[LLVMdev] Question about usage of LLVMLinkModules()

[Duncan P. N. Exon Smith]

> On 2015-Jun-11, at 12:30, Lefteris Karapetsas <lefteris at refu.co> wrote:
> 
> Hello Duncan,
> 
> I have only been experimenting with LLVM, never used the develop version and I initially joined the mailing list hoping to get some help with my particular issue. But it would be my pleasure to help root out this potential bug. I have not tried with the latest llvm trunk but `llvm-link` succesfully gives an error when trying to link the 2 modules together.
> 
> llvm-link llvm_module1.ll llvm_module2.ll
> ERROR: Linking globals named 'gstr_706834940': symbol multiply defined!
> 
> It must be an error in the C api wrapper. I am not familiar enough with the codebase to spot it fast though. As I said above the LLVM version is 3.6.1.
> 
> The C-wrapper of LLVMLinkModules seem to be using a DiagnosticInfo Handler to handle errors. The error is omitted in the core of LinkModules.cpp but for some reason does not appear outside when consumed by the C-API.

Yeah, definitely sounds like a bug in the LLVM C API then.  Can
you reproduce this on trunk?

It'd be great if you could file a PR, and attach the bitcode for
your modules.

> Lefteris(Eleftherios) Karapetsas
> Click here for my CV/portfolio website
> Also maintain a blog on various topics, mostly engineering
> 
> 
> On Thu, Jun 11, 2015 at 8:11 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> > On 2015-Jun-11, at 10:54, Lefteris Karapetsas <lefteris at refu.co> wrote:
> >
> > Hello Duncan,
> >
> > Thank you for your reply. I did figure it out in the end. It was a linkage conflict between the modules. There were some global types that I was declaring in both modules. After removing them, linking worked perfectly.
> >
> > Still it's a mystery why I got no error string pointing me to the reason the linking failed in the first place.
> 
> Does it (the missing error message) reproduce on trunk?
> 
> Does the error message get skipped just with the C API, or does
> something funny also happen for `llvm-link`?
> 
> I'm glad you sorted out your problem :), but that missing error
> message sounds like a real bug that it would be nice to root out.
> 
> 
> 
> >
> > Best Regards,
> >
> > Lefteris(Eleftherios) Karapetsas
> > Click here for my CV/portfolio website
> > Also maintain a blog on various topics, mostly engineering
> >
> >
> > On Thu, Jun 11, 2015 at 6:41 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> >
> > > On 2015-Jun-08, at 07:11, Lefteris Karapetsas <lefteris at refu.co> wrote:
> > >
> > > Hello all,
> > >
> > > I am using llvm as a backend to a hobby language I am making.  I have experimented with the C bindings since the project is written in C. I had no problems building a single lllvm module so far. I am not using a JIT compiler but at least at the moment am simply generating executables. The workflow I am using is to build a module with LLVMModuleCreateWithName() and populate it with the LLVMBuilder.
> > >
> > > After everything is in the module I simply output it into a file with LLVMPrintModuleToFile(). Then I am calling llc to convert the IR  to native machine code and finally gcc to create an executable out of it. All these from inside my program. It may not be the best way to generate an executable and I would really welcome as much advice as you can give me on that but it works.
> > >
> > > The issue comes when I actually create 2 different modules. One being a "standard" library of sorts and the other the main module which should use it. I create both modules, with no errors appearing if I use LLVMVerifyModule() on them.
> > >
> > > The problem is how do I link them? I tried to do it like this:
> > >
> > >             // if an error occurs LLVMLinkModules() returns true
> > >             if (true == LLVMLinkModules(main_module, stdlib_module, LLVMLinkerDestroySource, &error)) {
> > >                 bllvm_error("Could not link LLVM modules", error);
> > >                 goto end;
> > >             }
> > >
> > >
> > > But this fails (returns true), with no error. Even though both modules are valid. I understand that the above may be a really naive mistake, and that my whole approach may be wrong. This is why I would appreciate any links to tutorials/documents which would show the proper way to link 2 modules or some good old simple advice tips from any of you guys.
> >
> > Weird that you're not getting an error message.
> >
> > Best thing is probably to spin up a debugger (or add some printfs)
> > and see what's going on.
> >
> > One hypothetical point of failure: module linking requires that both
> > modules are in the same context.  If that's not the problem (and it
> > probably isn't, since I'd expect crashes there), my bet would be on
> > some sort of linkage conflict between the modules.
> >
> 
>