[llvm-dev] TBAA question

[Neil Henning via llvm-dev]

What are you querying the alias analysis on in the above example - between
the load and store?

How are you creating your MemoryLocation for those too?

On Fri, Nov 15, 2019 at 6:03 AM Venkataramanan Kumar via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Can someone  please clarify me on this?
>
>
>
> On Wed, 13 Nov 2019 at 22:25, Venkataramanan Kumar <
> venkataramanan.kumar.llvm at gmail.com> wrote:
>
>> Hi,
>>
>> I have the following LLVM IR.
>>
>> ----Snip---
>> %1 = getelementptr inbounds %struct._X, %struct._X* %0, i64 0, i32 3
>> %2 = load %struct._X*, %struct._X** %1, align 8, !tbaa !10
>> ....
>> .....
>> .......
>>
>> %3 = bitcast %struct._Y** %1 to i8**
>> store i8* null, i8** %3, align 8, !tbaa !9
>>
>> !9 = !{!7, !7, i64 0}
>> !7 = !{!"omnipotent char", !8, i64 0}
>> !8 = !{!"Simple C/C++ TBAA"}
>>
>> ----Snip----
>>
>> Load inst above is trying to access a field of _X which is a self
>> recursive pointer to same struct _X.
>> Store puts a null value for a pointer to struct _Y. It has been bitcasted
>> to i8** and store happens via char pointer.
>>
>>
>> When I query alias analysis although loads and stores are pointers to
>> different struct types. TBAA says may alias.
>> is it safe to assume the loaded pointer to struct _X do not alias with
>> the store , if we look further at bitcast and see the pointer to different
>> struct?
>>
>> is my understanding wrong here?
>>
>> regards,
>> Venkat,
>>
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-- 
Neil Henning
Senior Software Engineer Compiler
unity.com
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