1 files changed, 559 insertions, 6 deletions

diff --git a/lib/Transforms/Scalar/LoopSimplifyCFG.cpp b/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
index 2b83d3dc5f1b..2e5927f9a068 100644
--- a/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
+++ b/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
@@ -24,9 +24,12 @@
 #include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/MemorySSA.h"
+#include "llvm/Analysis/MemorySSAUpdater.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/IR/DomTreeUpdater.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Scalar/LoopPassManager.h"
@@ -38,9 +41,527 @@ using namespace llvm;
 
 #define DEBUG_TYPE "loop-simplifycfg"
 
-static bool simplifyLoopCFG(Loop &L, DominatorTree &DT, LoopInfo &LI,
-                            ScalarEvolution &SE) {
+static cl::opt<bool> EnableTermFolding("enable-loop-simplifycfg-term-folding",
+                                       cl::init(false));
+
+STATISTIC(NumTerminatorsFolded,
+          "Number of terminators folded to unconditional branches");
+STATISTIC(NumLoopBlocksDeleted,
+          "Number of loop blocks deleted");
+STATISTIC(NumLoopExitsDeleted,
+          "Number of loop exiting edges deleted");
+
+/// If \p BB is a switch or a conditional branch, but only one of its successors
+/// can be reached from this block in runtime, return this successor. Otherwise,
+/// return nullptr.
+static BasicBlock *getOnlyLiveSuccessor(BasicBlock *BB) {
+  Instruction *TI = BB->getTerminator();
+  if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
+    if (BI->isUnconditional())
+      return nullptr;
+    if (BI->getSuccessor(0) == BI->getSuccessor(1))
+      return BI->getSuccessor(0);
+    ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
+    if (!Cond)
+      return nullptr;
+    return Cond->isZero() ? BI->getSuccessor(1) : BI->getSuccessor(0);
+  }
+
+  if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
+    auto *CI = dyn_cast<ConstantInt>(SI->getCondition());
+    if (!CI)
+      return nullptr;
+    for (auto Case : SI->cases())
+      if (Case.getCaseValue() == CI)
+        return Case.getCaseSuccessor();
+    return SI->getDefaultDest();
+  }
+
+  return nullptr;
+}
+
+namespace {
+/// Helper class that can turn branches and switches with constant conditions
+/// into unconditional branches.
+class ConstantTerminatorFoldingImpl {
+private:
+  Loop &L;
+  LoopInfo &LI;
+  DominatorTree &DT;
+  ScalarEvolution &SE;
+  MemorySSAUpdater *MSSAU;
+
+  // Whether or not the current loop has irreducible CFG.
+  bool HasIrreducibleCFG = false;
+  // Whether or not the current loop will still exist after terminator constant
+  // folding will be done. In theory, there are two ways how it can happen:
+  // 1. Loop's latch(es) become unreachable from loop header;
+  // 2. Loop's header becomes unreachable from method entry.
+  // In practice, the second situation is impossible because we only modify the
+  // current loop and its preheader and do not affect preheader's reachibility
+  // from any other block. So this variable set to true means that loop's latch
+  // has become unreachable from loop header.
+  bool DeleteCurrentLoop = false;
+
+  // The blocks of the original loop that will still be reachable from entry
+  // after the constant folding.
+  SmallPtrSet<BasicBlock *, 8> LiveLoopBlocks;
+  // The blocks of the original loop that will become unreachable from entry
+  // after the constant folding.
+  SmallVector<BasicBlock *, 8> DeadLoopBlocks;
+  // The exits of the original loop that will still be reachable from entry
+  // after the constant folding.
+  SmallPtrSet<BasicBlock *, 8> LiveExitBlocks;
+  // The exits of the original loop that will become unreachable from entry
+  // after the constant folding.
+  SmallVector<BasicBlock *, 8> DeadExitBlocks;
+  // The blocks that will still be a part of the current loop after folding.
+  SmallPtrSet<BasicBlock *, 8> BlocksInLoopAfterFolding;
+  // The blocks that have terminators with constant condition that can be
+  // folded. Note: fold candidates should be in L but not in any of its
+  // subloops to avoid complex LI updates.
+  SmallVector<BasicBlock *, 8> FoldCandidates;
+
+  void dump() const {
+    dbgs() << "Constant terminator folding for loop " << L << "\n";
+    dbgs() << "After terminator constant-folding, the loop will";
+    if (!DeleteCurrentLoop)
+      dbgs() << " not";
+    dbgs() << " be destroyed\n";
+    auto PrintOutVector = [&](const char *Message,
+                           const SmallVectorImpl<BasicBlock *> &S) {
+      dbgs() << Message << "\n";
+      for (const BasicBlock *BB : S)
+        dbgs() << "\t" << BB->getName() << "\n";
+    };
+    auto PrintOutSet = [&](const char *Message,
+                           const SmallPtrSetImpl<BasicBlock *> &S) {
+      dbgs() << Message << "\n";
+      for (const BasicBlock *BB : S)
+        dbgs() << "\t" << BB->getName() << "\n";
+    };
+    PrintOutVector("Blocks in which we can constant-fold terminator:",
+                   FoldCandidates);
+    PrintOutSet("Live blocks from the original loop:", LiveLoopBlocks);
+    PrintOutVector("Dead blocks from the original loop:", DeadLoopBlocks);
