[llvm-dev] TBAA question

[Neil Henning via llvm-dev]

I'd need to see more of the IR if you could provide it to give a guess at
the problem!

On Fri, Nov 15, 2019 at 4:49 PM Venkataramanan Kumar <
venkataramanan.kumar.llvm at gmail.com> wrote:

> Hi Neil
>
> On Fri, 15 Nov 2019 at 15:26, Neil Henning <neil.henning at unity3d.com>
> wrote:
>
>> What are you querying the alias analysis on in the above example -
>> between the load and store?
>>
>
> I am interested to check if  the load "%2" is not changed inside the
> function (assuming there are no function calls).  I am checking to see if
> there are any store instructions in that function that could aliases  and
> modify the load.
>
>
>
>> How are you creating your MemoryLocation for those too?
>>
>
> I query using alias method as shown below.
>
> MemoryLocation Locld = MemoryLocation::get(Loadinst);
> MemoryLocation Locst = MemoryLocation::get(Storeinst);
>
> if (AA->alias(Loc,Locst) != llvm::NoAlias ) {
> ==> it reaches here
> }
>
> Also checked modref info.
> if(isModSet(AA->getModRefInfo(Storeinst, Loc))
> {
> ==> it reaches here
> }
>
> regards,
> Venkat,
>
>>
>> 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,
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>>
>> --
>> Neil Henning
>> Senior Software Engineer Compiler
>> unity.com
>>
>

-- 
Neil Henning
Senior Software Engineer Compiler
unity.com
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