[llvm-dev] TBAA question

[Venkataramanan Kumar via llvm-dev]

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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