Enabling openmp

[flatmax]

Is there a way to enable/compile with openmp support ?
For this task, I was thinking more specifically about execution in the nodejs environment and not the browser.

[flatmax]

It seems that you can builde libiomp out of tree, but will it work ?
https://openmp.llvm.org/README.txt

I have tried to build with the following flags, but no luck/love :
source /usr/local/emsdk/emsdk_env.sh
export EMSCRIPTEN=$EMSDK/upstream/emscripten
emcmake cmake -DCMAKE_INSTALL_PREFIX:PATH=$EMSCRIPTEN/system -DCMAKE_DISABLE_FIND_PACKAGE_CUDA=TRUE ..

[nihui]

In ncnn project, we implement a minimal openmp runtime for webassembly target
https://github.com/Tencent/ncnn/blob/master/src/simpleomp.cpp

It only works for

#pragma omp parallel for num_threads(N)

[ted537]

@nihui Impressive! How does this integrate with the OpenMP pragma? Currently, I'm guessing that __kmpc_fork_call() is the central component

[nkbai]

@nihui are there any documents about how to use simpleomp in emscripten?

[nihui]

add simpleomp source to your project and compile with -fopenmp

[Smithangshu]

Hi @nihui, I am trying to use simpleomp in my project with emscripten. But problem is that the simpleomp.cpp is dependent on platform.h but actually platform.h.in exists there instead of the platform.h. Can you please guide me about how to add the source of simpleomp correctly to my project?

[bottle2]

hey @Smithangshu and others
I successfully used ncnn's simpleomp.cpp!

I had to copy some parts of code. see:

• https://github.com/bottle2/raytracing/blob/master/cpu.h
• https://github.com/bottle2/raytracing/blob/master/platform.h

I also created this header to solve more linker errors:

• https://github.com/bottle2/raytracing/blob/master/simpleomp.h

in my particular case, my application is written in C and I just have a Makefile (I don't know CMake). hence

https://github.com/bottle2/raytracing/blob/master/Makefile
https://emscripten.org/docs/porting/pthreads.html

as seen in my Makefile, some caveats:

• flag -pthread must be used to compile every single translation unit
• when linking, specify e.g. -sPTHREAD_POOL_SIZE=navigator.hardwareConcurrency, otherwise if you use OpenMP in the main thread, it will deadlock or something
• I compile simpleomp.cpp separately with emcc -DNCNN_SIMPLEOMP=1 -c simpleomp.cpp, which will generate an object simpleomp.o, which can then be linked in the final application.

that's it. works like a charm

[MuTsunTsai]

Based on @bottle2's answer, I created this repo SimpleOMP that includes prebuilt static library and an usage example.