🛤 Exception handling

[binji]

This is a tracking issue for a post-MVP feature It will be updated as the issue progresses.

Topic Exception handling Champion Heejin Ahn (@aheejin) Status In progress Phase Feature proposal Linked issues https://github.com/WebAssembly/design/issues/417 Linked repositories github.com/WebAssembly/exception-handling

Details

The WebAssembly MVP supports four no-exception modes for C++:

• Compiler transforms throw to abort().
• Compiler-enforced -fno-exceptions mode (note caveats).
• Compiler conversion of exceptions to branching at all callsites.
• In a Web environment exception handling can be emulated using JavaScript exception handling, which can provide correct semantics but isn't fast.

These modes are suboptimal for code bases which rely on C++ exception handling, but are perfectly acceptable for C code, or for C++ code which avoids exceptions. This doesn't prevent developers from using the C++ standard library: their code will function correctly (albeit slower at times) as long as it doesn't encounter exceptional cases.

Post-MVP, WebAssembly will gain support for zero-cost exception handling.

In turn, this can be used to implement setjmp/longjmp, and can enable all of the defined behavior of setjmp/longjmp, namely unwinding the stack without calling C++ destructors. It does not, however, allow the undefined behavior case of jumping forward to a stack that was already unwound which is sometimes used to implement coroutines. Coroutine support is being considered separately.