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

[David Blaikie via cfe-users]

(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
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