document setjmp/longjmp support

SetjmpLongjmp.md

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+# C setjmp/longjmp support
+
+## Build an application
+
+To build an application using setjmp/longjmp, you need two things:
+
+* Enable the necessary LLVM translation (`-mllvm -wasm-enable-sjlj`)
+
+* Link the setjmp library (`-lsetjmp`)
+
+### Example without LTO
+
+```shell
+clang -Os -mllvm -wasm-enable-sjlj -o your_app.wasm your_app.c -lsetjmp
+```
+
+### Example with LTO
+
+```shell
+clang -Os -flto=full -mllvm -wasm-enable-sjlj -Wl,-mllvm,-wasm-enable-sjlj -o your_app.wasm your_app.c -lsetjmp
+```
+
+## Run an application
+
+To run the application built as in the previous section,
+you need to use a runtime with [exception handling proposal] support.
+
+Unfortunately, there are two incompatible versions of
+[exception handling proposal], which is commonly implemented by runtimes.
+
+* The latest version with `exnref`
+
+* The [phase3] version
+
+### Example with the latest exception handling proposal
+
+Because the current version of WASI-SDK produces an old version
+of [exception handling proposal] instructions, if your runtime
+implements the latest version of the proposal, you need to convert
+your module to the latest version.
+
+[toywasm] is an example of such runtimes.
+
+You can use binaryen `wasm-opt` command for the conversion.
+
+```shell
+wasm-opt --translate-to-exnref -all -o your_app.exnref.wasm your_app.wasm
+```
+
+Then you can run it with a runtime supporting the latest version of
+[exception handling proposal].
+
+```shell
+toywasm --wasi your_app.exnref.wasm
+```
+(You may need to enable the support with `-D TOYWASM_ENABLE_WASM_EXCEPTION_HANDLING=ON`.)
+
+### Example with the phase3 exception handling proposal (a bit older version)
+
+If your runtime supports the [phase3] version of
+[exception handling proposal], which is the same version as what WASI-SDK
+currently produces, you can run the produced module as it is.
+
+For example, the classic interpreter of [wasm-micro-runtime] is
+one of such runtimes.
+
+```shell
+iwasm your_app.wasm
+```
+(You may need to enable the support with `-D WAMR_BUILD_EXCE_HANDLING=1 -D WAMR_BUILD_FAST_INTERP=0`.)
+
+[exception handling proposal]: https://github.com/WebAssembly/exception-handling/
+[phase3]: https://github.com/WebAssembly/exception-handling/tree/main/proposals/exception-handling/legacy
+[toywasm]: https://github.com/yamt/toywasm
+[wasm-micro-runtime]: https://github.com/bytecodealliance/wasm-micro-runtime