[llvm-dev] PartialAlias: different start addresses

Daniel Berlin via llvm-dev
Sun, 16 Jul 2017 20:27:07 -0000

On Sun, Jul 16, 2017, 12:45 PM Nuno Lopes <nunoplopes at sapo.pt> 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)

Hence my question about whether it checked whether *either* was smaller, or
just in one direction.

  And how not enforcing that access sizes are equal
changes
> things with respect to commutativity either.  Maybe I completely missed
> your
> point..
>
> Nuno
>
>
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