[llvm-dev] PartialAlias: different start addresses

Daniel Berlin via llvm-dev
Sun, 16 Jul 2017 21:58:58 -0000

On Sun, Jul 16, 2017 at 2:34 PM, Nuno Lopes <nunoplopes at sapo.pt> wrote:

> On Sun, Jul 16, 2017, 12:45 PM Nuno Lopes 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).
>>>>
>>>
>>> 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.
>>>
>>
>> Historically, in llvm, we have guaranteed that alias(a, b) ==alias(b,
a)
>>
>> If it does:
>>
>> If start (a) == start (b)
>>   If size(b) < size(a)
>>       Return mustalias
>>   Return may or partial
>>
>> It will give different answers for alias(a, b) and alias (b, a)
>>
>
> Wait, but the size check is done in the clients, not in AA.
> AA only does (according to my understanding):
>
> If start (a) == start (b)
>     Return mustalias
> Return may or partial
>
>
That seems ... wrong to me, but i haven't thought very hard about it.
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