[LLVMdev] TBAA fail on optimization, why?

[Duncan Sands]

Hi Jun,

On 27/09/12 08:02, Jun Koi wrote:
> On Thu, Sep 20, 2012 at 5:18 PM, Duncan Sands <baldrick at free.fr> wrote:
>> Hi Jun, did you tell "opt" to make use of TBAA?  Also, please give complete
>> IR
>> that people can use to reproduce, and instructions on how to reproduce (eg
>> how
>> to run opt).
>>
>
> actually, i am still confused on which options should be given to
> "opt" for it to use TBAA. any hint?

-tbaa somewhere at the start.

> of course, we can simply use -O3, but i want to pinpoint exactly which
> passes are necessary for TBAA optimization to work.

The various alias analyses assume that you've run a certain minimum number of
optimizers on your bitcode, eg reg2mem.

Ciao, Duncan.

>
> thanks,
> Jun
>
>
>
>
>>> i have a simple code like below, in wich variable "aaa" does not alias
>>> to "bbb".
>>> i use TBAA to specify this, please see the code.
>>>
>>> then i ran this code thru LLVM optimization, and i expected that the
>>> second "store" instruction is eliminated.
>>> however, i am wrong: the second "store" instruction is still there
>>> after optimization.
>>>
>>> perhaps my TBAA setup is wrong somewhere? any hint, please?
>>>
>>> thanks!
>>> Jun
>>>
>>>
>>> @aaa = external global i32
>>> @bbb = external global i32
>>>
>>> ....
>>> %0 = load i32* @aaa, !tbaa !1
>>> %1 = load i32* @bbb, !tbaa !2
>>> %2 = add i32 %0, %1
>>> store i32 %2, i32* @aaa, !tbaa !2
>>>
>>> ; below line should be eliminated with optimization
>>> store i32 %1, i32* @bbb, !tbaa !1               ; <==== WHY
>>> optimization CANNOT ELIMINATE this insn?
>>>
>>> ...
>>> !tbaa = !{!0, !1, !2}
>>> !0 = metadata !{metadata !"tbaa_root"}
>>> !1 = metadata !{metadata !"aaa", metadata !0}
>>> !2 = metadata !{metadata !"bbb", metadata !0}
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>>
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