[LLVMdev] noalias and alias.scope metadata producers

[Hal Finkel]

----- 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?

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