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Re: Do not produce empty try-finally statements
- From: Michael Matz <matz at suse dot de>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: Jan Hubicka <hubicka at ucw dot cz>, Jakub Jelinek <jakub at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>, Jason Merrill <jason at redhat dot com>
- Date: Tue, 28 Jan 2014 13:22:06 +0100 (CET)
- Subject: Re: Do not produce empty try-finally statements
- Authentication-results: sourceware.org; auth=none
- References: <20140119224609 dot GB22057 at kam dot mff dot cuni dot cz> <20140120070735 dot GT892 at tucnak dot redhat dot com> <69409e4c-102f-4aaf-b79b-9ec982348162 at email dot android dot com> <20140120160717 dot GB4030 at kam dot mff dot cuni dot cz> <20140120161642 dot GB892 at tucnak dot redhat dot com> <20140120162243 dot GC4030 at kam dot mff dot cuni dot cz> <CAFiYyc2sQXF2AhwRUo5c1myeysgWDzLyMczkbHqKbROUvXtRKg at mail dot gmail dot com>
Hi,
On Tue, 28 Jan 2014, Richard Biener wrote:
> >> The EH optimizations involving cleanups with only clobbers in them
> >> are that if at the end of the cleanup after only CLOBBER stmts you
> >> would rethrow the exception externally, then the clobber isn't needed
> >> and the whole cleanup can be removed. And, if it rethrows somewhere
> >> internally, we can move the clobber stmts to the landing pad of
> >> wherever it would be caught.
> >
> > OK, I still do not see how ehclanup1 can then safely remove them
> > pre-inline given that the whole function body can be inlined into
> > another containing the outer EH region.
>
> That's true.
Yes, and I think they should be removed only after inlining.
(Alternatively the inliner could be extended to add clobbers when changing
an external-throw into an internal-throw, but well, ...)
> > If this is valid, why we can not just eliminate EH in those outer
> > clobber try..finally as part of ehlowering earlier?
>
> Probably we'd miss too many inlining cases from early inlining and the
> ehcleanup1 time is just a heuristic that works good enough for us?
No, removing even more clobbers would remove even more stack slot sharing.
If anything we should remove _less_ regions (as in the precondition for
Honzas sentence above, "If this is valid, ..." simply is false). AFAIU
this all is just a problem with O0 code quality. So, there's the obvious
solution: run ehcleanup also for O0.
Ciao,
Michael.