[llvm-dev] PartialAlias: different start addresses

Daniel Berlin via llvm-dev llvm-dev at lists.llvm.org
Sun Jul 16 15:46:10 PDT 2017

On Sun, Jul 16, 2017 at 2:58 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

> 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.

In particular, this would mean the documentation for partialalias *is*
correct :P.
IE it will only return partial if the starting addresses are not the same.

I actually would have expected something like:

if start(a) == start (b):
  if sizes equal:
    return mustalias
    return partialalias
if otherwise overlap
   return partial
return may

That said, i feel like both our existing code is trying to encode too much
info into these simple enum answers.

Maybe we should really be having something like:

enum aliaskind {
MustAlias,  // Exactly the same start and size
enum overlapkind {
FullOverlap, // Either the accesses are the same size, or one fully covers
the other
PartialOverlap, // They partially cover each other
Unknown // No idea

std::pair<aliaskind, overlapkind> alias(A, B) {
if (start(A) == start(B))
  if sizes unknown:
     return {PartialAliasSameStart, Unknown}
  if sizes equal:
     return {MustAlias, FullOverlap}
  if one smaller than the other
     return {PartialAliasSameStart, FullOverlap}

Then we don't have to muck around in the definitions of must/partial/may
too much, and the clients don't have to do too much when, for example, the
CSE/DSE like ones really want to know either "are these accessing exactly
the same memory location" *or* "can i eliminate this load/store in favor of
of an earlier/later one, maybe with some masking".

But maybe this doesn't need cleaning up.
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