[llvm-dev] Alias semantic

[Alexandre Isoard via llvm-dev]

On Thu, Mar 9, 2017 at 8:03 PM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 03/09/2017 01:39 PM, Alexandre Isoard via llvm-dev wrote:
>
> Hello fellow LLVM developers,
>
> I was wondering about the semantic of aliasing.
> Here are some examples where I am not sure if the two stores aliases:
>
> Example 1:
>
> for (int i = 2; i < n; ++i) {
>   store A[i];
>   store A[i-2];
> }
>
> Example 2:
>
> for (int i = 2; i < n; ++i)
>   store A[i];
> for (int i = 2; i < n; ++i)
>   store A[i-2];
>
> In the example 1, they do not alias in a single iteration, but they do
> alias at some point if n > 4.
>
>
> If you ask LLVM's AA whether 'store A[i]' and 'store A[i-2]' alias, it
> should say NoAlias. When two memory accesses have addresses that depend on
> common SSA values (the PHI node %i in this case), then it is interpreted as
> having the same value in both instructions.
>
> In example 2, you should get a MayAlias result, the PHI in both loops will
> be distinct SSA values (even though you've given them the same name in your
> example), and we can't prove that the memory accessed by the first store is
> disjoint from the memory accessed by the second.
>
> In any case, please don't confuse our AA for a loop dependence analysis.
> Are you looking for a loop dependence analysis?
>

That is most certainly what I am looking for.
I was wrongly assuming that AliasAnalysis would be a cheap
over-approximation, I was wrong.

Thanks.


>  -Hal
>
> That is, there is an execution path from one instance of the store to an
> instance of the other store such that they access the same memory cell.
>
> In example 2 it is easier to see the difference.
>
> I am somewhat in the fog here.
>
> Best regard.
>
> --
> *Alexandre Isoard*
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>


-- 
*Alexandre Isoard*
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