[LLVMdev] Incorrect result in LLVM Alias Analysis

[Adarsh HV]

Here's the bitcode of maptest.ll.
; ModuleID = 'maptest.c'
target datalayout =
"e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32-n8:16:32"
target triple = "i386-pc-linux-gnu"

define void @map(i32* %a) nounwind {
  %1 = alloca i32*, align 4
  store i32* %a, i32** %1, align 4
  %2 = load i32** %1, align 4
  store i32 20, i32* %2
  ret void
}

define i32 @main() nounwind {
  %1 = alloca i32, align 4
  %a = alloca i32*, align 4
  store i32 0, i32* %1
  %2 = call i8* @malloc(i32 4)
  %3 = bitcast i8* %2 to i32*
  store i32* %3, i32** %a, align 4
  %4 = load i32** %a, align 4
  store i32 15, i32* %4
  %5 = load i32** %a, align 4
  call void @map(i32* %5)
  ret i32 0
}

declare noalias i8* @malloc(i32) nounwind

Regards,
Adarsh H.V.


Adarsh HV wrote:
> 
> It contains the bitcode file(without any optimization) of the below
> program,
> void map(int *a)
>  {
>   *a=20;
> }
> int main(){
> int *a=(int *)malloc(sizeof(int));
> *a=15;
> map(a);
> return 0;
> }
> I want to check if the pointer operand of each store instruction aliases
> with the function's arguments. I have used below code for this,
> 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))
>                                    {
> 				             StoreInst *SI=dyn_cast<StoreInst>(j);
> 				             AliasAnalysis::Location LocA = AA.getLocation(SI);
> 				             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);
>                                                 }
>                                     }
>                               }
>                         }
> But I get 'May Alias' result for the store instruction that assigns the
> value of 'a' to 20 in the 'map' function.Is the result not supposed to be
> 'Must alias'? 
> 
> Duncan Sands wrote:
>> 
>> Hi Adarsh,
>> 
>>> I have used the follwing command,
>>> opt -load LLVComMan.so -ComMan -memdep -tbaa -mem2reg maptest.ll   -S
>>> What option other than -mem2reg should be included in this case to get
>>> the
>>> right results? Does the order in which I specify the optimizations to be
>>> run
>>> make a difference?
>> 
>> what is in maptest.ll?
>> 
>> Ciao, Duncan.
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> 
>> 
> 
> 

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