[LLVMdev] How to code catch-all in the new exception handling scheme?

[Duncan Sands]

Hi Bill,

>>> I'm looking at the docs, and while it refers to a "catch-all" clause,
>>
>> hopefully Bill will get rid of the first reference to "catch-all" since
>> that section is inaccurate.
>>
> I *think* this is now correct. Please check. :)

I still have some niggles:

   The unwinder delegates the decision of whether to stop in a call frame to
   that call frame's language-specific personality function. Not all personality
   functions guarantee that they will stop to perform cleanups.

This is incorrect: it is not the personality function that makes the decision,
it is who-ever is doing the unwinding.  For example if you use the Ada "throw"
method it will always run all C++ cleanups, even if that's all there is to do.
While if you use the C++ throw method it won't bother running Ada cleanups if
that is all there is to do.  All personality functions that I am familiar with
treat cleanups in the same way.

   For example, the GNU C++ personality function doesn't do so unless the
   exception is actually caught somewhere further up the stack.

As mentioned above, this has nothing to do with the personality function.

   When using this personality to implement EH for a language that guarantees
   that cleanups will always be run (e.g. Ada), be sure to indicate a catch-all
   in the landingpad instruction rather than just cleanups.

I suggest you just delete this last bit.  This isn't how Ada does it itself for
example.

>
>> it doesn't
>>> describe how to construct one.
>>
>> There is in general no such thing as a catch-all: it depends on the personality
>> function (i.e. the language), and, for example, the Ada language doesn't have
>> one (it has something that catches all Ada exceptions, but it won't catch C++
>> exceptions).
>>
> This is true in a sense that a catch-all is encoded the same way as any other catch type. So for C++, you would specify a catch-all like this:
>
>   %exn = landingpad { i8*, i32 } personality i32 (...)* __gxx_personality_v0
>             catch i8* @_ZTIi
>             catch i8* null
>
> A 'null' value is C++'s way of saying "this is a catch-all". Ada uses a global variable (function?) for this.

Yes, Ada has a global variable that plays this role.  However calling it
"catch-all" is misleading since it doesn't match any C++ exceptions (unlike
the C++ catch-all which also matches foreign exceptions).  This means that
the old exception handling scheme wouldn't always work properly if C++ code
was inlined into Ada code.  Not a big deal since it wouldn't work properly
anyway for all kinds of other reasons (eg: multiple personality functions in
the same function).

  To the new EH scheme, they are indistinguishable from a "normal" catch.
>
>> To get a cleanup add the "cleanup" flag to the landingpad instruction.  That
>> said, I'm ruminating on whether the cleanup flag should be removed: instead
>> there would always be an implicit cleanup.  As an optimization, the code
>> generators would not output a cleanup into the exception handling tables if
>> no useful code is executed when the cleanup is run.  This seems to be what
>> recent versions of GCC do.
>>
> I'm not a big fan of implicit behavior. :)

It would be explicitly documented, so then it wouldn't be implicit!  In fact
it would mean that "invoke" has the traditional LLVM semantics: if an exception
is unwound then control branches to the landing pad.  With the new scheme that
is currently no longer so - meaning that some of the PruneEH logic is currently
wrong :)  That said, I'm also a bit dubious about this "always has a cleanup"
idea.

  And it requires an optimization to "cleanup" (yeah, a pun...sue me) the 
extraneous code we generate, which won't happen at -O0 (right?).

The only effect of this optimization would be to not output a cleanup entry (0)
in the action table if its not needed.  That's pretty mild, and at -O0 who cares
if there are some pointless but harmless (except for slowing down unwinding)
cleanup entries in the action table?  I'm not sure what you mean by "the
extraneous code we generate".

  Though the optimization you mentioned here would be a nice thing to have with 
our current scheme.

My plan is to implement the optimization (clearing of the cleanup flag on the
landingpad instruction) at the IR level as a warm-up.  If it seems like a good
idea to eliminate the cleanup flag from the definition of landingpad instruction
then the logic can be moved to a codegen analysis instead, and used to see if
we can avoid adding a cleanup entry to the landing pad table.  Note that I
already implemented the optimization that if all a landing pad does is call
"resume", then the invoke+landingpad is removed in favour of a call, in case
that is what you are thinking of.

Ciao, Duncan.