[LLVMdev] Making llvm.eh.actions work

[Reid Kleckner]

llvm.eh.actions is basically a sequenced list of actions to take when an
invoke throws to the landing pad containing the actions call. The prototype
is just 'i8* llvm.eh.actions(...)', but it needs some structure. LLVM IR
supports three types of actions: cleanup, catch, and filter.
__CxxFrameHandler3 doesn't support filters, so it isn't really relevant
here.

Catch actions need three pieces of information:
1. The typeinfo to catch. This can be null for catch-all handlers.
2. The alloca that the exception object is stored into. This can be null if
there is no catch parameter.
3. The handler to run. This is required.

Cleanup actions only need one piece of information: the handler to run.

There are two main ways I can think of to encode this list.

The first is that we have a constant i32 parameter of 0 or 1 to indicate if
this is a cleanup or catch, followed by constants representing the
information required. We'd pass 3 args for catches and 1 for cleanups.

The second way is that we invent some kind of not-quite-a-catch-all
sentinel to use instead of the i32 argument. We could use i32 -1, i8*
undef, or something else. This all pretty confusing though.

The i32 argument approach extends to handle filters pretty naturally. It's
extensible, but it uses a lot of memory just to communicate a single bit.

The EH state number isn't encoded in the action list, it's computed later
using the algorithm you described in the "RFC: Native Windows C++ exception
handling" thread.

Does this seem reasonable?
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