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

[Duncan Sands]

Hi Talin,

> 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.

  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).

  In the old scheme, a selector value of 0 was
> used.

Technically speaking that's a cleanup not a catch-all.  C++ illustrates the
difference: if you throw an exception in a C++ program that only has cleanups
then the exception won't be unwound, instead the C++ unwinder code will just
terminate your program; however if there is a C++ catch-all (represented by
a catch with a null pointer in LLVM) then the exception will be unwound and
the landing pad branched to in the expected way.  The C++ catch-all is
represented by a catch clause with a null pointer argument in LLVM, but this
is C++ specific.  Note that the cleanup behaviour I mentioned is somewhat
built into the GCC unwinding library: when you call _Unwind_RaiseException,
if the search phase discovers that nothing matches the exception except for
cleanups all the way up the stack, then the call to _Unwind_RaiseException
returns (otherwise control branches to the landing pad that matched the
exception).  If you want cleanups to be run anyway, then you need to call
_Unwind_ForcedUnwind if _Unwind_RaiseException returns.  This is in contrast
to what happens if there is a catch-all: the call to _Unwind_RaiseException
will see that the exception matches the catch-all (because the personality
function says so) and it will cause execution to resume at the corresponding
landing pad.

  What's the corresponding technique for the new scheme?

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.

  Do I need to update
> my personality function to handle catch-alls (it wasn't needed in the previous
> scheme)?

Ciao, Duncan.