Exception support

[carlokok]

Feature

Cranelift currently doesn't support SEH (but has some support for finally I think). This is a feature request to support properly exception (landingpads on Posix, SEH on windows, maybe SLJL for others)

Benefit

WASM has plans to support exceptions, at which point it's going to be needed. Win64 has partial SEH support (iirc it just emits what's needed, nothing more), this would allow for c++ /java/.net exception support.

Implementation

Tricky one. I know LLVM has two alternative ways of doing this. I think with that in mind it should be possible to do a single implementation. What's basically needed is a way to say:
from here
[code]

till here is a protected block, on exceptions go to X, for finalization go to Y. For the handler itself it should be possible to define the personality to call.

Throw, Rethrow etc can all probably be done on a library level.

Alternatives

The only alternative is to not do it.

[bjorn3]

Duplicate of #1677

[carlokok]

I agree it's related, but unwind and catch are slightly distinct actions and your issue doesn't cover that?

[bjorn3]

Catch is implemented by returning _URC_HANDLER_FOUND instead of _URC_INSTALL_CONTEXT from the personality function and then jumping from the landing pad to the block handling the catch instead of calling _Unwind_Resume like for cleanup.

[bjorn3]

Given the above comments I think this can be closed as duplicate.

[carlokok]

I missed your reply last year but does that cover windows at all? Windows doesn't use personalities? At least not like that?

[bjorn3]

SEH seems to use exception handlers and termination handlers. I think catching an exception instead of doing cleanup is done by using an exception handler instead of termination handler. This is based on a cursory reading of the docs though, so I may be wrong. In any case both catching exceptions and cleanup needs the same codegen handling to support an alternative location to which a call can "return".