[LLVMdev] noalias and alias.scope metadata producers

Fri Sep 26 09:58:58 PDT 2014

----- Original Message -----
> From: "Samuel F Antao" <sfantao at us.ibm.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Johannes Doerfert" <doerfert at cs.uni-saarland.de>, "LLVM Dev" <llvmdev at cs.uiuc.edu>, "Tobias Grosser"
> <tobias at grosser.es>
> Sent: Friday, September 26, 2014 11:14:01 AM
> Subject: Re: [LLVMdev] noalias and alias.scope metadata producers
> 
> Hi Hal,
> 
> Thanks for the clarification.
> 
> Hal Finkel <hfinkel at anl.gov> wrote on 09/24/2014 06:42:10 PM:
> 
> > From: Hal Finkel <hfinkel at anl.gov>
> > To: Samuel F Antao/Watson/IBM at IBMUS
> > Cc: Tobias Grosser <tobias at grosser.es>, LLVM Dev
> > <llvmdev at cs.uiuc.edu>, Johannes Doerfert
> > <doerfert at cs.uni-saarland.de>
> > Date: 09/24/2014 06:42 PM
> > Subject: Re: [LLVMdev] noalias and alias.scope metadata producers
> > 
> > ----- Original Message -----
> > > From: "Samuel F Antao" <sfantao at us.ibm.com>
> > > To: "Johannes Doerfert" <doerfert at cs.uni-saarland.de>
> > > Cc: "Tobias Grosser" <tobias at grosser.es>, "Samuel F Antao"
> > <sfantao at us.ibm.com>, "LLVM Dev" <llvmdev at cs.uiuc.edu>
> > > Sent: Wednesday, September 24, 2014 3:28:25 PM
> > > Subject: Re: [LLVMdev] noalias and alias.scope metadata producers
> > > 
> > > 
> > > 
> > > Hal, Johannes,
> > > 
> > > 
> > > Thanks for the feedback. I have been digging into this a little
> > > bit
> > > more and was able to have some of this metadata being generated.
> > > Nevertheless, I am confused about the semantics of this metadata.
> > > Let me explain:
> > > 
> > > 
> > > I was expecting the alias metadata to complement the information
> > > that
> > > alias analysis passes compute. However, it seems that the alias
> > > information of the pointers used in memory instructions is
> > > assumed
> > > to be different form the information of these instructions
> > > themselves. Let me give you an example:
> > > 
> > > 
> > > 
> > > MayAlias: double* %arrayidx.i, double* %arrayidx6
> > > MayAlias: %4 = load double* %arrayidx.i, align 8, !tbaa !1,
> > > !alias.scope !7, !noalias !10 <-> store double %2, double*
> > > %arrayidx6, align 8, !tbaa !1
> > > 
> > > 
> > > becomes:
> > > 
> > > 
> > > 
> > > MayAlias: double* %arrayidx.i, double* %arrayidx6
> > > NoAlias: %4 = load double* %arrayidx.i, align 8, !tbaa !1,
> > > !alias.scope !8, !noalias !5 <-> store double %2, double*
> > > %arrayidx6, align 8, !tbaa !1, !alias.scope !5, !noalias !8
> > > 
> > > 
> > > after I annotate the store using arrayidx6 as not aliasing with
> > > the
> > > load using arrayidx.i. Shouldn't the alias information of the
> > > memory
> > > instructions be propagated to the used pointers by the alias
> > > analysis pass?
> > 
> > No, LLVM's current AA infrastructure does not do this kind of
> > backward inference from accesses to their pointers (you won't get
> > this from TBAA metadata either). Do you feel this would be useful?
> > 
> 
> My feeling is that the AliasAnalysis would be able to compute more
> accurate results if it propagates the information in the metadata. I
> see two advantages in doing that:
> 
> From a development viewpoint, it would be nice to have the alias
> information centralized in a single class and avoid more complex
> schemes to combine metadata and analysis information. E.g. LICM,
> gets the Alias Analysis, loads the metadata and forward the
> information to the tracker that combines both. Maybe there is a good
> reason to use this scheme that I am not aware, but using AA only
> would be more elegant and easier to maintain.
> From a performance viewpoint, the ability to make AA as accurate as
> possible by propagating the metadata information may result in
> significant improvements. In particular for LLVM IR clients that can
> infer a lot of alias information from given language properties,
> they can use that to heavily annotate the code. In those cases AA
> would be almost irrelevant as the metadata contain most information.
> 
> What do you think about this? If one thinks this is a good direction
> to go I'd be happy to help doing that.

