[LLVMdev] Infinite loop when adding a new analysis pass

[Junjie Gu]

I think I know what goes wrong.  LICM acutally requires another
transformation : LoopSimplify, which does not preserve MyAnalysis.  So
LICM first schedules MyAnalysis before running LICM, then schedule
LoopSimplify, but scheduling LoopSimplify causes MyAnalysis to be
removed because it is not preserved, And LICM keeps repeating this
schedule & remove, and thus infinite loop.

It's nice for LLVM to add mechanism to detect this kind of conflicting
requirements....

On Mon, Aug 22, 2011 at 3:03 PM, Junjie Gu <jgu222 at gmail.com> wrote:
> I am trying to add an analysis pass as a FunctionPass,  and let LICM
> (LoopPass) depends upon it.  So in LICM.cpp, I have the following:
>
>    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
>      AU.setPreservesCFG();
>      AU.addRequired<DominatorTree>();
>      AU.addRequired<LoopInfo>();
>      AU.addRequiredID(LoopSimplifyID);
>      AU.addRequired<AliasAnalysis>();
>      AU.addRequired<MyAnalysis>();                         //  Add this
>      AU.addPreserved<AliasAnalysis>();
>      AU.addPreserved("scalar-evolution");
>      AU.addPreservedID(LoopSimplifyID);
>    }
>
>
> char LICM::ID = 0;
> INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
> INITIALIZE_PASS_DEPENDENCY(DominatorTree)
> INITIALIZE_PASS_DEPENDENCY(LoopInfo)
> INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
> INITIALIZE_PASS_DEPENDENCY(MyAnalysis)
>             // add this dependency
> INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
> INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
>
>
> Howerver,  I got an infinite loop when trying
>
> opt -O3 t.bc -o out.bc
>
> Where t.bc is created using clang from
>  int main() { return 0; }
>
> What am I missing ?
>
> All files are attacahed (include/Analysis/MyAnalysis.h,
> lib/Analysis/MyAnanlysis.cpp,  and diffs to the existing files).
>
> Thanks
> Junjie
>