mold behavior linking c++20 differs from c++17 for the same project

[marekr]

So recently I've migrated Kicad from C++17 to C++20. And I have it compiling under MSVC, Clang (with lld) on both Linux and macOS, and gcc with gold on Linux.

But gcc+mold and clang+mold are running into a linker error that is unique to the usage of mold.

[build] mold: error: undefined symbol: Kiface()
[build] >>> referenced by eda_draw_frame.cpp
[build] >>>               common/libcommon.a(eda_draw_frame.cpp.o):(EDA_DRAW_FRAME::loadCanvasTypeSetting(APP_SETTINGS_BASE*))>>> referenced by eda_draw_frame.cpp
[build] >>>               common/libcommon.a(eda_draw_frame.cpp.o):(EDA_DRAW_FRAME::saveCanvasTypeSetting(EDA_DRAW_PANEL_GAL::GAL_TYPE))>>> referenced by eda_base_frame.cpp
[build] >>>               common/libcommon.a(eda_base_frame.cpp.o):(EDA_BASE_FRAME::config() const)>>> referenced 3 more times
[build] 

The basic summary in our mess is

libcommon.a references a extern called
extern KIFACE_BASE& Kiface();

It is used by a few objects built as part of libcommon.a. The definition of KIFACE_BASE also exists in libcommon.a

The actual Kiface() function only exists in another static library called gerbview_kiface_objects which the gerbview executable does not link to because there's a gerbview_kiface that instead uses it. In fact none of objects that gerbview is normally looking to link from libcommon.a should be referencing Kiface().

The oddity is this only happens with mold being used to link C++20, changing the project back to C++17 by flipping the std definition makes it work.

I installed the latest mold:

$ mold -v
mold 2.30.0 (c7f6a91da512ef8a2634a1bef5d4ba82104659fe; compatible with GNU ld)

Is there a C++ standard related change that would affect specifically affect only mold?

[rui314]

Can I reproduce the issue just by building kicad with gcc and mold? What is you gcc version?

[rui314]

Please try again with dfa26d0.

[marekr]

That seems to fix it, thanks

[rui314]

I had to revert it back to the original code in a3b4192 because of regressions.

Could you make a change to your build file so that libcommon.a will not be passed to the linker when linking these particular programs? Passing an archive file that shouldn't be used and let the linker to ignore that file is a bit fragile; unless you are stress-testing linker compatibility, you don't want to do that.

[marekr]

Passing an archive file that shouldn't be used and let the linker to ignore that file is a bit fragile; unless you are stress-testing linker compatibility, you don't want to do that.

I already made that change the other night to some of the targets where common.a became no longer necessary due to other refactoring. There's one target remaining that actually has not only bad link dependencies but improper code added that drove the dependency so it's taking me longer to figure out how to unwind that mess.

But why does C++17 vs. C++20 break it?

[rui314]

I don't know why, but some C++ std symbols are in libcommon.a only when compiled as C++20 as follows:

user:/build/kicad/build/c20$ mold @rsp -y _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
trace-symbol: /usr/lib/x86_64-linux-gnu/libwx_gtk3u_stc-3.2.so: reference to _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
trace-symbol: /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.so: definition of _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
trace-symbol: common/libcommon.a(PS_plotter.cpp.o): definition of _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
trace-symbol: common/libcommon.a(eda_draw_frame.cpp.o): definition of _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
trace-symbol: /usr/lib/x86_64-linux-gnu/libwx_gtk3u_stc-3.2.so keeps common/libcommon.a(PS_plotter.cpp.o) for _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
trace-symbol: libs/kimath/libkimath.a(md5_hash.cpp.o): definition of _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc

user:/build/kicad/build/c17$ mold @rsp -y _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
trace-symbol: common/libkicommon.so.8.99.0: reference to _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
trace-symbol: /usr/lib/gcc/x86_64-linux-gnu/13/libstdc++.so: definition of _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc

user:/build/kicad-c17/build/c17$ c++filt _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE9push_backEc
std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::push_back(char)

[dsa-t]

Is there another way to make mold work with C++20 and KiCad?

