int main()
>>> {
>>> std::cout << "App" << std::endl;
>>> bar();
>>> }
>>> ////////////////////////////////////////
>>>
>>>
>>> When I'm using llvm-ar and llvm-ranlib to create static library
>>> everything works fine:
>>>
>>> $ clang++ -flto=thin -c bar.cpp -o bar.o
>>> $ llvm-ar cr bar.a bar.o
>>> $ llvm-ranlib bar.a
>>> $ clang++ -flto=thin -c app.cpp -o app.o
>>> $ clang++ -flto=thin -o app app.o bar.a
>>>
>>>
>>> However using default ar and ranlib provided by my GNU/Linux
>>> distribution results in linking failure:
>>>
>>> $ clang++ -flto=thin -c bar.cpp -o bar.o
>>> $ ar cr bar.a bar.o
>>> $ ranlib bar.a
>>> $ clang++ -flto=thin -c app.cpp -o app.o
>>> $ clang++ -flto=thin -o app app.o bar.a
>>> bar.a: *error adding symbols: Archive has no index; run ranlib to add
>>> one*
>>> clang: error: linker command failed with exit code 1 (use -v to see
>>> invocation)
>>>
>>>
>>> Note that for GCC using default ar and ranlib works fine:
>>>
>>> $ g++ -flto -c bar.cpp -o bar.o
>>> $ ar cr bar.a bar.o
>>> $ ranlib bar.a
>>> $ g++ -flto -c app.cpp -o app.o
>>> $ g++ -flto -o app app.o bar.a
>>>
>>>
>>> Obviously using gcc-ar and gcc-ranlib also works fine:
>>>
>>> $ g++ -flto -c bar.cpp -o bar.o
>>> $ gcc-ar cr bar.a bar.o
>>> $ gcc-ranlib bar.a
>>> $ g++ -flto -c app.cpp -o app.o
>>> $ g++ -flto -o app app.o bar.a
>>>
>>>
>>> My question is: *Can I use ar and ranlib provided by GNU/Linux with
>>> clang++?*
>>>
>>
>> Not when using LTO, so far as I know. Otherwise, yes. (no idea if
>> llvm-ar/llvm-ranlib are totally general purpose replacements for those
>> tools, but I guess they are roughly/generally so)
>>
>>
>>>
>>> If not, then to properly setup clang as a default compiler on GNU/Linux
>>> machine,
>>> I should not only run update-alternatives for cc and c++ commands,
>>> but also somehow do that for ar and ranlib commands?
>>>
>>> $ sudo update-alternatives --config cc
>>> $ sudo update-alternatives --config c++
>>>
>>>
>>> That's very problematic since update-alternatives doesn't work with ar
>>> and ranlib:
>>>
>>> $ sudo update-alternatives --config ar
>>> update-alternatives: error: no alternatives for ar
>>> $ sudo update-alternatives --config ranlib
>>> update-alternatives: error: no alternatives for ranlib
>>>
>>>
>> Looks like you can teach update-alternatives to support new commands with
>> the --install option?
>>
>>
>>>
>>> If default ar and ranlib is expected to not work with clang++,
>>> then how should I setup Clang as a default compiler on GNU/Linux?
>>>
>>> Thanks, Mateusz Zych
>>> _______________________________________________
>>> cfe-users mailing list
>>> cfe-users at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
>>>
>>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>  408-460-2413
>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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