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

Tobias Hieta via llvm-dev
Wed, 22 Jul 2020 06:47:14 -0000

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