implementing escape analysis

Thu Feb 16 17:44:00 GMT 2006

Andrew Haley wrote:
> Diego Novillo writes:
> > Andrew Haley wrote:
> > 
> > > http://www.research.ibm.com/people/g/gupta/escape.ps
> > > 
> > > But I probably wouldn't recommend you to do it in a gcj-specific way.
> > > gcj is merely the Java language front-end for gcc, and there may be a
> > > plan for escape analysis that will suit all gcc's languages that can
> > > use it.
> > > 
> > Oh, excellent. It's been in my wish list for a while. I agree with
> > Andrew, this analysis should not be done in the individual front ends.
> > At worst, I could envision langhooks if we needed to get some
> > FE-specific information.
> > 
> > Choi's paper is a good start. I remember it being quite reasonable, but
> > I am not aware of recent results in the literature. We already have
> > some primitive (well, embarrassing) support for determining escape
> > points. See tree-ssa-alias.c:is_escape_site.
>> I remember the weird thing about that paper (it's been a long time
> since I read it) is that it didn't talk much about inheritance. It
> seems to me that inheritance is one of the greatest problems for an
> object-oriented language...
>> > Essentially, a good escape analysis technique should probably be
> > incorporated at that point. I have not yet thought about the details,
> > but I'd be happy to help with the implementation.
>> In theory it doesn't look all that difficult do do in the simple
> cases, but a good implementation would need a lot of knowledge about
> the way the class library behaves, and therefore what might escape. A
> database, I suppose. But just a skeleton implementation that does a
> few simple things would be nice.
>
Is is possible to do a conversion of a heap allocation to a stack 
allocation in a language independent manner in GCC?
I keep threatening to do this for gcj, but was thinking that the best 
place to do it may be the front end. Once in tree-ssa form it is more 
difficult to figure out that a call to _Jv_AllocObject followed by a 
call to the constructor is the equivalent of the 'new' operator.
Also replacing calls to a general purpose type 
(StringBuilder/StringBuffer) to an optimized version 
(gnu.gcj.runtime.StringBuffer) requires very large changes to the 
tree-ssa form, but in the front-end it is quite simple.
The ability to do a good job of both of these transformations has to be 
based on (at least a simplified form of) escape analysis of the runtime 
library.
David Daney