[LLVMdev] TBAA fail on optimization, why?

[Jun Koi]

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?

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

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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