[llvm-dev] How to debug a missing symbol with ThinLTO?

[Tobias Hieta via llvm-dev]

David,

Thanks for looking into this. I did a small reproduction on my machine
outside of my build system. So here is how to reproduce:

Download https://downloads.xiph.org/releases/ogg/libogg-1.3.4.tar.xz
Download llvm-10.0.1 macOS binary

export PATH=<path to llvm/bin>:$PATH
export SDKROOT=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk

untar libogg
AR=llvm-ar CC=clang CXX=clang++ CFLAGS="-flto=thin
-mmacosx-version-min=10.9" LDFLAGS=-flto=thin ./configure
--disable-shared --enable-static
make -j20

Then try to link to the library in a small C++ program - I used this:

#include <iostream>
#include "ogg/ogg.h"

using namespace std;

int main()
{
ogg_stream_state os;
if (ogg_stream_init(&os, 123) == 0)
cout << "Initialized stream succesfully" << endl;

return 0;
}

And from the libogg directory I linked to it like this:

clang++ -o test -flto=thin test.cpp src/.libs/libogg.a -I include

undef: _ogg_stream_init
Undefined symbols for architecture x86_64:
"_ogg_stream_init", referenced from:
_main in 0.x86_64.thinlto.o
ld: symbol(s) not found for architecture x86_64
clang-10: error: linker command failed with exit code 1 (use -v to see
invocation)

hope this helps - thanks!

On Wed, Jul 22, 2020 at 9:11 AM David Blaikie <dblaikie at gmail.com> wrote:
>
> Got a link to the source/build instructions?
>
> This sort of thing happens more often in C++ with templates where one
> object depends (incorrectly) on an implicit instantiation created in
> another object, rather than carrying its own instantiation.
>
> Not sure what might cause it in C code.
>
> On Tue, Jul 21, 2020 at 11:47 PM Tobias Hieta via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hello,
> >
> > I am building libogg with clang (10.0.1) on macOS and if I pass
> > "-flto=thin" to C and LDFLAGS it will not link correctly claiming
> > missing symbols when linking to the archive (libogg.a).
> >
> > undef: _ogg_stream_init
> > Undefined symbols for architecture x86_64:
> > "_ogg_stream_init", referenced from:
> > _main in lto.o
> >
> > Removing lto=thin fixes the problem. Inspecting the AR libs with
> > llvm-nm I see the symbol there (but without address):
> >
> > not working archive:
> > ---------------- T _ogg_stream_init
> >
> > working archive:
> > 0000000000000200 T _ogg_stream_init
> >
> > My guess is that this output is correct since the archive contains
> > bitcode in the thin lto case and otherwise it's the finished object.
> >
> > It seems to me that the LTO decides to not include this symbol? It's
> > defined like this:
> >
> > extern int ogg_stream_init(ogg_stream_state *os,int serialno);
> >
> > llvm-ar is used to create the archive.
> >
> > Is there any good way to debug this?
> >
> > Thanks,
> > Tobias
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev