[LLVMdev] RFC: Exception Handling Proposal II

[Bill Wendling]

On Nov 24, 2010, at 4:58 AM, John McCall wrote:

>> If that's so, how do you encode which which landing pad is to be
>> followed per region?
>> 
>> Consider the following code:
>> 
>> try {
>> Foo f();
>> f.run(); // can throw exception
>> Bar b();
>> b.run(); // can throw exception
>> Baz z();
>> z.run(); // can throw exception
>> } catch (...) {
>> }
> 
> I assume you don't mean these to be function declarations. :)
> 
>> The object 'f' is in a different cleanup area than 'b' which, in turn
>> is in a different area than 'z'. These three regions should point to
>> three different landing pads (or different offsets in the same landing
>> pad), which (I believe) are encoded in IR by being declared after
>> different dispatch instructions, all of which within the same region.
> 
> Nope.  Three regions, three landing pads, three dispatch instructions.
> (actually four if Foo::Foo() can throw).  The Baz-destructing region
> chains to the Bar-destructing region which chains to the Foo-destructing
> region which chains to the catching region;  the first three are
> cleanup-only.
> 
Ah ha! I think I had a different mental model than you did. Or at least I remembered things differently from the discussion. :-) For me, there is one dispatch per region, which is why I had the region number associated with the invokes as well as the "unwind to" edge coming from them. (See my response to Renato for a description.) I'll think more about your model...

>> If that's so, why do you still have the invoke call? Why should you
>> treat call-exceptions any differently than instruction-exceptions?
> 
> One of my favorite things about this design is that it's totally
> independent of what exactly is allowed to throw.  I'm really not sure
> how best to represent other throwing instructions, except that I'm
> pretty confident that we don't want anything as heavyweight as
> invoke.  There's a pretty broad range of possibilities — we could
> make invoke-like instructions for all of them (ick...), or we could
> tag individual instructions with regions, or we could mark basic
> blocks as unwinding to particular places.  But we can wrestle
> with that independently of deciding to adopt explicitly-chained
> landing pads.
> 
Exactly! :-)

-bw