[llvm-dev] How to debug a missing symbol with ThinLTO? (original) (raw)

Tobias Hieta via llvm-dev llvm-dev at lists.llvm.org
Wed Jul 22 00:28:54 PDT 2020

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 #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: oggstreaminit > Undefined symbols for architecture x8664: > "oggstreaminit", 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 oggstreaminit > > working archive: > 0000000000000200 T oggstreaminit > > 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 oggstreaminit(oggstreamstate *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

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