If idf2vrml/dxf2idf are linked to common, there's some code in common expecting Kiface(), but Kiface() it's not implemented in idf2vrml/dxf2idf.

[797/939  81% :: 131.712] Linking CXX executable utils/idftools/idf2vrml
FAILED: utils/idftools/idf2vrml 
: && /usr/bin/clang++-14 -fstandalone-debug -pthread -Wall -Wsuggest-override -Winconsistent-missing-override -Werror=vla -Wimplicit-fallthrough -Werror=return-type -Wshadow -Wsign-compare -Wmissing-field-initializers -Wempty-body -Wreorder -Wmismatched-tags -Wpessimizing-move -Wredundant-move -Wno-psabi -g3 -ggdb3 -fuse-ld=mold utils/idftools/CMakeFiles/idf2vrml.dir/idf2vrml.cpp.o -o utils/idftools/idf2vrml -L/usr/local/lib -Wl,-rpath,/usr/local/lib:<kicad>/build/Debug/common/gal:<kicad>/build/Debug/common:<kicad>/build/Debug/api:  utils/idftools/libidf3.a  libs/kimath/libkimath.a  common/libcommon.a  /usr/lib/x86_64-linux-gnu/libGL.so  /usr/lib/x86_64-linux-gnu/libGLU.so  -L/usr/local/lib  -pthread  -lwx_gtk3u_gl-3.2  -lwx_gtk3u_aui-3.2  -lwx_gtk3u_html-3.2  -lwx_gtk3u_core-3.2  -lwx_baseu_net-3.2  -lwx_baseu-3.2  -lwx_gtk3u_propgrid-3.2  -lwx_baseu_xml-3.2  -lwx_gtk3u_stc-3.2  -lwx_gtk3u_richtext-3.2  scripting/libscripting.a  common/libcommon.a  scripting/libscripting.a  thirdparty/libcontext/liblibcontext.a  common/gal/libkigal.so.8.99.0  /usr/lib/x86_64-linux-gnu/libGLEW.so  /usr/lib/x86_64-linux-gnu/libcairo.so  /usr/lib/x86_64-linux-gnu/libpixman-1.so  /usr/lib/x86_64-linux-gnu/libGL.so  /usr/lib/x86_64-linux-gnu/libGLU.so  thirdparty/nanosvg/libnanosvg.a  thirdparty/dxflib_qcad/libdxflib_qcad.a  thirdparty/tinyspline_lib/libtinyspline_lib.a  thirdparty/nanodbc/libnanodbc.a  -lodbc  /usr/lib/x86_64-linux-gnu/libboost_locale.so.1.74.0  /usr/lib/x86_64-linux-gnu/libboost_chrono.so.1.74.0  /usr/lib/x86_64-linux-gnu/libboost_system.so.1.74.0  /usr/lib/x86_64-linux-gnu/libboost_thread.so.1.74.0  /usr/lib/x86_64-linux-gnu/libboost_atomic.so.1.74.0  common/libkicommon.so.8.99.0  libs/kimath/libkimath.a  thirdparty/clipper2/libclipper2.a  -lm  libs/kiplatform/libkiplatform.a  libs/core/libcore.a  -L/usr/local/lib  -pthread  -lwx_gtk3u_gl-3.2  -lwx_gtk3u_aui-3.2  -lwx_gtk3u_html-3.2  -lwx_gtk3u_core-3.2  -lwx_baseu_net-3.2  -lwx_baseu-3.2  -lwx_gtk3u_propgrid-3.2  -lwx_baseu_xml-3.2  -lwx_gtk3u_stc-3.2  -lwx_gtk3u_richtext-3.2  -lgtk-3  -lgdk-3  -lpangocairo-1.0  -lpango-1.0  -lharfbuzz  -latk-1.0  -lcairo-gobject  -lcairo  -lgdk_pixbuf-2.0  -lgio-2.0  -lgobject-2.0  -lglib-2.0  -lsecret-1  -lgio-2.0  -lgobject-2.0  -lglib-2.0  -lsecret-1  thirdparty/fmt/libfmtd.a  api/libkiapi.so.8.99.0  /usr/lib/x86_64-linux-gnu/libprotobuf.so  /usr/lib/x86_64-linux-gnu/libcurl.so  /usr/lib/x86_64-linux-gnu/libgit2.so  /usr/lib/x86_64-linux-gnu/libfreetype.so  /usr/lib/x86_64-linux-gnu/libharfbuzz.so  /usr/lib/x86_64-linux-gnu/libfontconfig.so  /usr/lib/x86_64-linux-gnu/libpython3.10.so && :
mold: error: undefined symbol: Kiface()
>>> referenced by ds_data_model.cpp
>>>               common/libcommon.a(ds_data_model.cpp.o):(DS_DATA_MODEL::ResolvePath(wxString const&, wxString const&))>>> referenced by eda_base_frame.cpp
>>>               common/libcommon.a(eda_base_frame.cpp.o):(EDA_BASE_FRAME::config() const)>>> referenced by eda_base_frame.cpp
>>>               common/libcommon.a(eda_base_frame.cpp.o):(EDA_BASE_FRAME::sys_search())>>> referenced 3 more times