I think that the benefit is very unclear; we derive software maintainability benefits from the current modular system, and even within this system, the different AA passes can feed information to each other.

Most users of AA are really interested in whether they can reorder, or prove redundant, some loads or stores or function calls, making the results returned for the pointers themselves somewhat secondary. If you'd like to argue for a different setup, you'd need to provide some clear use cases that the current infrastructure fails to address.

Thanks again,
Hal

> 
> Thanks again,
> Samuel
> 
> > -Hal
> > 
> > > Is this something that was not implemented (if so I'd
> > > be happy to work on this) or is my interpretation of the
> > > semantics
> > > wrong?
> > > 
> > > 
> > > Thanks again!
> > > Samuel
> > > 
> > > 
> > > 
> > > 
> > > 2014-09-19 21:34 GMT-04:00 Johannes Doerfert <
> > > doerfert at cs.uni-saarland.de > :
> > > 
> > > 
> > > Hey Samuel,
> > > 
> > > I'm not sure if this is interesting for you but maybe it is:
> > > 
> > > Polly can emit these metadata for loop nests we can analyze. It
> > > is
> > > based
> > > on runtime alias checks, thus versioning. We are currently fixing
> > > the
> > > bugs
> > > in the runtime alias check generation we merged yesterday but
> > > after
> > > that
> > > (or if you like earlier) I can submit the annotation patch for
> > > review.
> > > Some limited tests on polybench benchmarks (e.g., 3mm) without
> > > any
> > > polyhedral
> > > optimizations, thus only parallel and noalias annotations, showed
> > > up
> > > to
> > > 20% improvement.
> > > 
> > > Best regards,
> > > Johannes
> > > 
> > > 
> > > 
> > > On 09/19, Samuel F Antao wrote:
> > > > 
> > > > 
> > > > Hi all,
> > > > 
> > > > In LLVM language reference I read that one can use noalias and
> > > > alias.scope
> > > > metadata to provide more detailed information about pointer
> > > > aliasing.
> > > > However, I was unable to obtain any LLVM IR annotations using
> > > > this
> > > > metadata
> > > > from any LLVM optimization pass or Clang frontend (am I missing
> > > > something?).
> > > > 
> > > > If I understand it correctly, this information would complement
> > > > the
> > > > type-based alias information and whatever mechanisms the alias
> > > > analysis
> > > > passes in LLVM compute from the input program. I was wondering
> > > > if
> > > > the
> > > > coverage provided by these two components is already acceptable
> > > > or
> > > > if there
> > > > is work that can be done in LLVM IR clients like clang to
> > > > provide
> > > > more
> > > > information with proper noalias and alias.scope annotations.
> > > > 
> > > > Any comments?
> > > > 
> > > > Thanks!
> > > > Samuel
> > > 
> > > > _______________________________________________
> > > > LLVM Developers mailing list
> > > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > > 
> > > 
> > > --
> > > 
> > > Johannes Doerfert
> > > Researcher / PhD Student
> > > 
> > > Compiler Design Lab (Prof. Hack)
> > > Saarland University, Computer Science
> > > Building E1.3, Room 4.26
> > > 
> > > Tel. +49 (0)681 302-57521 : doerfert at cs.uni-saarland.de
> > > Fax. +49 (0)681 302-3065 :
> > > http://www.cdl.uni-saarland.de/people/doerfert
> > > 
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > > 
> > > 
> > > 
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > > 
> > 
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> > 
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory