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

[Mike Stump]

On Wednesday, May 7, 2003, at 03:19 AM, Ranjit Mathew wrote:
> Now I have an issue somewhat related to this
> discussion that I hope someone would be able to
> shed some light on: on Windows, the Cygwin/MinGW
> targets have had to give up on DW2 EH in favour of
> SJLJ purely because exceptions could not be
> thrown from a callback function, across the
> Windows Event Dispatcher stack to the handler
> of the exception.

This can be made to work given an integrated sjlj/dwarf2 EH scheme.  At 
transition points into and out of code compiled by other compilation 
systems (or -fno-exception code), we introduce a sjlj cleanup point 
there.  For, take for example:

foo() {
   DispatchMessage();
}

we have to know that DispatchMessage is going to go to `outside', 
imagine if you will:

#pragma gcc EH doesnt have unwind tables: DispatchMessage

foo() {
   ...
   DispatchMessage();
   ...
}

then it is a simple process to generate:

foo() {
	...
	sjljtry {
		DispatchMessage();
	} catch (...) {
		rethrow;
	}
	...
}

instead.  We can get back `into' the unwind tables by doing the lj back 
to this point.

> In fact, this seems to be a fundamental limitation
> of the DW2 EH mechanism and not just on Windows.

An unimplemented feature, I'd say, not a fundamental limitation.  
Though, such a change would be probably have ABI impact, as only the 
enhanced thrower would be able to fall back to doing the throw when it 
runs out of unwind information.