llvm dev - Aug 2012 - [LLVMdev] Creating DAGs

All,

I apologize if this is an inappropriate question for this mailing
list.  If so, please recommend an appropriate place to post the
question.  I'm also somewhat new to LLVM, so I could have some pretty
fundamental misunderstandings about what I am trying to do.  I have
searched for information on the llvm.org website (user's guide,
programmer's manual and doxygen documentation), and looked through the
3.1 source code.  I have also searched the archives of this list.

My question is: Is it possible to create a DAG from an llvm::Module?
I would like to do this independent of a specific architecture using
LLVM IR.  In particular, I am interested in using a DAG to compute the
Strahler number (http://en.wikipedia.org/wiki/Strahler_number) of the
graph to determine the minimum number of register needed to evaluate
an expression tree.

I have read through the documentation on SelectionDAG.  However, this
seems to require a TargetMachine.  Is there a way to create a graph
without a target?  This may be a fundamental lack of understanding on
my part, if so please point me to something that might clarify.

Best Regards,
Ben

--
Dr.-Ing. Benjamin Karl Bergen
Los Alamos National Laboratory
P.O. Box 1663, MS B287
Los Alamos, NM 87545

(505) 465-9316 (m)
(505) 665-4939 (f)

On Wed, Aug 08, 2012 at 11:24:27 -0600, Ben Bergen
wrote:
> I apologize if this is an inappropriate question for this mailing
> list.
This is the correct list.
> My question is: Is it possible to create a DAG from an llvm::Module?
> I would like to do this independent of a specific architecture using
> LLVM IR.  In particular, I am interested in using a DAG to compute the
> Strahler number (http://en.wikipedia.org/wiki/Strahler_number) of the
> graph to determine the minimum number of register needed to evaluate
> an expression tree.
If you are actually interested in processor registers, I don't think it
makes sense to perform this computation at the LLVM IR level. The backend
changes the code too much to map such results from the IR to the generated
machine code, regardless of the platform. Are you after processor registers?
Then the ScheduleDAG you found is the one to use.

-- 
Gergö Barany, research assistant                 gergo at complang.tuwien.ac.at
Institute of Computer Languages      http://www.complang.tuwien.ac.at/gergo/
Vienna University of Technology                       Tel: +43-1-58801-58522
Argentinierstrasse 8/E185, 1040 Wien, Austria         Fax: +43-1-58801-18598