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

[Ahmed Bougacha]

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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