[llvm-dev] Alias Analysis with inbound GEPs

[Demikhovsky, Elena via llvm-dev]

> Would this proposal help: https://reviews.llvm.org/D22793 ?
> Yes, it looks like that would help.

Thank you. I’ll try to get more info from the author.
Btw, Intel compiler vectorizes this loop and, probably, it is the right thing to do.

-           Elena

From: Hal Finkel [mailto:hfinkel at anl.gov]
Sent: Tuesday, July 26, 2016 21:54
To: Mehdi Amini <mehdi.amini at apple.com>
Cc: Demikhovsky, Elena <elena.demikhovsky at intel.com>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Alias Analysis with inbound GEPs


________________________________
From: "Mehdi Amini" <mehdi.amini at apple.com<mailto:mehdi.amini at apple.com>>
To: "Hal Finkel" <hfinkel at anl.gov<mailto:hfinkel at anl.gov>>
Cc: "Elena Demikhovsky" <elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>, "llvm-dev" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: Tuesday, July 26, 2016 1:37:28 PM
Subject: Re: [llvm-dev] Alias Analysis with inbound GEPs

On Jul 25, 2016, at 10:16 AM, Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:


________________________________
From: "Elena via llvm-dev Demikhovsky" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
To: "llvm-dev" <llvm-dev at lists.llvm.org<mailto: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 ?
Yes, it looks like that would help.

 -Hal

—
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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Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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