[LLVMdev] Exception Handling Tables Question

[Duncan Sands]

Hi Bill,

>>> Yeah. The logic will need tweaking for sure. I'm also concerned about the
>>> _Unwind_resume() call. GCC emits a call site region for it in the exception
>>> table. We...kind of do that. It looks like it's being included in one of the
>>> "this is a region which isn't in a try-catch block, but it has a call in it,
>>> so lets add it to the exception table" areas.
>> isn't that exactly how it should be?  This is a call that will unwind
>> out of the function, so C++ requires it to have a call site region just
>> like any other call that we want to let unwind out of the function.  I
>> don't see why it needs any special logic.  If I understood right the
>> change you want to make is that if a call is known not to unwind then
>> you want to omit adding a call-site entry if that saves some space in
>> the call site table, which seems irrelevant to this, or am I missing
>> something?  By the way, LLVM "nounwind" calls are different to GCC no
>> throw regions IIRC.  If an exception is thrown in a GCC no throw region
>> then it must (C++) result in a call to "terminate".  These are not
>> mapped to "nounwind", instead we create explicit "catch-all" filter
>> expressions for this (IIRC).  In LLVM it is undefined what happens if
>> a call is marked "nounwind" but nonetheless an exception unwinds out
>> of it.  Thus you can add call-site entries for nounwind calls,
>> or not add them, as you like - whatever is most convenient (eg: saves
>> the most space).  An interesting optimization which we don't do is to
>> identify which calls correspond to a "catch-all" filter and not generate
>> an entry for them in the call-site table (no need to add the filter
>> either) - this saves space and the C++ runtime knows to handle this
>> just like if we added the filter explicitly.
>>
> There's a miscommunication here. :-) The _Unwind_resume call isn't
> marked with "nounwind", however it's not called through an "invoke"
> instruction, only a regular "call" instruction. From what I can see,
> the only reason it falls within a call site in the exception table is
> because we're generating call sites for areas of code without
> "invokes".

yes, and that's absolutely the right thing to do!  Due to a strange
design choice, if a call does *not* have an entry in the call-site
table, and the call unwinds, then the runtime calls std::terminate.
All calls that may throw (like _Unwind_Resume!) must have an entry
in the call-site table.

If I implement my optimization to eliminate these call site
> entries which don't have "invoke" calls in them, then the
> _Unwind_resume call won't have an entry into the exception table, and
> that would be bad.

As explained above, you can only eliminate call-site entries for
nounwind calls.  Having a call-site entry has nothing to do with
whether a call is an invoke or not, it is to do with whether the
call can throw an exception or not.

Ciao,

Duncan.