[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 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,-debugsnapshot" 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 #include "ogg/ogg.h" using namespace std; int main() { oggstreamstate os; if (oggstreaminit(&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: oggstreaminit_ _Undefined symbols for architecture x8664:_ _"oggstreaminit", referenced from:_ _main in 0.x8664.thinlto.o_ _ld: symbol(s) not found for architecture x8664_ _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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