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Type: Bug
Resolution: Won't Do
Priority: P2: Important
Fix Version/s: None
Affects Version/s: 6.9.2
Component/s: Build System: CMake
Labels:
- Reported_by_support_standard
- cmake_upstream
Environment:
Windows 10 22H2 host; pre-built Qt for WASM kit

Platform/s:

WebAssembly
Commits:
897034bc9 (dev), d6d832d0d (dev), e7680f055 (6.10), 7e2f198f1 (tqtc/lts-6.8), 531c49d30 (6.10), 0677b635a (tqtc/lts-6.8)

Code
CMakeLists.txt

cmake_minimum_required(VERSION 3.16)
project(MyProject)

find_package(Qt6 COMPONENTS Core REQUIRED)
qt_add_executable(MyProject main.cpp)

target_link_libraries(MyProject PRIVATE Qt6::Core)
target_link_options(MyProject PRIVATE -sMAIN_MODULE=1)

main.cpp

#include <QCoreApplication>

int main(int argc, char* argv[])
{
    QCoreApplication app(argc, argv);
    return app.exec();
}

Outcomes

When built in Release mode, the following command is used:

C:\emscripten-core\emsdk\upstream\emscripten\em++.bat -O3 -DNDEBUG -sMAIN_MODULE=1 -s MODULARIZE=1 -s EXPORT_NAME=MyProject_entry -s EXPORTED_RUNTIME_METHODS=UTF16ToString,stringToUTF16,JSEvents,specialHTMLTargets,FS,callMain -s INITIAL_MEMORY=50MB -s MAXIMUM_MEMORY=4GB -s MAX_WEBGL_VERSION=2 -s WASM_BIGINT=1 -s STACK_SIZE=5MB -s ALLOW_MEMORY_GROWTH -sERROR_ON_UNDEFINED_SYMBOLS=1 -sFETCH CMakeFiles/MyProject.dir/main.cpp.o -o MyProject.js  C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a  C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a  -lembind  C:/Qt/6.9.2/wasm_singlethread/lib/libQt6BundledZLIB.a  C:/Qt/6.9.2/wasm_singlethread/lib/libQt6BundledPcre2.a

Notice that C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a appears twice.

Problems
Normally, without -sMAIN_MODULE=1, the duplication doesn't cause any problems. However, a customer wanted their Qt app to be dlopen()'ed by a larger WASM app so they added the linker flag. Unfortunately, this caused duplicated symbol errors:

wasm-ld: error: duplicate symbol: qDetectCpuFeatures
>>> defined in C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a(qsimd.cpp.o)
>>> defined in C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a(qsimd.cpp.o)

wasm-ld: error: duplicate symbol: qt_cpu_features
>>> defined in C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a(qsimd.cpp.o)
>>> defined in C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a(qsimd.cpp.o)

wasm-ld: error: duplicate symbol: qDumpCPUFeatures()
>>> defined in C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a(qsimd.cpp.o)
>>> defined in C:/Qt/6.9.2/wasm_singlethread/lib/libQt6Core.a(qsimd.cpp.o)

...

Workarounds

Manually run the command above with the duplicated entry removed. This causes the build to complete successfully.

Alternatively, using qmake seems to work too (QMAKE_LFLAGS += -sMAIN_MODULE=1)