[llvm-dev] PartialAlias: different start addresses

[Daniel Berlin via llvm-dev]

On Sat, Jul 15, 2017 at 5:35 AM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 07/15/2017 04:51 AM, Nuno Lopes wrote:
>
>> On 07/14/2017 04:37 PM, Nuno Lopes wrote:
>>>
>>>> Thank you all for your replies.
>>>> So here seems to be an agreement that the documentation for
>>>> PartialAlias is incorrect.
>>>>
>>>> Daniel: now you got me wondering about MustAlias. This is what the docs
>>>> say:
>>>> "The MustAlias response may only be returned if the two memory objects
>>>> are *guaranteed to always start at exactly the same location*"
>>>>
>>>> This statement is regardless of the access sizes.  For example, in SCEV
>>>> AA:
>>>> // If they evaluate to the same expression, it's a MustAlias.
>>>> if (AS == BS)
>>>>  return MustAlias;
>>>>
>>>> AS/BS are scev expressions for the pointers. So no check for the access
>>>> size.
>>>>
>>>> So, does must needs to check for access sizes?  If so, SCEV AA is buggy
>>>> and the documentation needs tweaking.
>>>>
>>>
>>> I'm under the impression that there is code that depends on the size
>>> check, but I don't trust my recollection in this regard. SCEV AA is known
>>> to cause miscompiles, IIRC, maybe you just found out why ;)
>>>
>>
>> It's true that the CFL AAs have this code:
>> if (LocA.Ptr == LocB.Ptr)
>>  return LocA.Size == LocB.Size ? MustAlias : PartialAlias;
>>
>>
>> I grepped for clients of MustAlias:
>> ~/llvm/lib/Transforms $ grep -Rl MustAlias .
>> ./ObjCARC/ObjCARCOpts.cpp
>> ./ObjCARC/ProvenanceAnalysis.cpp
>> ./Scalar/DeadStoreElimination.cpp
>> ./Scalar/GVN.cpp
>> ./Scalar/LICM.cpp
>> ./Scalar/LoopVersioningLICM.cpp
>> ./Scalar/MemCpyOptimizer.cpp
>> ./Scalar/MergedLoadStoreMotion.cpp
>> ./Scalar/NewGVN.cpp
>> ./Utils/VNCoercion.cpp
>>
>> I glanced over all the uses in these files and I couldn't find any usage
>> that requires sizes to match.  Actually most clients check access sizes
>> themselves. Most don't need equal sizes, just need one to be smaller than
>> the other.
>>
>
Does aliasing actually check both ways?
Otherwise, alias (A, B) will give different results than alias (B, A).
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