[llvm-dev] Alias Analysis with inbound GEPs

[Mehdi Amini via llvm-dev]

> On Jul 25, 2016, at 10:16 AM, Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> From: "Elena via llvm-dev Demikhovsky" <llvm-dev at lists.llvm.org>
> To: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Monday, July 25, 2016 9:45:55 AM
> Subject: [llvm-dev] Alias Analysis with inbound GEPs
> 
> Hi,
>  
> I’m checking aliasing of two pointers:
>  
>   %GEP1 = getelementptr inbounds %struct.s, %struct.s* %0, i64 0, i32 1, i64 %indvars.iv41, i64 %indvars.iv39
>   %GEP2 = getelementptr inbounds %struct.s, %struct.s* %0, i64 0, i32 16
>  
> The result I got is “PartialAlias” because the indices of the GEP1 are variable.
> That seems like a bug. PartialAlias should only be returned when we can prove a partial overlap. Otherwise, MayAlias should be returned.
> Shouldn’t the “inbounds” keyword mean that the access to sub-array is also in-bounds?
> No. inbounds applies only to the whole object.

Would this proposal help: https://reviews.llvm.org/D22793 <https://reviews.llvm.org/D22793> ?

— 
Mehdi


> I’m trying to reach “NoAlias” consensus between GEP1 and GEP2.
> Did the original code come from C or C++? What are we modeling here?
> 
>  -Hal
>  
> Thanks.
>  
> Elena
>  
>  
>  
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> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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