PMTopLevelManager::schedulePass/// Schedule pass P for execution. Make sure that passes required by
 /// P are run before P is run. Update analysis info maintained by
 /// the manager. Remove dead passes. This is a recursive function.
 /*调度操作P。确保操作P所依赖的操作都已经执行完毕。更新分析信息*/
 void PMTopLevelManager::schedulePass(Pass *P) {
 df df df d df df d
   // TODO : Allocate function manager for this pass, other wise required set
   /// may be inserted into previous function manager
 
   // Give pass a chance to prepare the stage.
   P->preparePassManager(activeStack);
 
   // If P is an analysis pass and it is available then do not
   // generate the analysis again. Stale analysis info should not be
   // available at this point.
   const PassInfo *PI =
     PassRegistry::getPassRegistry()->getPassInfo(P->getPassID());
   if (PI && PI->isAnalysis() && findAnalysisPass(P->getPassID())) {
     delete P;
     return;
   }
 
   AnalysisUsage *AnUsage = findAnalysisUsage(P);
 
   bool checkAnalysis = true;
   while (checkAnalysis) {
     checkAnalysis = false;
 
     const AnalysisUsage::VectorType &RequiredSet = AnUsage->getRequiredSet();
     for (AnalysisUsage::VectorType::const_iterator I = RequiredSet.begin(),
            E = RequiredSet.end(); I != E; ++I) {
 
       Pass *AnalysisPass = findAnalysisPass(*I);
       if (!AnalysisPass) {
         const PassInfo *PI = PassRegistry::getPassRegistry()->getPassInfo(*I);
         assert(PI && "Expected required passes to be initialized");
         AnalysisPass = PI->createPass();
         if (P->getPotentialPassManagerType () ==
             AnalysisPass->getPotentialPassManagerType())
           // Schedule analysis pass that is managed by the same pass manager.
           schedulePass(AnalysisPass);
         else if (P->getPotentialPassManagerType () >
                  AnalysisPass->getPotentialPassManagerType()) {
           // Schedule analysis pass that is managed by a new manager.
           schedulePass(AnalysisPass);
           // Recheck analysis passes to ensure that required analyses that
           // are already checked are still available.
           checkAnalysis = true;
         }
         else
           // Do not schedule this analysis. Lower level analsyis
           // passes are run on the fly.
           delete AnalysisPass;
       }
     }
   }
 
   // Now all required passes are available.
   addTopLevelPass(P);
 }