Document a setjmp/longjmp convention

SetjmpLongjmp.md

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+# C setjmp/longjmp in WebAssembly
+
+## Overview
+
+This document describes a convention to implement C setjmp/longjmp via
+[WebAssembly exception-handling proposal].
+
+This document also briefly mentions another convention based on JavaScript
+exceptions.
+
+[WebAssembly exception-handling proposal]: https://github.com/WebAssembly/exception-handling
+
+## Runtime ABI
+
+### Linear memory structures
+
+This convention uses a few structures on the WebAssembly linear memory.
+
+#### Reserved area in jmp_buf
+
+The first 6 words of C jmp_buf is reserved for the use by the runtime.
+("words" here are C pointer types specified in the [C ABI].)
+It should have large enough alignment to store C pointers.
+The actual contents of this area is private to the runtime implementation.
+
+[C ABI]: BasicCABI.md
+
+##### Notes about the size of reserved area in jmp_buf
+
+Emscripten has been using 6 `unsigned long`s. (`unsigned long [6]`)
+
+GCC and Clang uses `intptr_t [5]` for their [setjmp/longjmp builtins].
+It isn't relevant right now though, because LLVM's WebAssembly target
+doesn't provide these builtins.
+
+[setjmp/longjmp builtins]: https://gcc.gnu.org/onlinedocs/gcc/Nonlocal-Gotos.html
+
+#### __WasmLongjmpArgs
+
+An equivalent of the following structure is used to associate necessary
+data to the WebAssembly exception.
+
+```c
+struct __WasmLongjmpArgs {
+  void *env; // a pointer to jmp_buf
+  int val;
+};
+```
+
+The lifetime of this structure is rather short. It lives only during a
+single longjmp execution.
+A runtime can use a part of `jmp_buf` for this structure. It's also ok to use
+a separate thread-local storage to place this structure. A runtime without
+multi-threading support can simply place this structure in a global variable.
+
+### Exception tag
+
+This convention uses a WebAssembly exception to perform a non-local jump
+for C `longjmp`.
+
+The exception is created with an exception tag named `__c_longjmp`.
+The name is used for both of [static linking](Linking.md) and
+[dynamic linking](DynamicLinking.md).
+The type of exception tag is `(param i32)`. (Or, `(param i64)` for [memory64])
+The parameter is the address of the `__WasmLongjmpArgs` structure on the
+linear memory.
+
+[memory64]: https://github.com/WebAssembly/memory64
+
+### Functions
+
+```c
+void __wasm_setjmp(jmp_buf env, uint32_t label, void *func_invocation_id);
+uint32_t __wasm_setjmp_test(jmp_buf env, void *func_invocation_id);
+void __wasm_longjmp(jmp_buf env, int val);
+```
+
+`__wasm_setjmp` records the necessary data in the `env` so that it can be
+used by `__wasm_longjmp` later.
+`label` is a non-zero identifier to distinguish setjmp call-sites within
+the function. Note that a C function can contain multiple setjmp() calls.
+`func_invocation_id` is the identifier to distinguish invocations of this
+C function. Note that, when a C function which calls setjmp() is invoked
+recursively, setjmp/longjmp needs to distinguish them.
+
+`__wasm_setjmp_test` tests if the longjmp target belongs to the current
+function invocation. if it does, this function returns the `label` value
+saved by `__wasm_setjmp`. Otherwise, it returns 0.
+
+`__wasm_longjmp` is similar to C `longjmp`.
+If `val` is 0, it's `__wasm_longjmp`'s responsibility to convert it to 1.
+It performs a long jump by filling a `__WasmLongjmpArgs` structure and
+throwing an exception with its address. The exception is created with
+the `__c_longjmp` exception tag.
+
+## Code conversion
+
+The C compiler detects `setjmp` and `longjmp` calls in a program and
+converts them into the corresponding WebAssembly exception-handling
+instructions and calls to the above mentioned runtime ABI.
+
+### Functions calling setjmp()
+
+On the function entry, the compiler would generate the logic to create
+the identifier of this function invocation, typically by performing an
+equivalent of `alloca(1)`. Note that the alloca size is not important
+because the pointer is merely used as an identifier and never be dereferenced.
+
+Also, the compiler converts C `setjmp` calls to `__wasm_setjmp` calls.
+
+For each setjmp callsite, the compiler allocates non-zero identifier called
+"label". The label value passed to `__wasm_setjmp` is recorded by the
+runtime and returned by later `__wasm_setjmp_test` when processing a longjmp
+to the corresponding jmp_buf.
+
+Also, for code blocks which possibly call `longjmp` directly or indirectly,
+the compiler generates instructions to catch and process exceptions with
+the `__c_longjmp` exception tag accordingly.
