Do not produce empty try-finally statements

[Jan Hubicka]

> 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.  

In that case I think I shoud look into detecting clobber only EH in inliner and
do not account it into the size/time estimates.  I always wondered why tramp3d
becomes so harder for inliner with EH enabled, I seem to get it now ;)

> (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.

It is also problem of inliner quality decisions and memory use/compile time.
The in-memory representation of unnecesary EH is quite big.

I am quite ignorant in this area, but for -O0 can't we simply disable all
clobbers?

Honza
> 
> 
> Ciao,
> Michael.