[LLVMdev] alias result

[Hal Finkel]

----- Original Message -----
> From: "Haopeng Liu" <hyliuhp at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Monday, February 16, 2015 1:59:44 PM
> Subject: Re: [LLVMdev] alias result
> 
> Oh, got it. Thanks for your explain. I misunderstand what getLocation
> does.
> 
> If I would like to get the result of alias analysis referring to the
> stored value, what should I do in llvm?

if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
  Value *V = SI->getValueOperand();
  if (V->getType()->isPointerTy()) {
    // Now you know that you're storing a pointer-typed value V.
    Loc = AliasAnalysis::Location(V);
  }
}

 -Hal

> 
> 
> On 2/16/15 1:42 PM, Hal Finkel wrote:
> > ----- Original Message -----
> >> From: "Haopeng Liu" <hyliuhp at gmail.com>
> >> To: "Hal Finkel" <hfinkel at anl.gov>
> >> Cc: llvmdev at cs.uiuc.edu
> >> Sent: Monday, February 16, 2015 1:38:40 PM
> >> Subject: Re: [LLVMdev] alias result
> >>
> >> Yes, in my example, %1 and %2 point to t. %3 points to t2.
> >>
> >> But t and t2 point to the same var, is it? That's where I'm
> >> confused.
> > So you believe that:
> >>>>>> (1): store i64* %thd, i64** %t, align 8    //int64 *t = &thd;
> > ...
> >>>>>> (3): store i64* %1, i64** %t2, align 8     //int64 *t2 = t;
> > calling AA.getLocation on these two instructions, and then calling
> > alias on them should not return NoAlias? I think your confusion is
> > regarding what getLocation does. It is not referring to the stored
> > value, but the location where those values are being stored (%t
> > and %t2 in this case). Those won't alias, because %t and %t2 point
> > to disjoint local stack locations.
> >
> >   -Hal
> >
> >>
> >> On 2/16/15 12:37 PM, Hal Finkel wrote:
> >>> ----- Original Message -----
> >>>> From: "Haopeng Liu" <hyliuhp at gmail.com>
> >>>> To: "Hal Finkel" <hfinkel at anl.gov>
> >>>> Cc: llvmdev at cs.uiuc.edu
> >>>> Sent: Monday, February 16, 2015 12:37:06 PM
> >>>> Subject: Re: [LLVMdev] alias result
> >>>>
> >>>> t and t2 are two pointers defined in .c file.
> >>>>
> >>>> The definitions in .bc are:
> >>>> %thd = alloc i64, align 8
> >>>> %t = alloca i64*, align 8
> >>>> %t2 = alloca i64*, align 8
> >>>>
> >>>> .c file likes this:
> >>>> int thd;
> >>>> int *t = &thd;
> >>>> int *t2 = t;
> >>> Okay, those point to distinct local stack allocations. Why do you
> >>> expect them to alias?
> >>>
> >>>    -Hal
> >>>
> >>>> On 2/16/15 12:28 PM, Hal Finkel wrote:
> >>>>> ----- Original Message -----
> >>>>>> From: "Haopeng Liu" <hyliuhp at gmail.com>
> >>>>>> To: llvmdev at cs.uiuc.edu
> >>>>>> Sent: Monday, February 16, 2015 12:12:18 PM
> >>>>>> Subject: [LLVMdev] alias result
> >>>>>>
> >>>>>> Hi all,
> >>>>>>
> >>>>>> I am implementing a pass which needs aliasanalysis in llvm.
> >>>>>>
> >>>>>> My pass firstly records all store or load instructions as
> >>>>>> follows:
> >>>>>> loc[cnt++] = AA.getLocation(si/li);
> >>>>>>
> >>>>>> Then, get each pairwise alias result.
> >>>>>> AliasAnalysis::AliasResult ar = AA.alias(loc[i], loc[j]);
> >>>>>> switch(ar){ case 0,1,2,3;}
> >>>>>>
> >>>>>> And the command is: opt -load mypass.so -mypass -basicaa
> >>>>>> test.bc
> >>>>>>
> >>>>>> The store/load in test.bc:
> >>>>>>
> >>>>>> (1): store i64* %thd, i64** %t, align 8    //int64 *t = &thd;
> >>>>>> (2): %1 = load i64** %t, align 8             //load t
> >>>>>> (3): store i64* %1, i64** %t2, align 8     //int64 *t2 = t;
> >>>>>> (4): %2 = load i64** %t, align 8             //load t
> >>>>>> (5): %3 = load i64** %t2, align 8           //load t2
> >>>>>>
> >>>>>> It seems that all these 5 instructions should be aliased to
> >>>>>> each
> >>>>>> other.
> >>>>>>
> >>>>>> But the result should that only (1,2) (1,4) (2,4) (3,5) are
> >>>>>> must
> >>>>>> alias.
> >>>>>> Others are no alias.
> >>>>> Where do %t and %t2 come from?
> >>>>>
> >>>>>     -Hal
> >>>>>
> >>>>>> Who can explain these results? Any hits would be appreciated.
> >>>>>> Thank
> >>>>>> you.
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> _______________________________________________
> >>>>>> 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