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

Wed Jul 22 11:42:56 PDT 2020

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

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> 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> 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> 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
>> 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
>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>
>
>
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