[LLVMdev] RFC: New Exception Handling Proposal

[Jay Foad]

> Please read and let me know your opinions!

This definitely seems like an improvement on the current situation.

>            .---------.
>            | convoke |
>            `---------'
>                 |
>                 v
>     .-----------------------.
>     |                       |
>     v                       |
>  %normal   .----------------+---------------.
>            |                |      ...      |
>            v                v               v
>       select.1 = 1   select.2 = 2    select.n = n
>            |                |               |
>            `----------------+---------------'
>                             |
>                             v
>          .----------------------------------------.
>          | %sel = phi [%select.1, ..., %select.n] |
>          |     %eh_ptr = llvm.eh.exception()      |
>          |           [cleanup code]               |
>          |           switch on %sel               |
>          `----------------------------------------'
>                             |
>                             v
>                  .--------------------.
>                  |          |   ...   |
>                  v          v         v
>               %catch.1   %catch.2  %catch.n

One thing I don't like about this (not that it's any worse than the
current situation) is that the llvm.eh.exception() call is now quite a
long way away from the landing pads.

As I understand it, when you generate native code, at each landing pad
the exception pointer appears magically in a well-known native
register. Do you think there's a way to model this more closely in the
LLVM IR? (You could just specify that the llvm.eh.exception call has
to be at the start of a landing pad, but I think that is too prone to
being broken by optimisation passes.)

To illustrate why I don't like having the llvm.eh.exception() call
dissociated from the landing pads, what is the *exact* definition of
llvm.eh.exception:

- Does there have to be a one-to-one correspondence between convokes
and llvm.eh.exception calls?
- Does llvm.eh.exception have to be dominated by landing pads?
- Does it just return the exception pointer from the *most recent*
landing pad that execution flowed through?

etc.

Maybe (this has just occurred to me) the llvm.eh.exception call could
explicitly refer to a convoke, which must dominate it?

Thanks,
Jay.