[LLVMdev] RFC: Exception Handling Proposal II

[Bill Wendling]

On Nov 24, 2010, at 6:41 PM, John McCall wrote:

> On Nov 24, 2010, at 5:07 PM, Bill Wendling wrote:
> 
>> On Nov 24, 2010, at 11:18 AM, John McCall wrote:
>> 
>>> I think my model has some nice conceptual advantages;  for one, it gives you the constraint that only EH edges and dispatch instructions can lead to landing pads, which I think will simplify what EH preparation has to do.  But I could be convinced.
>> 
>> Notice though that we would need to keep both the EH edge from the invoke and the region numbers, which you said was redundant (in your first email). :-) Consider if there were several cleanup landing pads leading in a chain, through successive dispatches, down to the last dispatch that decides which catch to execute:
> 
>> lp0:
>>  call void @A::~A(%A* %x)
>>  dispatch region 0 [], resume label %lp1
>> lp1:
>>  call void @B::~B(%B* %y)
>>  dispatch region 1 [], resume label %lp2
>> lp2:
>>  call void @C::~C(%C* %z)
>>  dispatch region 2 [], resume label %lp3
>> lp3:
>>  dispatch region 3 [filter i8* null]   # no resume edge
>> 
>> If we have inlining of any of those d'tor calls, we may lose the fact that the dispatch in, say, lp1 is a cleanup that lands onto the region 2 dispatch in lp2.
> 
> What you mean is that, given a resume or invoke edge, we need to be able to find the dispatch for the target region.  There are ways to make that happen without tagged edges;  for example, you could make the landing pad a special subclass of BasicBlock with a pointer to the dispatch, although that'd be a fairly invasive change.  Tagging the edges solves the problem for clients with a handle on an edge;  clients that want to go from (say) a dispatch to its landing pad(s) will still have trouble.

It's not that troublesome. The dispatch would give you the region number. All objects in the function with that region number will point to the landing pad(s) for that region.

>> We know from experience that once this information is lost, it's *really* hard to get it back again. That's what DwarfEHPrepare.cpp is all about, and I want to get rid of that pass because it's a series of hacks to get around our poor EH support.
> 
> While I agree that the hacks need to go, there is always going to be some amount of custom codegen for EH just to get the special data to flow properly from landing pads to the eh.exception intrinsic and the dispatch instruction.  My design would give you some very powerful assumptions to work with to implement that:  both the intrinsic calls and the dispatch would always be dominated by the region's landing pad, which would in turn only be reachable via specific edges.  I don't know how you're planning on implementing this without those assumptions, but if you say you don't need them, that certainly diminishes the appeal of my proposal.

We actually have the "reachable via specific edges" check in our code right now. But when we tried to allow exceptions to be marked as proper "cleanups", the assumption was violated. So I'm wary of making this same assumption twice.

But anyway, I think that I can gather the necessary information from the region numbers and the invokes' "unwind to" edges to create the EH tables. The only intrinsic call that should remain is the one that gets the EH pointer. And that's only needed by the catch blocks.

Perhaps I'm missing something? :-)

>> On a personal level, I'm not a big fan of forcing constraints on code when they aren't needed. We had problems in the past (with inlining into a cleanup part of an EH region) with enforcing the invariant that only unwind edges may jump to a landing pad. If we go back to the example above, if @C::~C() were inlined, it could come to pass that the dispatch is placed into a separate basic block and that the inlined code branches into that new basic block thus violating the constraint.
> 
> I'm not suggesting that the landing pad has to be the same block as the block with the dispatch instruction.  That's an obviously unreasonable constraint;  in fact, it wouldn't hold for the vast majority of C++ cleanups, since we generally have to invoke destructors so we can terminate if they throw.  Since — unlike present-day branches between cleanups — dispatch edges will be initially opaque to the optimizer, I don't see much danger of it producing invalid code from otherwise-valid transformations.

Okay, I misread. But then we're back to a disconnect of information. If you have lp0 jumping into lp1, how does it know that that's the dispatch for region 1? We would have to implement the BasicBlock subclassing that you mentioned above, because it's yet another piece of information that needs to be tightly coupled between instructions. The subclassing of BasicBlock has some issues which need to be thought out more. In particular, it requires augmenting the IR with basic block attributes, defining semantics on those, supporting them throughout the compiler, etc. It's a large change in and of itself, and would be no longer orthogonal to the EH changes.

-bw