gcc 16.x speculative devirtualization is problematic (ArchLinux 2026.07.01, Fedora 44...)

[m0rb]

gcc's -fdevirtualize-speculatively is on by default with optimization levels >= 2 and generates direct references to vtables of derived types

Fedora 44 and Arch bork almost exactly the same way:

[673/678] Linking CXX executable /home/morb/build/Prey2006/output/linux/prey06
FAILED: [code=1] /home/morb/build/Prey2006/output/linux/prey06
: && /usr/bin/c++ -Werror=dangling-reference -Woverloaded-virtual -Wno-class-memaccess -Wno-c++20-compat -O2 -g -DNDEBUG -g -O3 -ffp-contract=off -fomit-frame-pointer -fno-math-errno -fno-trapping-math -Wl,--dependency-file=CMakeFiles/prey06.dir/link.d CMakeFiles/prey06.dir/aas/AASFile.cpp.o CMakeFiles/prey06.dir/aas/AASFile_optimize.cpp.o CMakeFiles/prey06.dir/aas/AASFile_sample.cpp.o CMakeFiles/prey06.dir/aas/AASFileManager.cpp.o CMakeFiles/prey06.dir/renderer/Cinematic.cpp.o CMakeFiles/prey06.dir/renderer/DeviceContext.cpp.o CMakeFiles/prey06.dir/renderer/GuiModel.cpp.o CMakeFiles/prey06.dir/renderer/Image_files.cpp.o CMakeFiles/prey06.dir/renderer/Image_init.cpp.o CMakeFiles/prey06.dir/renderer/Image_load.cpp.o CMakeFiles/prey06.dir/renderer/Image_process.cpp.o CMakeFiles/prey06.dir/renderer/Image_program.cpp.o CMakeFiles/prey06.dir/renderer/Interaction.cpp.o CMakeFiles/prey06.dir/renderer/Material.cpp.o CMakeFiles/prey06.dir/renderer/MegaTexture.cpp.o 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CMakeFiles/prey06.dir/tools/imgui/materialeditor/MaterialPropTreeView.cpp.o CMakeFiles/prey06.dir/tools/imgui/materialeditor/MaterialPreviewPropView.cpp.o CMakeFiles/prey06.dir/tools/imgui/materialeditor/MaterialPreviewView.cpp.o CMakeFiles/prey06.dir/tools/imgui/materialeditor/StageView.cpp.o CMakeFiles/prey06.dir/tools/imgui/materialeditor/MEOptions.cpp.o CMakeFiles/prey06.dir/tools/imgui/materialeditor/MEMainFrame.cpp.o CMakeFiles/prey06.dir/tools/imgui/materialeditor/FindDialog.cpp.o CMakeFiles/prey06.dir/tools/edit_stub.cpp.o CMakeFiles/prey06.dir/tools/debugger/DebuggerBreakpoint.cpp.o CMakeFiles/prey06.dir/tools/debugger/DebuggerServer.cpp.o CMakeFiles/prey06.dir/tools/debugger/DebuggerScript.cpp.o CMakeFiles/prey06.dir/tools/debugger/debugger.cpp.o -o /home/morb/build/Prey2006/output/linux/prey06  libidlib.a  /usr/lib64/libopenal.so  -lSDL2  -ldl  -lrt  -lgcc_s_asneeded  -lgcc_s_asneeded && :
/usr/bin/ld.bfd: CMakeFiles/prey06.dir/renderer/Material.cpp.o: in function `hhProfilerGraph::~hhProfilerGraph()':
/home/morb/build/Prey2006/neo/renderer/Cinematic.h:119:(.text+0x404): undefined reference to `vtable for hhProfilerGraph'
/usr/bin/ld.bfd: /home/morb/build/Prey2006/neo/renderer/Cinematic.h:119:(.text+0x4114): undefined reference to `vtable for hhProfilerGraph'
/usr/bin/ld.bfd: /home/morb/build/Prey2006/neo/renderer/Cinematic.h:119:(.text._ZN15hhProfilerGraphD0Ev[_ZN15hhProfilerGraphD5Ev]+0x7): undefined reference to `vtable for hhProfilerGraph'
/usr/bin/ld.bfd: CMakeFiles/prey06.dir/renderer/Material.cpp.o: in function `hhProfilerGraph::~hhProfilerGraph()':
/home/morb/build/Prey2006/neo/renderer/Cinematic.h:119:(.text._ZN15hhProfilerGraphD2Ev[_ZN15hhProfilerGraphD5Ev]+0x3): undefined reference to `vtable for hhProfilerGraph'
/usr/bin/ld.bfd: CMakeFiles/prey06.dir/renderer/RenderSystem_init.cpp.o: in function `hhProfilerGraph::~hhProfilerGraph()':
/home/morb/build/Prey2006/neo/renderer/Cinematic.h:119:(.text+0x1333): undefined reference to `vtable for hhProfilerGraph'
/usr/bin/ld.bfd: CMakeFiles/prey06.dir/renderer/RenderSystem_init.cpp.o:/home/morb/build/Prey2006/neo/renderer/Cinematic.h:119: more undefined references to `vtable for hhProfilerGraph' follow
collect2: error: ld returned 1 exit status
[676/678] Building CXX object CMakeFiles/prey06ded.dir/ui/Window.cpp.o
ninja: build stopped: subcommand failed.

A current workaround is for users to add -fno-devirtualize-speculatively to their CXXFLAGS (i.e. cmake -B build neo -DCMAKE_CXX_FLAGS="-fno-devirtualize-speculatively")

Eventually this will require attention as more users out there will have newer compiler suites / toolchains with aggressive optimizations.