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

[Mateusz Zych via cfe-users]

Hi Teresa, David, thanks for your reply!

Right, installing *llvm-dev* package (
https://packages.ubuntu.com/bionic/llvm-dev)

   - $ sudo apt install llvm-dev

and adding *LLVMgold.so* symbolic link to /usr/lib/bfd-plugins worked
great. 😀

   - $ sudo ln -s /usr/lib/LLVMgold.so /usr/lib/bfd-plugins/LLVMgold.so

I'm wondering if adding this symbolic link and LLVMgold.so
should be part of installation of Clang on GNU/Linux (Ubuntu in my case)?
I mean I was expecting Clang to work no problem after installing just clang
package.

Best regards, Mateusz Zych

2018-08-13 18:52 GMT+02:00 Teresa Johnson <tejohnson at google.com>:

> 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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