[LLVMdev] alias analysis in backend

[Jonas Paulsson]

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 .. ?

/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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