Considering support setjmp & longjmp?

[Brooooooklyn]

setjmp.h is not supported in wasi-libc at this moment: WebAssembly/wasi-libc#432
If you could support setjmp.h in emnapi, it would be very useful for C++ projects compiled to wasi target, such as https://github.com/AOMediaCodec/libavif.

For references:

• wasix implemented setjmp.h here: https://github.com/wasix-org/wasix-libc/blob/main/libc-bottom-half/sources/__wasilibc_stack.c
• https://twitter.com/kripken/status/1173993718262534145

[toyobayashi]

OK. Though I think setjmp/longjmp is out of the scope of emnapi, I'll investigate how it works in WASIX and Emscripten, then consider providing prebuilt static library as temporary solution before wasi-libc have it.

Happy dragon new year!

[toyobayashi]

WASIX and kripken's demo rely on the Asyncify feature of binaryen, which will affect runtime performance and increases code size.

Emscripten has two implementations of setjmp/longjmp, the default way relies on generated JavaScript glue code by emcc, the other relies on WebAssembly Exception Handling feature, they both require passing additional flags to llvm backend to let llvm transform the setjmp() / longjmp() call to additional runtime code.

I've created a separate repository (https://github.com/toyobayashi/wasm-sjlj) to test if the wasm EH way works in WASI, included the prebuilt libraries. I used the implementation in emscripten repo, and pass -mllvm -wasm-enable-sjlj, it looks good. Since it is not relative with Node-API, I'm not sure if it should go emnapi's repo.

Refs:

• https://reviews.llvm.org/D115893
• https://github.com/llvm/llvm-project/blob/586ecdf205aa8b3d162da6f955170a6736656615/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp#L187

Could you try it to build libavif?

[Brooooooklyn]

@toyobayashi it works! thanks for your help!

[Brooooooklyn]

Can you publish a npm package that contains lib/libsetjmp.a lib/libsetjmp-mt.a and include/setjmp.h files?

[toyobayashi]

Done. https://www.npmjs.com/package/wasm-sjlj