[LLVMdev] Basic AliasAnalysis: Can GEPs with the same base but different constant indices into a struct alias?

[Daniel Berlin]

Are you counting unions, or literally just structs?


On Tue Jan 20 2015 at 12:34:38 PM Ahmed Bougacha <ahmed.bougacha at gmail.com>
wrote:

> Hi all,
>
> This is covered by (struct-path aware) TBAA, but BasicAA disagrees.
> See the attached testcase, where it prevents us from removing the
> redundant load.
> For arbitrary GEPs, we can't decide based on constant indices (because
> of e.g., &A[0][1] and &A[1][0], with *A a one-element array).  BasicAA
> has some logic to "try to distinguish something like &A[i][1] against
> &A[42][0]".  For the testcase, it works for instance when the struct
> has an even number of floats.
>
> However, I think we can safely say GEPs with different constant
> indices into a struct can't alias, at least when:
> - the GEP is inbounds
> - the access sizes are such that there is no overlap
> - the struct index is the final one
>
> That is, these can't alias:
>     gep i1, j1, k1, 0
>     gep i2, j2, k2, 1
> If this is a struct pointer:
>     gep i1, j1, k1
>
> I couldn't come up with a counterexample that would prevent us from
> doing this in BasicAA.
> I was surprised it wasn't; surely I missed something?
>
> Thanks!
>
> -Ahmed
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