[llvm-dev] PartialAlias: different start addresses

Nuno Lopes via llvm-dev llvm-dev at lists.llvm.org
Sun Jul 16 12:45:18 PDT 2017

>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).

Sorry for the delay.
I'm not sure I understood what you wrote, sorry.  What you wrote is true in 
general, but I don't see how MustAlias in particular is worse than the other 
AA results.  And how not enforcing that access sizes are equal changes 
things with respect to commutativity either.  Maybe I completely missed your 


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