Alias Analysis in LLVM

Question

I am trying to find the alias between a store instruction's pointer operand and function arguments. This is the code,

virtual void getAnalysisUsage(AnalysisUsage &AU) const {

    AU.addRequiredTransitive<AliasAnalysis>();
    AU.addPreserved<AliasAnalysis>();
}

virtual bool runOnFunction(Function &F) {

    AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

    for(Function::iterator i=F.begin();i!=F.end();++i){
        for(BasicBlock::iterator j=i->begin();j!=i->end();++j)
        {
            if(dyn_cast<StoreInst>(j)){
                const StoreInst *SI=dyn_cast<StoreInst>(j);

                AliasAnalysis::Location LocA = AA.getLocation(SI);

                const Value *si_v= SI->getPointerOperand();

                for(Function::arg_iterator k=F.arg_begin(); k!=F.arg_end();++k)
                {
                    Value *v=dyn_cast<Value>(k);

                    AliasAnalysis::Location loc=AliasAnalysis::Location(v);
                    AliasAnalysis::AliasResult ar=AA.alias(LocA,loc);

                    switch(ar)
                    {
                    case 0:errs()<<  "NoAlias\n";
                    break;
                    ///< No dependencies.
                    case 1:errs()<<"MayAlias\n";    ///< Anything goes
                    break;
                    case 2: errs()<<"PartialAlias\n";///< Pointers differ, but pointees overlap.
                    break;

                    case 3: errs()<<"MustAlias\n";
                    }
               }
   }

   return true;
}
};
}

But I get MayAlias result even if the store instruction's pointer operand is not referencing the function argument. Is there something wrong with the logic? Are there any files in the LLVM source code that contain code to do something similar. Thanks:)

Answer 1

The default alias analysis method from the AA group is basicaa, which always return "may alias". Try specifying an AA method (--globalsmodref-aa, -scev-aa,..) instead of letting opt use the default.

Something like this: opt -globalsmodref-aa -your_pass ...