clang: error: linker command failed with exit code 1 (use -v to see invocation)
[797/939  81% :: 131.802] Linking CXX executable utils/idftools/dxf2idf
FAILED: utils/idftools/dxf2idf 
: && /usr/bin/clang++-14 -fstandalone-debug -pthread -Wall -Wsuggest-override -Winconsistent-missing-override -Werror=vla -Wimplicit-fallthrough -Werror=return-type -Wshadow -Wsign-compare -Wmissing-field-initializers -Wempty-body -Wreorder -Wmismatched-tags -Wpessimizing-move -Wredundant-move -Wno-psabi -g3 -ggdb3 -fuse-ld=mold utils/idftools/CMakeFiles/dxf2idf.dir/dxf2idfmain.cpp.o utils/idftools/CMakeFiles/dxf2idf.dir/dxf2idf.cpp.o -o utils/idftools/dxf2idf -L/usr/local/lib -Wl,-rpath,/usr/local/lib:<kicad>/build/Debug/common/gal:<kicad>/build/Debug/common:<kicad>/build/Debug/api:  thirdparty/dxflib_qcad/libdxflib_qcad.a  utils/idftools/libidf3.a  libs/kimath/libkimath.a  common/libcommon.a  /usr/lib/x86_64-linux-gnu/libGL.so  /usr/lib/x86_64-linux-gnu/libGLU.so  -L/usr/local/lib  -pthread  -lwx_gtk3u_gl-3.2  -lwx_gtk3u_aui-3.2  -lwx_gtk3u_html-3.2  -lwx_gtk3u_core-3.2  -lwx_baseu_net-3.2  -lwx_baseu-3.2  -lwx_gtk3u_propgrid-3.2  -lwx_baseu_xml-3.2  -lwx_gtk3u_stc-3.2  -lwx_gtk3u_richtext-3.2  scripting/libscripting.a  common/libcommon.a  scripting/libscripting.a  thirdparty/dxflib_qcad/libdxflib_qcad.a  thirdparty/libcontext/liblibcontext.a  common/gal/libkigal.so.8.99.0  /usr/lib/x86_64-linux-gnu/libGLEW.so  /usr/lib/x86_64-linux-gnu/libcairo.so  /usr/lib/x86_64-linux-gnu/libpixman-1.so  /usr/lib/x86_64-linux-gnu/libGL.so  /usr/lib/x86_64-linux-gnu/libGLU.so  thirdparty/nanosvg/libnanosvg.a  thirdparty/tinyspline_lib/libtinyspline_lib.a  thirdparty/nanodbc/libnanodbc.a  -lodbc  /usr/lib/x86_64-linux-gnu/libboost_locale.so.1.74.0  /usr/lib/x86_64-linux-gnu/libboost_chrono.so.1.74.0  /usr/lib/x86_64-linux-gnu/libboost_system.so.1.74.0  /usr/lib/x86_64-linux-gnu/libboost_thread.so.1.74.0  /usr/lib/x86_64-linux-gnu/libboost_atomic.so.1.74.0  common/libkicommon.so.8.99.0  libs/kimath/libkimath.a  thirdparty/clipper2/libclipper2.a  -lm  libs/kiplatform/libkiplatform.a  libs/core/libcore.a  -L/usr/local/lib  -pthread  -lwx_gtk3u_gl-3.2  -lwx_gtk3u_aui-3.2  -lwx_gtk3u_html-3.2  -lwx_gtk3u_core-3.2  -lwx_baseu_net-3.2  -lwx_baseu-3.2  -lwx_gtk3u_propgrid-3.2  -lwx_baseu_xml-3.2  -lwx_gtk3u_stc-3.2  -lwx_gtk3u_richtext-3.2  -lgtk-3  -lgdk-3  -lpangocairo-1.0  -lpango-1.0  -lharfbuzz  -latk-1.0  -lcairo-gobject  -lcairo  -lgdk_pixbuf-2.0  -lgio-2.0  -lgobject-2.0  -lglib-2.0  -lsecret-1  -lgio-2.0  -lgobject-2.0  -lglib-2.0  -lsecret-1  thirdparty/fmt/libfmtd.a  api/libkiapi.so.8.99.0  /usr/lib/x86_64-linux-gnu/libprotobuf.so  /usr/lib/x86_64-linux-gnu/libcurl.so  /usr/lib/x86_64-linux-gnu/libgit2.so  /usr/lib/x86_64-linux-gnu/libfreetype.so  /usr/lib/x86_64-linux-gnu/libharfbuzz.so  /usr/lib/x86_64-linux-gnu/libfontconfig.so  /usr/lib/x86_64-linux-gnu/libpython3.10.so && :
mold: error: undefined symbol: Kiface()
>>> referenced by ds_data_model.cpp
>>>               common/libcommon.a(ds_data_model.cpp.o):(DS_DATA_MODEL::ResolvePath(wxString const&, wxString const&))>>> referenced by eda_base_frame.cpp
>>>               common/libcommon.a(eda_base_frame.cpp.o):(EDA_BASE_FRAME::config() const)>>> referenced by eda_base_frame.cpp
>>>               common/libcommon.a(eda_base_frame.cpp.o):(EDA_BASE_FRAME::sys_search())>>> referenced 3 more times

