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

Wed Jul 22 13:10:38 PDT 2020

Looks like your static library is not even pulled into the link command so the static library is not even in the snapshot. From the link command in the snapshot, the static library is not on the command line from snapshot:

/Applications/Xcode-11.3.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld -Z -demangle -object_path_lto /var/folders/5c/85r7gp0909j5jbytzds1j7b40000gn/T/thinlto-bf51d2 -lto_library data_files/libLTO.dylib -no_deduplicate -dynamic -arch x86_64 -macosx_version_min 10.15.0 -debug_snapshot -filelist objects/filelist dylibs/libSystem.tbd dylibs/libc++.tbd

Steven

> On Jul 22, 2020, at 11:42 AM, Tobias Hieta <tobias at plexapp.com> wrote:
> 
> Hello,
> 
> Thanks for the reply Teresa and Steven,
> 
> I uploaded the snapshot to GDrive: https://drive.google.com/file/d/11Ngst9FOnVL4fWYlKalzoFXL--B1SaOb/view?usp=sharing <https://drive.google.com/file/d/11Ngst9FOnVL4fWYlKalzoFXL--B1SaOb/view?usp=sharing>
> 
> My ld version is:
> 
> ➜ ld -v
> @(#)PROGRAM:ld  PROJECT:ld64-530
> BUILD 18:57:17 Dec 13 2019
> configured to support archs: armv6 armv7 armv7s arm64 arm64e arm64_32 i386 x86_64 x86_64h armv6m armv7k armv7m armv7em
> LTO support using: LLVM version 11.0.0, (clang-1100.0.33.17) (static support for 23, runtime is 23)
> TAPI support using: Apple TAPI version 11.0.0 (tapi-1100.0.11)
> 
> Since that said it was using LTO from Xcode I thought that might be the problem - but when I run clang with -v to see the linker invocation it seems to pass the correct libLTO.dylib:
>  
> "/usr/bin/ld" -demangle -object_path_lto /var/folders/5c/85r7gp0909j5jbytzds1j7b40000gn/T/thinlto-a8ae97 -lto_library /Users/tobias/tmp/clang+llvm-10.0.1-x86_64-apple-darwin/lib/libLTO.dylib -no_deduplicate -dynamic -arch x86_64 -platform_version macos 10.15.0 10.15 -syslibroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -o test /var/folders/5c/85r7gp0909j5jbytzds1j7b40000gn/T/test-d0eb14.o src/.libs/libogg.a -debug_snapshot -lc++ -lSystem /Users/tobias/tmp/clang+llvm-10.0.1-x86_64-apple-darwin/lib/clang/10.0.1/lib/darwin/libclang_rt.osx.a
> 
> Thanks again,
> Tobias
> 
> On Wed, Jul 22, 2020 at 7:20 PM Steven Wu <stevenwu at apple.com <mailto:stevenwu at apple.com>> wrote:
> This is usually a problem that is not using llvm-ar. I cannot reproduce this problem with either llvm 10.0 or TOT version. Which linker version are you using? You can also try pass "-Wl,-debug_snapshot" to the command where the error produces and then locate the "*.ld-snapshot" in /tmp directory and attach that as a reproducer.
> 
> Steven
> 
>> On Jul 22, 2020, at 8:41 AM, Teresa Johnson <tejohnson at google.com <mailto:tejohnson at google.com>> wrote:
>> 
>> Adding Steven Wu who can hopefully help as this is MacOS. Unfortunately, I don't have a way to run the MacOS compiler myself, and it uses the old LTO API which I am less familiar with. Typically these issues happen if you don't use llvm-ar, but it looks like you are using that so I'm not sure.
>> 
>> Teresa
>> 
>> On Wed, Jul 22, 2020 at 12:29 AM Tobias Hieta <tobias at plexapp.com <mailto:tobias at plexapp.com>> wrote:
>> 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 <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 <mailto: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 <mailto: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 <mailto:llvm-dev at lists.llvm.org>
>> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>> 
>> 
>> -- 
>> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com <mailto:tejohnson at google.com> |
> 

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