llvm dev - Jul 2020 - How to debug a missing symbol with ThinLTO?

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

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

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