+
+When catching the exception, the compiler-generated logic calls
+`__wasm_setjmp_test` to see if the exception is for this invocation
+of this function.
+If it is, `__wasm_setjmp_test` returns the non-zero label value recorded by
+the last `__wasm_setjmp` call for the jmp_buf. The compiler-generated logic
+can use the label value to pretend a return from the corresponding setjmp.
+Otherwise, `__wasm_setjmp_test` returns 0. In that case, the
+compiler-generated logic should rethrow the exception by calling
+`__wasm_longjmp` so that it can be eventually caught by the right function.
+
+For an example, a C function like this would be converted like
+the following pseudo code.
+```c
+void f(void) {
+  jmp_buf env;
+  if (!setjmp(env)) {
+    might_call_longjmp(env);
+  }
+}
+```
+
+```wat
+  $func_invocation_id = alloca(1)
+
+  ;; 100 is a label generated by the compiler
+  call $__wasm_setjmp($env, 100, $func_invocation_id)
+
+  block
+    block (result i32)
+      try_table (catch $__c_longjmp 0)
+        call $might_call_longjmp
+      end
+      ;; might_call_longjmp didn't call longjmp
+      br 1
+    end
+    ;; might_call_longjmp called longjmp
+    pop __WasmLongjmpArgs pointer from the operand stack
+    $env = __WasmLongjmpArgs.env
+    $val = __WasmLongjmpArgs.val
+    $label = $__wasm_setjmp_test($env, $func_invocation_id)
+    if ($label == 0) {
+      ;; not for us. rethrow.
+      call $__wasm_longjmp($env, $val)
+    }
+    ;; ours.
+    ;; somehow jump to the block corresponding to the $label
+    ...
+    ...
+  end
+```
+
+### Longjmp calls
+
+The compiler converts C `longjmp` calls to `__wasm_longjmp` calls.
+
+## Dynamic-linking consideration
+
+In case of [dynamic-linking], it's the dynamic linker's responsibility
+to provide the exception tag for this convention with the name
+"env.__c_longjmp". Modules should import the tag so that cross-module
+longjmp works.
+
+[dynamic-linking]: DynamicLinking.md
+
+## Emscripten JavaScript-based exceptions
+
+Emscripten has a mode to use JavaScript-based exceptions instead of
+WebAssembly exceptions. In that mode, `emscripten_longjmp` function,
+which throws a JavaScript exception, is used instead of `__wasm_longjmp`.
+
+```c
+void emscripten_longjmp(uintptr_t env, int val);
+```
+
+The compiler translates C function calls which possibly ends up with
+calling `longjmp` to indirect calls via a JavaScript wrapper which
+catches the JavaScript exception.
+
+## Implementations
+
+* LLVM (19 and later) has a pass ([WebAssemblyLowerEmscriptenEHSjLj.cpp])
+  to perform the convertion mentioned above.  It can be enabled with the
+  `-mllvm -wasm-enable-sjlj` option.
+
+  Note: as of writing this, LLVM produces a bit older version of
+  exception-handling instructions. (`try`, `delegate`, etc)
+  binaryen has a conversion from the old instructions to the latest
+  instructions. (`try_table` etc.)
+
+* Emscripten (3.1.57 or later) has the runtime support ([emscripten_setjmp.c])
+  for the convention documented above.
+
+* wasi-libc has the runtime support ([wasi-libc rt.c]) for the convention
+  documented above.
+
+[WebAssemblyLowerEmscriptenEHSjLj.cpp]: https://github.com/llvm/llvm-project/blob/70deb7bfe90af91c68454b70683fbe98feaea87d/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
+
+[emscripten_setjmp.c]: https://github.com/emscripten-core/emscripten/blob/7d66497d96cdcffa394ad67d87f7118137edf9ab/system/lib/compiler-rt/emscripten_setjmp.c
+
+[wasi-libc rt.c]: https://github.com/WebAssembly/wasi-libc/blob/d03829489904d38c624f6de9983190f1e5e7c9c5/libc-top-half/musl/src/setjmp/wasm32/rt.c
+
+## Future directions
+
+* `__WasmLongjmpArgs` can be replaced with WebAssembly multivalue.
+
+* Or, alternatively, we can make `__wasm_setjmp_test` take the
+  `__WasmLongjmpArgs` pointer so that we can drop the `__WasmLongjmpArgs`
+  structure layout from the ABI.
+
+* It might be simpler for the complier-generated catching logic to rethrow
+  the exception with the `rethrow`/`throw_ref` instruction instead of
+  calling `__wasm_longjmp`. Or, it might be simpler to make
+  `__wasm_setjmp_test` rethow the exception internally.
+
+* If/When WebAssembly exception gets more ubiquitous, we might want to move
+  the runtime to compiler-rt.