llvm dev - Jul 2017 - PartialAlias: different start addresses

Hi,

I going through the alias analysis documentation 
(http://llvm.org/docs/AliasAnalysis.html) and noticed the following in the 
definition of PartialAlias:
"
The PartialAlias response is used when the two memory objects are known to 
be overlapping in some way, but *do not start at the same address*.
"

Is it really required that the objects do no start at the same address?  if 
that's the case the AA algorithm would need to prove that.
I'm asking this because:
1) This condition seems very strict and I don't think it's met in a few 
places I found by manual inspection
2) I couldn't find any client that uses this information in a meaningful 
way. Is there a use case for PartialAlias w/ strict distinct starting 
addresses?

Thanks,
Nuno

On Fri, Jul 14, 2017 at 12:50 PM, Nuno Lopes via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I going through the alias analysis documentation
> (http://llvm.org/docs/AliasAnalysis.html) and noticed the following in the
> definition of PartialAlias:
> "
> The PartialAlias response is used when the two memory objects are known to
> be overlapping in some way, but *do not start at the same address*.
> "
>
> Is it really required that the objects do no start at the same address?  if
> that's the case the AA algorithm would need to prove that.
> I'm asking this because:
> 1) This condition seems very strict and I don't think it's met in a
few
> places I found by manual inspection
If I read the definition correctly, at least our Andersens' AA
implementation violates it.
see:

AliasResult CFLAndersAAResult::alias(const MemoryLocation &LocA,
                                     const MemoryLocation &LocB) {
  if (LocA.Ptr == LocB.Ptr)
    return LocA.Size == LocB.Size ? MustAlias : PartialAlias;


(i.e. the two objects are overlapping here *and* start at the same address).

On 07/14/2017 03:00 PM, Davide Italiano via llvm-dev
wrote:
> On Fri, Jul 14, 2017 at 12:50 PM, Nuno Lopes via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Hi,
>>
>> I going through the alias analysis documentation
>> (http://llvm.org/docs/AliasAnalysis.html) and noticed the following in
the
>> definition of PartialAlias:
>> "
>> The PartialAlias response is used when the two memory objects are known
to
>> be overlapping in some way, but *do not start at the same address*.
>> "
>>
>> Is it really required that the objects do no start at the same address?
if
>> that's the case the AA algorithm would need to prove that.
>> I'm asking this because:
>> 1) This condition seems very strict and I don't think it's met
in a few
>> places I found by manual inspection
> If I read the definition correctly, at least our Andersens' AA
> implementation violates it.
> see:
>
> AliasResult CFLAndersAAResult::alias(const MemoryLocation &LocA,
>                                       const MemoryLocation &LocB) {
>    if (LocA.Ptr == LocB.Ptr)
>      return LocA.Size == LocB.Size ? MustAlias : PartialAlias;
>
>
> (i.e. the two objects are overlapping here *and* start at the same
address).
I've never noticed that part of the definition myself. I'm fairly 
certain that's not what we actually implement.

  -Hal
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory