[Interest] Qt 6.2.2 Webassembly - sign conversion errors

[Lorn Potter]

Hi,
Emscripten uses clang/LLVM and not gcc, if makes any difference to you.

Setting /etc/alternatives has no effect on emscripten as essentially we 
are cross compiling.

Perhaps you might try adding
  CONFIG += c++17
If you are using c++-17 ?

emcc -v

  $> emcc -v
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 
2.0.14
clang version 13.0.0 
(/opt/s/w/ir/cache/git/chromium.googlesource.com-external-github.com-llvm-llvm--project 
5f3c99085d4c2ebf57fd0586b013b02e32a8e20b)
Target: wasm32-unknown-emscripten
Thread model: posix


On 27/12/2021 9:56 PM, Axel Spoerl wrote:
> I can narrow down the issue as follows:
> 
> 
> The problem at hand occurs on a compile server, compiling desktop 
> executables for Qt 5.15.2 (with GCC-7), Qt 6.2.2 (with GCC-9). To make 
> both configurations work, QMAKE_CXX=g++-7 / QMAKE_CXX=g++-9 arguments 
> are added in the respective Qt Creator build steps (which is necessary 
> for Qt 6.2.2 even if g++-9 is specified as compiler in the kit itself).
> 
> 
> Emscripten (intended as a 3rd compile target) has been installed 
> according to https://doc.qt.io/qt-6/wasm.html 
> <https://doc.qt.io/qt-6/wasm.html>  and 
> https://emscripten.org/docs/getting_started/index.html 
> <https://emscripten.org/docs/getting_started/index.html>
> 
> The emscripten installation is able to compile the latest openssl pulled 
> from github.
> 
> 
> No matter which GCC version is chosen in /etc/alternatives and no matter 
> which emscripten version is activated (3.0.1 or 2.0.14):  
> .../upstream/emscripten/cache/sysroot/include/c++ only contains the v1 
> directory.
> 
> The server's /usr/include/c++ contains v1, 7 and 9 according to the GCC 
> versions installed. This is where my emscripten knowledge ends and I can 
> only experiment (e.g. by manually linking the 9 directory into the 
> emscripten cache, which is being ignored).
> 
> 
> Compiling basic C++ code with #include <cmath> works for GCC 7 and 
> produces the errors mentioned earlier for GCC 9 (i.e. Qt 6.2.2).
> 
> 
> Makefiles are generated by qmake out of a single .pro file. The .pro 
> file contains a WASM specific section to link against its own libcrypto 
> and libssl. Apart from the compilers used, all Makefiles look almost 
> identical.
> 
> 
> => It appears to me as if emscripten uses a default GCC version, which 
> in case of GCC 9 being expected is the wrong one. This leads to the lib 
> incompatibility as described by Thiago. I have not found options for 
> em++, 'emsdk install' or 'emsdk activate', that provide a GCC selection. 
> That may well be attributable to a lack of knowledge on my side.
> 
> 
>  > Well, Qt is a bit relevant if the qt-cmake of the Qt 6.2.2 WASM 
> installation
> 
>  > results in broken make files.
> 
>  >
> 
>  > So the question is perhaps: Axel, assuming that you installed emscripten
> 
>  > following the instruction and that you can verify that you can compile
> 
>  > basic C++ code as Thiago suggested, how do you then generate your make
> 
>  > files?
> 
>  >
> 
>  > Volker
> 
>  >
> 
> 
> _______________________________________________
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> Interest at qt-project.org
> https://lists.qt-project.org/listinfo/interest

-- 
Lorn Potter
Freelance Qt Developer. Platform Maintainer Qt WebAssembly, Maintainer 
QtSensors
Author, Hands-on Mobile and Embedded Development with Qt 5