[LLVMdev] RFC: Exception Handling Rewrite

[John McCall]

On Aug 2, 2011, at 10:28 AM, Duncan Sands wrote:
>>>> Is it intended that "cleanup ty_1, ty_2" potentially be different to
>>>> "cleanup ty_1 cleanup ty_2"?  Perhaps this is useful for funky personality
>>>> functions.
>>>> 
>>> Yeah. One can basically interleave the catches and filters. But having two catch or two filter clauses in a row should be semantically the same as the clauses being merged into one. (E.g., your examples would be equivalent.)
>> 
>> Let me rephrase the question then.  Why not make the grammar be either:
>> 
>>>>>  <clause>   :=
>>>>>       cleanup
>>>>>    |  catch<ty_1>
>>>>>    |  filter<ty_1>
>> 
>> .. forcing "catch" or "filter" before each entry.  If you don't like that, why not make the grammar be something like:
>> 
>>>>> %res = landingpad<somety>   personality<ty>   <pers_fn>  (catch<ty>+)? (filter<ty>+)?
>> 
>> Is there anything specific about the ordering of catch or filter clauses that affect semantics?  If so, the first alternative seems cleaner.  If not, the second does.
> 
> yes there is something about the order :)  When front-ends output code, all
> catches come before filters, but after inlining you can get: catches, filters,
> catches, filters etc and this ordering needs to be preserved.  Also, filters
> are different from catches: filter ty_1, ty_2, ..., ty_N matches any exception
> that would not be caught by any of ty_1, ty_2, ..., ty_N.  This is quite
> different to filter ty_1, filter ty_2, ..., filter ty_N.  For example, suppose
> you throw ty_2.  Then the filter ty_1, ty_2, ..., ty_N wouldn't match that,
> but filter ty_1 would.
> 
> So filter ty_1, ..., ty_N makes sense; but I don't think catch ty_1, ..., ty_N
> makes sense.

Agreed.  Other notes: a zero-type filter is meaningful (and pretty
common — it's the result of 'throw()'), but the ordering of the cleanup
bit is not, so I would suggest this grammar:

Instruction ::= 'landingpad' Type 'personality' TypeAndValue 'cleanup'? LandingPadClause*
LandingPadClause ::= 'catch' TypeAndValue
LandingPadClause ::= 'filter' TypeAndValue*

John.