[llvm-dev] Iteration distance failed in DependenceAnalysis C++ API

[Shuo Wang via llvm-dev]

Hi LLVM-ers,

     (1) I tried to use the following command line to get the iteration distance of a loop-carried dependence of a simple program : 
           
 	   command line:      opt -basicaa -da -analyze dis_test.ll

           program: 	for ( int i=0; j < n; ++i ) {
                           	     b[i] = b[i-2] + 3;
			  	 }
           results:  da analyze — none!
			 da analyze — consistent anti [-2] (This is what I expected!!)
			 da analyze — none!
    
     (2) However, when I tried to use the C++ API to do the same thing, the distance result is null.

           My pass: 

	class IterationDistance : public DependenceAnalysis { 
  		...
 	        void getAnalysisUsage(AnalysisUsage &AU) const { 
			 AU.setPreservesAll(); 
			 AU.addRequired<AliasAnalysis>(); 				 		 				 AU.addRequired<LoopInfo>(); 
			 AU.addRequired<ScalarEvolution>(); 		 		 		 	     
                         AU.addRequired<DependenceAnalysis>();
		}
                ...
	   	DependenceAnalysis *DA = &(getAnalysis<DependenceAnalysis>());
		auto D = DA->depends(Src, Des, true);
		if (D->isAnti()) {
                    unsigned Levels = D->getLevels();
                    errs() << "levels = " << Levels << "\n";
                    char Direction;
                    for (unsigned II = 1; II <= Levels; ++II) {
                        ...
			const SCEV *Distance = D->getDistance(II);
                        const SCEVConstant *SCEVConst = dyn_cast_or_null<SCEVConstant>(Distance);
                        ...
		}

           Results: levels = 0;
                         Distance is null.

     (3) My guess is that the "levels = 0” which makes the Distance cannot be got. Can anyone help me with this? 

Thanks!
Shvo
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