[llvm-dev] Alias semantic

[Hal Finkel via llvm-dev]

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?

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

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