Porting vanilla (minimal external API usage, no SIMD, no th…

1. Beware of code which does unaligned memory reads/writes (which "may be slow on some CPU architectures")
2. Test with -fsanitize=undefined -fsanitize=address
3. Link with "-s ALLOW_MEMORY_GROWTH=1" and "-s INITIAL_MEMORY=X", X is a multiple of 64k, allows the C++ heap to grow.

(I completely avoid unaligned memory reads/writes because when compiled to asm.js they "can fail silently".)

4. Link with "-s MALLOC=emmalloc" to reduce compiled size.
5. I usually test with -O1 because -O0 can take very long to load/execute.
6. For debugging link with -s DEMANGLE_SUPPORT=1 and -s ASSERTIONS=1

7. C++ printf() outputs to the Developer Console: Chrome Settings->More tools->Developer tools
8. If something crashes, try running in Firefox which may explain the error differently.
9. I use Web Server for Chrome for development:
https://t.co/3drLehhSlL

The C FILE I/O functions work, but on a virtual file system:
https://t.co/iyJyAAVBiu

You can package up individual files from a directory that gets preloaded before your module executes.

To wrap C++ functions, classes, types etc. to expose them to Javascript code, look for "EMSCRIPTEN_BINDINGS". It's quite slick and mostly automatic.

Reading an integer property on a Javascript object from C++:

const emscripten::val& srcBuffer;

unsigned int length = srcBuffer["length"].as();

Copying from a Javascript Uint8Array to a std::vector:

And the reverse: copying from a std::vector to a Javascript Uint8Array (which was sized in Javascript to be >= the size you want to copy):

To JavaScripte/WebAssembly wizards all of this is probably the most basic beginning stuff. However, to native C++ coders not very familiar with these technologies (like me), all of this takes a while to figure out.

I use "emcmake cmake ../" in my build directory, with my CMakeLists.txt file in the parent folder. It sets environment variables that CMake needs to use emcc (the compiler).

All the heavy lifting that I do (data compression, decompression, transcoding)- stuff that doesn't require external API access - is in C/C++. Pretty much everything else, like the UI, WebGL usage, etc. is written in Javascript.

WebAssembly SIMD is the tech you want to use for SIMD in the web world. Personally, I use the intrinsic function wrappers. Functionality wise it seems vaguely somewhere in between SSE v2 and v4.

Multithreading support is still the wild west. I wouldn't depend on it being available yet in all browsers:
https://t.co/hyK6E6Pvso

#emscripten #webassembly