[LLVMdev] GC questions.

Tobias Nurmiranta spyck at lysator.liu.se
Thu Jul 22 04:57:57 PDT 2004

On Thu, 22 Jul 2004, Chris Lattner wrote:
> > While I was editing LowerGC.cpp I made a little test (not part of this
> > patch, but the diff with LowerGC.cpp in cvs is attached). I've added a new
> > intrinsic called llvm.gcroot_value(sbyte*, sbyte*), which takes a pointer
> > directly instead and transforms it into an alloca. The idea is the
> ...
> > In this way, the frontends don't have to explictly alloca the gcroots,
> > but instead just use the intrinsic, which hides how the gcroots are
> > implemented.
> Unfortunately this won't really work.  It might be reasonable to include
> something like this in the front-end that you're working on, but including
> this at the LLVM level is a bad idea.  The basic idea of the alloca model
> is that it directly exposes to the optimizer the fact that GC pointers may
> change at unpredictable times, which is something that memory can do, but
> SSA registers do not do.

Ok. But if the gcroot_value transformation in done before any
optimizations, I get the same result as if I had used gcroot and alloca.
So maybe I can move it to a independent pass which I then use with my

Hmm, it felt nicer to mark raw LLVM pointers as roots and then transform
them, since I then didn't have to see the overhead in my frontend :).

Btw, how hard would it be to implement the system described in this
paper[1], support for collection at every instruction? You would need to
get the gc root information with you all the way down to machine code and
register allocation. Just curious :).

,	Tobias

[1] http://portal.acm.org/citation.cfm?id=301652&dl=ACM&coll=portal

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