Compiling OpenMP to WebAssembly

[Thomas Schoenhofer]

Hi,

I am trying to compile a multi threaded application to WebAssembly. The application uses OpenMP for multithreading.

I have already downloaded the source files for OpenMP and compiled it for my host machine using make. With the following command I can get it to link with a simple demo application on my machine:

g++ -Wall -Werror -pedantic main.o -o main.x /$PATH_TO_OPENMP/build/runtime/src/libgomp.a -pthread -lstdc++ -Wl,--no-as-needed -ldl

I then tried to compile OpenMP to the llvm bytecode format used by Emscripten. To do so I tried to run 'emmake make', so that the emscripten framework executes the OpenMP makefiles with a suitable compiler. As emscripten does not like shared object files I compiled it to static library .a files. This works and actually gives me object files to which I can link.

I then wanted to link my demo application with the following command

em++ -Wall -Werror -pedantic main.o -o main.html /home/main/data/Programming/openMP/openmp_web/build/runtime/src/libgomp.a -pthread -lstdc++ -Wl,--no-as-needed -ldl

But I get these warnings, that it couldn't link to OpenMP files:

shared:WARNING: object /tmp/emscripten_temp_ONa0eU_archive_contents/kmp_atomic.cpp.o is not a valid object file for emscripten, cannot link
                              .
                              .
shared:WARNING: object /tmp/emscripten_temp_ONa0eU_archive_contents/kmp_str.cpp.o is not a valid object file for emscripten, cannot link
shared:WARNING: object /tmp/emscripten_temp_ONa0eU_archive_contents

So I figured I must have compiled OpenMP with the wrong compiler. I then tried to change the compiler when building the library by using the following commands:

cmake -DCMAKE_C_COMPILER=emcc -DCMAKE_CXX_COMPILER=em++ -DLIBOMP_LIB_TYPE=normal -DLIBOMP_ENABLE_SHARED=OFF -DCMAKE_BUILD_TYPE=Release -DLIBOMP_ARCH=x86_64 OPENMP_STANDALONE_BUILD=1 ..
emmake make

But this just gives strange errors on some missing system variables

/home/main/data/Programming/openMP/openmp_web/runtime/src/kmp_platform.h:82:2: error: Unknown OS 
/home/main/data/Programming/openMP/openmp_web/runtime/src/kmp_platform.h:203:2: error: Unknown or unsupported architecture 
In file included from /home/main/data/Programming/openMP/openmp_web/runtime/src/kmp_alloc.cpp:13: 
In file included from /home/main/data/Programming/openMP/openmp_web/runtime/src/kmp.h:77: /home/main/data/Programming/openMP/openmp_web/runtime/src/kmp_os.h:171:2: 
error: "Can't determine size_t printf format specifier."

Does anyone have an idea on what I could do differently?

[Sam Clegg]

I looks like that kmp_platform.h doesn't yet have support for
emscripten. You probably want to add an "ifdef __EMSCRIPTEN__" block
there.

> Does anyone have an idea on what I could do differently?
>

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