[GVNHoist] computeInsertionPoints() miscalculates IDF

Fix for https://bugs.llvm.org/show_bug.cgi?id=38912.

In GVNHoist::computeInsertionPoints() we iterate over the Value
Numbers and calculate the Iterated Dominance Frontiers without
clearing the IDFBlocks vector first. IDFBlocks ends up accumulating
an insane number of basic blocks, which bloats the compilation time
of SemaChecking.cpp with ubsan enabled.

Differential Revision: https://reviews.llvm.org/D51980

llvm-svn: 342055

llvm/lib/Transforms/Scalar/GVNHoist.cpp

@@ -155,7 +155,6 @@ struct CHIArg {
 
 using CHIIt = SmallVectorImpl<CHIArg>::iterator;
 using CHIArgs = iterator_range<CHIIt>;
-using CHICache = DenseMap<BasicBlock *, SmallPtrSet<Instruction *, 4>>;
 using OutValuesType = DenseMap<BasicBlock *, SmallVector<CHIArg, 2>>;
 using InValuesType =
     DenseMap<BasicBlock *, SmallVector<std::pair<VNType, Instruction *>, 2>>;
@@ -767,7 +766,6 @@ class GVNHoist {
     ReverseIDFCalculator IDFs(*PDT);
     OutValuesType OutValue;
     InValuesType InValue;
-    CHICache CachedCHIs;
     for (const auto &R : Ranks) {
       const SmallVecInsn &V = Map.lookup(R);
       if (V.size() < 2)
@@ -786,6 +784,7 @@ class GVNHoist {
       // which currently have dead terminators that are control
       // dependence sources of a block which is in NewLiveBlocks.
       IDFs.setDefiningBlocks(VNBlocks);
+      IDFBlocks.clear();
       IDFs.calculate(IDFBlocks);
 
       // Make a map of BB vs instructions to be hoisted.
@@ -798,8 +797,7 @@ class GVNHoist {
         for (unsigned i = 0; i < V.size(); ++i) {
           CHIArg C = {VN, nullptr, nullptr};
            // Ignore spurious PDFs.
-          if (DT->properlyDominates(IDFB, V[i]->getParent()) &&
-              CachedCHIs[IDFB].insert(V[i]).second) {
+          if (DT->properlyDominates(IDFB, V[i]->getParent())) {
             OutValue[IDFB].push_back(C);
             LLVM_DEBUG(dbgs() << "\nInsertion a CHI for BB: " << IDFB->getName()
                               << ", for Insn: " << *V[i]);