clang: error: linker command failed with exit code 1 (use -v to see invocation)

[rui314]

That's exactly what we were discussing in the above thread, no? My understanding is that efforts are underway to fix it on the KiCad side.

[dsa-t]

Yes, we're still looking at ways to solve this in KiCad, but why does it work fine with other linkers in C++20 mode?

[rui314]

I don't know why it worked with C++17 and doesn't with C++20 (look at my last comment in the above thread for the symbol differences), but the thing is that the same set of symbols are defined by the .so and the .a, and it's somewhat arbitrary for the linker to pick up which one.

[ninetailedtori]

mold: error: undefined symbol: PCB_IO_EASYEDAPRO::PCB_IO_EASYEDAPRO()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans115.ltrans.o:(std::_Function_handler<PCB_IO* (), registerEasyEDAProPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&))
mold: error: undefined symbol: PCB_IO_CADSTAR_ARCHIVE::PCB_IO_CADSTAR_ARCHIVE()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans114.ltrans.o:(std::_Function_handler<PCB_IO* (), registerCadstarArchivePlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&) [clone .lto_priv.0])
mold: error: undefined symbol: PCB_IO_ALTIUM_CIRCUIT_MAKER::PCB_IO_ALTIUM_CIRCUIT_MAKER()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans114.ltrans.o:(std::_Function_handler<PCB_IO* (), registerAltiumCircuitMakerPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&) [clone .lto_priv.0])
mold: error: undefined symbol: vtable for PCB_IO_ODBPP
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans115.ltrans.o:(std::_Function_handler<PCB_IO* (), registerODBPPPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&))
mold: error: undefined symbol: PCB_IO_EASYEDA::PCB_IO_EASYEDA()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans115.ltrans.o:(std::_Function_handler<PCB_IO* (), registerEasyEDAPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&))
mold: error: undefined symbol: PCB_IO_ALTIUM_DESIGNER::PCB_IO_ALTIUM_DESIGNER()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans114.ltrans.o:(std::_Function_handler<PCB_IO* (), registerAltiumDesignerPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&) [clone .lto_priv.0])
mold: error: undefined symbol: PCB_IO_SOLIDWORKS::PCB_IO_SOLIDWORKS()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans115.ltrans.o:(std::_Function_handler<PCB_IO* (), registerSolidworksPCBPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&))
mold: error: undefined symbol: PCB_IO_FABMASTER::PCB_IO_FABMASTER()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans115.ltrans.o:(std::_Function_handler<PCB_IO* (), registerFabmasterPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&))
