[llvm-commits] [PATCH] Extend BasicAliasAnalysis to recognize cyclic NoAlias phis in loops

[Hongbin Zheng]

On Thu, Aug 30, 2012 at 11:26 PM, Tobias Grosser <tobias at grosser.es> wrote:
> On 08/30/2012 04:48 PM, Arnold Schwaighofer wrote:
>>
>> Ping.
>>
>> On 8/9/2012 10:54 AM, Arnold Schwaighofer wrote:
>>>
>>> Hi,
>>>
>>> BasicAliasAnalysis should recognize phis that have incoming values from
>>> outside the loop that are known to not alias and whose other incoming
>>> values do not change this fact because they perform congruent operations.
>>>
>>> Example: We want to prove that ptr_phi and ptr_phi2 do not alias each
>>> other.
>
>
> Hi Arnold,
>
> I have really no idea about the alias analysis, but I was surprised to see
> that you add some kind of loop analysis to the basic alias analysis. From my
> point of view the scev alias analysis would be the better match. Running
> -scev-aa on your test case still gives:
>
> AliasSet[0x1530080, 3] may alias, Mod/Ref
>     Pointers: (i32* %ptr_phi, 4), (i32* %ptr2_phi, 4), (i32* %coeff, 4)
>
> AliasSet[0x1530300, 1] must alias, Mod        forwarding to 0x1530080
>
> Asking myself, why this is the case I looked at the scev values of
> %ptr_phi and %ptr2_phi.
>
> %ptr_phi = phi i32* [ %ptr, %entry ], [ %ptr_inc, %while.body ]
>   -->  {(4 + %ptr2)<nsw>,+,4}<nw><%while.body>
> %ptr2_phi = phi i32* [ %ptr2, %entry ], [ %ptr2_inc, %while.body ]
>   -->  {%ptr2,+,4}<nw><%while.body>
>
> Digging a little bit into the debugger, the scev-aa can even figure out that
> the offset of between these two pointers is 4. However, for some reason it
> does not prove that they don't alias. Do you understand why the scev-aa can
> not handle this?
Does this means %coeff may alias %ptr2_phi and %coeff may alias %ptr_phi?
>
> Cheers
> Tobi
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits