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

[jingu@codeplay.com via llvm-dev]

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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