mold: error: undefined symbol: vtable for PCB_IO_IPC2581
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans115.ltrans.o:(std::_Function_handler<PCB_IO* (), registerIPC2581Plugin::{lambda()#1}>::_M_invoke(std::_Any_data const&))
mold: error: undefined symbol: PCB_IO_ALTIUM_CIRCUIT_STUDIO::PCB_IO_ALTIUM_CIRCUIT_STUDIO()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans114.ltrans.o:(std::_Function_handler<PCB_IO* (), registerAltiumCircuitStudioPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&) [clone .lto_priv.0])
mold: error: undefined symbol: PCB_IO_PCAD::PCB_IO_PCAD()
>>> referenced by <artificial>
>>>               /tmp/ccYtoaI1.ltrans115.ltrans.o:(std::_Function_handler<PCB_IO* (), registerPcadPlugin::{lambda()#1}>::_M_invoke(std::_Any_data const&))
collect2: error: ld returned 1 exit status
make[2]: *** [qa/tests/spice/CMakeFiles/qa_spice.dir/build.make:1096: qa/tests/spice/qa_spice] Error 1
make[1]: *** [CMakeFiles/Makefile2:8121: qa/tests/spice/CMakeFiles/qa_spice.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
/home/tori/.cache/yay/kicad-git/src/kicad-git/common/dialog_about/AboutDialog_main.cpp: In function ‘buildKicadAboutBanner’:
/home/tori/.cache/yay/kicad-git/src/kicad-git/common/dialog_about/AboutDialog_main.cpp:58:13: note: variable tracking size limit exceeded with ‘-fvar-tracking-assignments’, retrying without
   58 | static void buildKicadAboutBanner( EDA_BASE_FRAME* aParent, ABOUT_APP_INFO& aInfo )
      |             ^
[100%] Built target qa_pcbnew_tools
/home/tori/.cache/yay/kicad-git/src/kicad-git/common/dialog_about/AboutDialog_main.cpp: In function ‘buildKicadAboutBanner’:
/home/tori/.cache/yay/kicad-git/src/kicad-git/common/dialog_about/AboutDialog_main.cpp:58:13: note: variable tracking size limit exceeded with ‘-fvar-tracking-assignments’, retrying without
   58 | static void buildKicadAboutBanner( EDA_BASE_FRAME* aParent, ABOUT_APP_INFO& aInfo )
      |             ^
[100%] Built target qa_api
/home/tori/.cache/yay/kicad-git/src/kicad-git/common/dialog_about/AboutDialog_main.cpp: In function ‘buildKicadAboutBanner’:
/home/tori/.cache/yay/kicad-git/src/kicad-git/common/dialog_about/AboutDialog_main.cpp:58:13: note: variable tracking size limit exceeded with ‘-fvar-tracking-assignments’, retrying without
   58 | static void buildKicadAboutBanner( EDA_BASE_FRAME* aParent, ABOUT_APP_INFO& aInfo )
      |             ^
[100%] Built target qa_pcbnew
make: *** [Makefile:166: all] Error 2
==> ERROR: A failure occurred in build().
    Aborting...
 -> error making: kicad-git-exit status 4

Adding my case here as well.