[LLVMdev] alias result

[Haopeng Liu]

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.


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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>>