[LLVMdev] Exception Handling Tables Question

Bill Wendling isanbard at gmail.com
Thu Sep 17 13:11:06 PDT 2009

On Thu, Sep 17, 2009 at 3:40 AM, Duncan Sands
<duncan.sands at math.u-psud.fr> wrote:
> 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". 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.

That was all I'm saying here.


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