[LLVMdev] alias result

[Haopeng Liu]

Thanks for your suggestion. I tried your method.

%thd = alloc i64, align 8
%t = alloc i64*, align 8
%t2 = alloc i64*, align8

1). store i64* %thd, i64** %t, align 8         //int64 *t = &thd;
2). store i64* %thd, i64** %t2, align 8      // int64 *t2 = &thd;

It can find that t and t2 point to the same var by using 
getValueOperand() and isPointerTy(). Then judge that %t and %t2 are must 
alias.

But another example is that:

%thd = alloc i64, align 8
%t = alloc i64*, align 8
%t2 = alloc i64*, align8

1). store i64* %thd, i64** %t, align 8         //int64 *t = &thd;
2). %1 = load i64** %t, align 8
3). store i64* %1, i64** %t2, align 8        // int64 *t2 = t; (%t2=%1=%t)

How to judge that (%t, %1, %t2) are must alias in this case?

-Haopeng


On 2/16/15 2:38 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 1:59:44 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.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
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>>>>>>>>
>>