llvm dev - Jul 2020 - How to debug a missing symbol with ThinLTO?

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
<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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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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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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