[llvm-dev] GlobalsAA from GVN

[Mehdi Amini via llvm-dev]

> On Dec 3, 2015, at 4:21 AM, Vaivaswatha Nagaraj via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Thank you both for the inputs. I've created a patch for the same, please review:
> http://reviews.llvm.org/D15185 <http://reviews.llvm.org/D15185>
> 
> >You can specifically insert it into the pass pipeline, but in this case, we should just fix SLP to preserve the analysis.
> @Hal, I tried doing this but it didn't seem to work. I added the GlobalsAA pass right before GVN but it didn't seem to help. In retrospect, looking at the way pass manager works, it looks like it wasn't of much use. Since GlobalsAA is a ModulePass, and FunctionPasses that are scheduled even later are run first

I don’t expect this: the PassManager should honor the order you insert passes. 
I confirmed with: opt -debug-pass=Structure  -globals-aa -instcombine -slp-vectorizer -globals-aa -gvn

 ModulePass Manager
    CallGraph Construction
    Globals Alias Analysis
    FunctionPass Manager
      Basic Alias Analysis (stateless AA impl)
      Function Alias Analysis Results
      Dominator Tree Construction
      Combine redundant instructions
      Natural Loop Information
      Scalar Evolution Analysis
      Function Alias Analysis Results
      SLP Vectorizer
    CallGraph Construction
    Globals Alias Analysis
    FunctionPass Manager
      Dominator Tree Construction
      Basic Alias Analysis (stateless AA impl)
      Function Alias Analysis Results
      Memory Dependence Analysis
      Global Value Numbering
      Module Verifier


— 
Mehdi




> , those FunctionPasses that invalidate it do so for good. I do not know if there is some other way to force it though.
> 
> Thanks,
> 
>   - Vaivaswatha
> 
> On Thu, Dec 3, 2015 at 4:53 PM, Hal Finkel <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
> ----- Original Message -----
> > From: "Vaivaswatha Nagaraj via llvm-dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> > To: "James Molloy" <james at jamesmolloy.co.uk <mailto:james at jamesmolloy.co.uk>>
> > Cc: "LLVM Dev" <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> > Sent: Thursday, December 3, 2015 5:20:34 AM
> > Subject: Re: [llvm-dev] GlobalsAA from GVN
> >
> > Hi James,
> >
> > Thanks for the help. From the log, I could infer that SLP vectorizer
> > is not preserving alias analysis, preventing GVN from getting the
> > info. Although the first function to get compiled has GlobalsAA
> > available during GVN, rest of them do not as SLP vectorizer run on
> > that function invalidates GlobalsAA which is a module pass. Is there
> > a way to force re-computation of a particular analysis?
> 
> You can specifically insert it into the pass pipeline, but in this case, we should just fix SLP to preserve the analysis.
> 
>  -Hal
> 
> >
> > As a side note, I didn't have this problem on release_37.
> >
> > Thanks a lot.
> >
> > Regards,
> >
> >
> >
> >
> >
> > - Vaivaswatha
> >
> >
> > On Thu, Dec 3, 2015 at 3:02 PM, James Molloy <
> > james at jamesmolloy.co.uk <mailto:james at jamesmolloy.co.uk> > wrote:
> >
> >
> >
> > Hi Vaivaswatha,
> >
> >
> > GlobalsAA is not an immutable pass because it needs to cache queries
> > to avoid them being unusably slow. It therefore relies on passes
> > explicitly preserving it. Most of the passes in the scalar pipeline
> > have been modified to setPreserved<GlobalsAA>() and I know the pass
> > gets preserved at least until LICM.
> >
> >
> > You can use -debug-pass=Executions to determine at what point
> > GlobalsAA is not preserved (which pass clobbers it) - you'd then
> > need to add GlobalsAA to its preserved pass list.
> >
> >
> > Cheers,
> >
> >
> > James
> >
> >
> >
> >
> > On Wed, 2 Dec 2015 at 09:35 Vaivaswatha Nagaraj via llvm-dev <
> > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > wrote:
> >
> >
> >
> >
> >
> >
> >
> >
> > Hi,
> >
> > I've noticed that alias analysis queries arising from GVN do not use
> > the results from GlobalsAA.
> > The last call to AAResultsWrapperPass::runOnFunction() before GVN
> > does not add GlobalsAAWrapperPass due to unavailability. This leads
> > to the alias queries from GVN not having any globals mod-ref info.
> >
> > Is this a known issue? and is there any way to have globals mod-ref
> > info available for GVN?
> >
> > Thanks,
> >
> >
> >
> >
> >
> >
> >
> >
> > - Vaivaswatha
> > _______________________________________________
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> >
> >
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> >
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
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