From 67a00b872a323e97b266b20909f882c6a4453d19 Mon Sep 17 00:00:00 2001
From: Alexandros Lamprineas <alexandros.lamprineas@arm.com>
Date: Wed, 12 Sep 2018 14:28:23 +0000
Subject: [PATCH] [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
---
 lib/Transforms/Scalar/GVNHoist.cpp | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/lib/Transforms/Scalar/GVNHoist.cpp b/lib/Transforms/Scalar/GVNHoist.cpp
index 10cdcd59e42..d031d675642 100644
--- a/lib/Transforms/Scalar/GVNHoist.cpp
+++ b/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 @@ private:
     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 @@ private:
       // 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 @@ private:
         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]);