[PlaceSafepoints] Use analysis infrastructure to get dominator tree

[reames]

Summary

The old code computed dominators for every loop. This was terribly slow with no good reason. Just use the standard infrastructure for analysis passes.

lib/Transforms/Scalar/PlaceSafepoints.cpp

struct PlaceBackedgeSafepointsImpl : public LoopPass {
   PlaceBackedgeSafepointsImpl(bool CallSafepoints = false)
       : LoopPass(ID), CallSafepointsEnabled(CallSafepoints) {
     initializePlaceBackedgeSafepointsImplPass(*PassRegistry::getPassRegistry());
   }
 
   bool runOnLoop(Loop *, LPPassManager &LPM) override;
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<DominatorTreeWrapperPass>();
     AU.addRequired<ScalarEvolution>();
     // We no longer modify the IR at all in this pass.  Thus all
     // analysis are preserved.
     AU.setPreservesAll();
   }
 };
 }
 
while (!worklist.empty()) {
     BasicBlock *BB = worklist.back();
     worklist.pop_back();
     scanOneBB(&*BB->begin(), end, calls, seen, worklist);
   }
 }
 
 bool PlaceBackedgeSafepointsImpl::runOnLoop(Loop *L, LPPassManager &LPM) {
   ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
+  DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 
   // Loop through all loop latches (branches controlling backedges).  We need
   // to place a safepoint on every backedge (potentially). 
   // Note: In common usage, there will be only one edge due to LoopSimplify
   // having run sometime earlier in the pipeline, but this code must be correct
   // w.r.t. loops with multiple backedges.
   BasicBlock *header = L->getHeader();
 
-  // TODO: Use the analysis pass infrastructure for this.  There is no reason
-  // to recalculate this here.
-  DominatorTree DT;
-  DT.recalculate(*header->getParent());
-
   SmallVector<BasicBlock*, 16> LoopLatches;
   L->getLoopLatches(LoopLatches);
   for (BasicBlock *pred : LoopLatches) {
     assert(L->contains(pred));
 
     // Make a policy decision about whether this loop needs a safepoint or
     // not.  Note that this is about unburdening the optimizer in loops, not
     // avoiding the runtime cost of the actual safepoint.
     if (!AllBackedges) {
       if (mustBeFiniteCountedLoop(L, SE, pred)) {
         if (TraceLSP)
           errs() << "skipping safepoint placement in finite loop\n";
         FiniteExecution++;
         continue;
       }
       if (CallSafepointsEnabled &&
-          containsUnconditionalCallSafepoint(L, header, pred, DT)) {
+          containsUnconditionalCallSafepoint(L, header, pred, *DT)) {
         // Note: This is only semantically legal since we won't do any further
         // IPO or inlining before the actual call insertion..  If we hadn't, we
         // might latter loose this call safepoint.
         if (TraceLSP)
           errs() << "skipping safepoint placement due to unconditional call\n";
         CallInLoop++;
         continue;
       }

 char PlaceSafepoints::ID = 0;
 
 ModulePass *llvm::createPlaceSafepointsPass() { return new PlaceSafepoints(); }
 
 INITIALIZE_PASS_BEGIN(PlaceBackedgeSafepointsImpl,
                       "place-backedge-safepoints-impl",
                       "Place Backedge Safepoints", false, false)
 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 INITIALIZE_PASS_END(PlaceBackedgeSafepointsImpl,
                     "place-backedge-safepoints-impl",
                     "Place Backedge Safepoints", false, false)
 
 INITIALIZE_PASS_BEGIN(PlaceSafepoints, "place-safepoints", "Place Safepoints",
                       false, false)
 INITIALIZE_PASS_END(PlaceSafepoints, "place-safepoints", "Place Safepoints",
                     false, false)