[LLVMdev] alias analysis in backend

[Hal Finkel]

----- Original Message -----
> From: "Jonas Paulsson" <jonas.paulsson at ericsson.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Wednesday, April 17, 2013 12:22:49 AM
> Subject: RE: [LLVMdev] alias analysis in backend
> 
> Hi Hal,
> 
> Thanks. How about a symbol with two different immediate offsets - the
> Value* would be the same, right? I don't see how
> AliasAnalysis::Location would handle this... And BasicAliasAnalysis
> does
> 
> if (V1 == V2) return MustAlias;
> 
> , so I'm not sure how this would be done .. ?

If you run with -enable-misched -enable-aa-sched-mi 

then you'll get this logic from the end of MIsNeedChainEdge:

  // The following interface to AA is fashioned after DAGCombiner::isAlias
  // and operates with MachineMemOperand offset with some important
  // assumptions:
  //   - LLVM fundamentally assumes flat address spaces.
  //   - MachineOperand offset can *only* result from legalization and
  //     cannot affect queries other than the trivial case of overlap
  //     checking.
  //   - These offsets never wrap and never step outside
  //     of allocated objects.
  //   - There should never be any negative offsets here.
  //
...

  int64_t MinOffset = std::min(MMOa->getOffset(), MMOb->getOffset());
  int64_t Overlapa = MMOa->getSize() + MMOa->getOffset() - MinOffset;
  int64_t Overlapb = MMOb->getSize() + MMOb->getOffset() - MinOffset;

  AliasAnalysis::AliasResult AAResult = AA->alias(
  AliasAnalysis::Location(MMOa->getValue(), Overlapa,
                          MMOa->getTBAAInfo()),
  AliasAnalysis::Location(MMOb->getValue(), Overlapb,
                          MMOb->getTBAAInfo()));

  return (AAResult != AliasAnalysis::NoAlias);

 -Hal

> 
> /Jonas
> 
> 
> > -----Original Message-----
> > From: Hal Finkel [mailto:hfinkel at anl.gov]
> > Sent: Tuesday, April 16, 2013 7:35 PM
> > To: Jonas Paulsson
> > Cc: llvmdev at cs.uiuc.edu
> > Subject: Re: [LLVMdev] alias analysis in backend
> > 
> > ----- Original Message -----
> > > From: "Jonas Paulsson" <jonas.paulsson at ericsson.com>
> > > To: llvmdev at cs.uiuc.edu
> > > Sent: Tuesday, April 16, 2013 11:24:36 AM
> > > Subject: [LLVMdev] alias analysis in backend
> > >
> > >
> > >
> > >
> > >
> > > Hi,
> > >
> > >
> > >
> > > I would like to implement alias analysis in my backend. I would
> > > like
> > > to for example get the result that two stack-accesses with
> > > different
> > > offsets (indexes), would return noAlias. However, I'm somewhat
> > > confused as there is no notion of offset for the Location object.
> > >
> > >
> > >
> > > I would also like to call ScheduleDAGInstr::buildSchedGraph()
> > > with
> > > this AliasAnalysis and have MIsNeedsChainEdge() return false in
> > > this
> > > case.
> > >
> > >
> > >
> > > What should I do? Adding a MemoryOperand to such an instruction
> > > seems
> > > right, but it doesn't seem to fit quite. What Value would be
> > > referenced?
> > 
> > I think that they should have pseudo-source values, see:
> > include/llvm/CodeGen/PseudoSourceValue.h
> > 
> > I was under the impression that different pseudo source values from
> > different frame indices already have this no-alias property. If
> > they
> > don't, then this seems like a nice general improvement that would
> > benefit
> > all backends.
> > 
> >  -Hal
> > 
> > > BasicAliasAnalysis returns MustAlias for the same Value,
> > > e g 'Stack'.
> > >
> > >
> > >
> > > Should I implement a target AliasAnalysis, perhaps derived from
> > > BasicAliasAnalysis, and make it required for my pass that will be
> > > using it?
> > >
> > >
> > >
> > > If not, could I make this work with BasicAliasAnalysis by adding
> > > the
> > > right memory operands?
> > >
> > >
> > >
> > > Thanks,
> > >
> > >
> > >
> > > Jonas Paulsson
> > >
> > >
> > >
> > >
> > >
> > >
> > > _______________________________________________
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> > >
>