Re: __attribute__((cleanup(function)) versus try/finally

["Ranjit Mathew" <rmathew at hotmail dot com>]

> > The same issue was pointed out (albeit in passing)
> > when Aldy Hernandez had submitted his __try/__finally
> > patch to GCC for C.
>
> And I responded at the time claiming that the semantics of
> the two extensions are identical.

Very similar, but not identical, AFAICT:

   http://msdn.microsoft.com/library/en-us/vccelng/htm/state_14.asp

The __finally block is executed in case of both "normal" C++ exceptions
as well as "faults" such as accessing a NULL pointer, dividing by zero,
etc.

Of course this would tie in with integrating SEH support into GCC,
a brave attempt at which can be found at:

   http://reactos.wox.org/index.php?page=gccseh

in which case my point becomes moot.


> > Since the Win32 API stack does not have DW2 EH
> > unwind information, the program just terminates
> > when an exception is thrown (if using DW2 EH).
> >
> > SJLJ does not have any such problems.
> >
> > In fact, this seems to be a fundamental limitation
> > of the DW2 EH mechanism and not just on Windows.
>
> Yes.  Such is the price you pay for not saving all of the
> registers up front when an exception is *not* thrown.

IIRC, till some time ago "-fsjlj-exceptions" used to be available
that should allow such fringe cases to work albeit with a
performance hit.

Otherwise DW2 EH is *way better* than SJLJ as you all
know very well and should be the default.

Does anyone know why "fsjlj-exceptions" support was
dropped and made a configure time option?


> > MD_FALLBACK_FRAME_STATE_FOR looks a bit promising
> > but how do we generalise it to any "foreign" caller?
>
> Eh, MD_FALLBACK_FRAME_STATE_FOR isn't really generalizable.
> You could perhaps hack it to parse SEH unwind data, but...

SEH is an SJLJ type scheme:

http://www.microsoft.com/msj/0197/Exception/Exception.aspx


> Another possibility is to give up on unwind-dw2.c entirely
> and write a new unwinder that uses the SEH bits in the Win32
> API directly, all the time.  Then (1) you'd be able to unwind
> through Windows routines and (2) there'd be no "confusion"
> about the supposed "differences" in try/finally.

Possible, but it feels awful to throw away the fantastic DW2 EH...

:-(

Ranjit.

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