[llvm-dev] Question about Alias Analysis with restrict keyword

[jingu@codeplay.com via llvm-dev]

Ah, the recursive phi and gep caused the may-alias... Eli made my brain 
re-work. Thank you very much! To be sure, If we add "-basicaa-recphi" 
option, the example does not generate may-alias with restrict keyword.


On 19/06/18 10:33, jingu at codeplay.com via llvm-dev wrote:
> Hi Eli,
>
> I appreciate your good comments. It is very helpful.
>
> Thanks again,
>
> JinGu Kang
>
>
> On 18/06/18 23:17, Friedman, Eli wrote:
>> On 6/18/2018 8:16 AM, jingu at codeplay.com wrote:
>>>
>>> On BasicAAResult::aliasGEP()
>>>
>>> 1297     // Check to see if these two pointers are related by the 
>>> getelementptr
>>> 1298     // instruction.  If one pointer is a GEP with a non-zero 
>>> index of the other
>>> 1299     // pointer, we know they cannot alias.
>>> 1300
>>> 1301     // If both accesses are unknown size, we can't do anything 
>>> useful here.
>>> 1302     if (V1Size == MemoryLocation::UnknownSize &&
>>> 1303         V2Size == MemoryLocation::UnknownSize)
>>> 1304       return MayAlias;
>>
>> The check for UnknownSize is here because it's impossible to 
>> disambiguate the offsets of two pointers which both have unknown 
>> size, so the rest of BasicAAResult::aliasGEP can't do anything useful.
>>
>>> 1305
>>> 1306     AliasResult R = aliasCheck(UnderlyingV1, 
>>> MemoryLocation::UnknownSize,
>>> 1307                                AAMDNodes(), V2, 
>>> MemoryLocation::UnknownSize,
>>> 1308                                V2AAInfo, nullptr, UnderlyingV2);
>>>
>>> On line 1306, we can see the V1Size and V2Size are set up with 
>>> "MemoryLocation::UnknownSize" and it causes may-alias. On line 1302 
>>> and 1033, there are checking code for 
>>> "MemoryLocation::UnknownSize"... I am not sure whether it is bug or 
>>> not... Can someone let me know why it passes 
>>> "MemoryLocation::UnknownSize" please? If I missed something, please 
>>> let me know.
>>>
>>
>> UnderlyingV1 is the base of the GEP.  So this is asking whether the 
>> base of the GEP aliases V2.  It uses UnknownSize because the offset 
>> is unknown; it can't tell which bytes are accessed, so it's using an 
>> approximation instead.
>>
>> We could theoretically try to be a bit more clever here if the size 
>> is known and the GEP offset is constant: we could instead pass 
>> V1Size+GEP1BaseOffset.  Not sure that would have much effect in 
>> practice, though.
>>
>> -Eli
>>
>

-- 
JINGU KANG
Software Engineer, Compilers
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