[LLVMdev] alias result

[Haopeng Liu]

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?


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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>>>>>>
>>