+    PrintOutSet("Live exit blocks:", LiveExitBlocks);
+    PrintOutVector("Dead exit blocks:", DeadExitBlocks);
+    if (!DeleteCurrentLoop)
+      PrintOutSet("The following blocks will still be part of the loop:",
+                  BlocksInLoopAfterFolding);
+  }
+
+  /// Whether or not the current loop has irreducible CFG.
+  bool hasIrreducibleCFG(LoopBlocksDFS &DFS) {
+    assert(DFS.isComplete() && "DFS is expected to be finished");
+    // Index of a basic block in RPO traversal.
+    DenseMap<const BasicBlock *, unsigned> RPO;
+    unsigned Current = 0;
+    for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I)
+      RPO[*I] = Current++;
+
+    for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
+      BasicBlock *BB = *I;
+      for (auto *Succ : successors(BB))
+        if (L.contains(Succ) && !LI.isLoopHeader(Succ) && RPO[BB] > RPO[Succ])
+          // If an edge goes from a block with greater order number into a block
+          // with lesses number, and it is not a loop backedge, then it can only
+          // be a part of irreducible non-loop cycle.
+          return true;
+    }
+    return false;
+  }
+
+  /// Fill all information about status of blocks and exits of the current loop
+  /// if constant folding of all branches will be done.
+  void analyze() {
+    LoopBlocksDFS DFS(&L);
+    DFS.perform(&LI);
+    assert(DFS.isComplete() && "DFS is expected to be finished");
+
+    // TODO: The algorithm below relies on both RPO and Postorder traversals.
+    // When the loop has only reducible CFG inside, then the invariant "all
+    // predecessors of X are processed before X in RPO" is preserved. However
+    // an irreducible loop can break this invariant (e.g. latch does not have to
+    // be the last block in the traversal in this case, and the algorithm relies
+    // on this). We can later decide to support such cases by altering the
+    // algorithms, but so far we just give up analyzing them.
+    if (hasIrreducibleCFG(DFS)) {
+      HasIrreducibleCFG = true;
+      return;
+    }
+
+    // Collect live and dead loop blocks and exits.
+    LiveLoopBlocks.insert(L.getHeader());
+    for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
+      BasicBlock *BB = *I;
+
+      // If a loop block wasn't marked as live so far, then it's dead.
+      if (!LiveLoopBlocks.count(BB)) {
+        DeadLoopBlocks.push_back(BB);
+        continue;
+      }
+
+      BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(BB);
+
+      // If a block has only one live successor, it's a candidate on constant
+      // folding. Only handle blocks from current loop: branches in child loops
+      // are skipped because if they can be folded, they should be folded during
+      // the processing of child loops.
+      if (TheOnlySucc && LI.getLoopFor(BB) == &L)
+        FoldCandidates.push_back(BB);
+
+      // Handle successors.
+      for (BasicBlock *Succ : successors(BB))
+        if (!TheOnlySucc || TheOnlySucc == Succ) {
+          if (L.contains(Succ))
+            LiveLoopBlocks.insert(Succ);
+          else
+            LiveExitBlocks.insert(Succ);
+        }
+    }
+
+    // Sanity check: amount of dead and live loop blocks should match the total
+    // number of blocks in loop.
+    assert(L.getNumBlocks() == LiveLoopBlocks.size() + DeadLoopBlocks.size() &&
+           "Malformed block sets?");
+
+    // Now, all exit blocks that are not marked as live are dead.
+    SmallVector<BasicBlock *, 8> ExitBlocks;
+    L.getExitBlocks(ExitBlocks);
+    for (auto *ExitBlock : ExitBlocks)
+      if (!LiveExitBlocks.count(ExitBlock))
+        DeadExitBlocks.push_back(ExitBlock);
+
+    // Whether or not the edge From->To will still be present in graph after the
+    // folding.
+    auto IsEdgeLive = [&](BasicBlock *From, BasicBlock *To) {
+      if (!LiveLoopBlocks.count(From))
+        return false;
+      BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(From);
+      return !TheOnlySucc || TheOnlySucc == To;
+    };
+
+    // The loop will not be destroyed if its latch is live.
+    DeleteCurrentLoop = !IsEdgeLive(L.getLoopLatch(), L.getHeader());
+
+    // If we are going to delete the current loop completely, no extra analysis
+    // is needed.
+    if (DeleteCurrentLoop)
+      return;
+
+    // Otherwise, we should check which blocks will still be a part of the
+    // current loop after the transform.
+    BlocksInLoopAfterFolding.insert(L.getLoopLatch());
+    // If the loop is live, then we should compute what blocks are still in
+    // loop after all branch folding has been done. A block is in loop if
+    // it has a live edge to another block that is in the loop; by definition,
+    // latch is in the loop.
+    auto BlockIsInLoop = [&](BasicBlock *BB) {
+      return any_of(successors(BB), [&](BasicBlock *Succ) {
+        return BlocksInLoopAfterFolding.count(Succ) && IsEdgeLive(BB, Succ);
+      });
+    };
+    for (auto I = DFS.beginPostorder(), E = DFS.endPostorder(); I != E; ++I) {
+      BasicBlock *BB = *I;
+      if (BlockIsInLoop(BB))
+        BlocksInLoopAfterFolding.insert(BB);
+    }
+
+    // Sanity check: header must be in loop.
+    assert(BlocksInLoopAfterFolding.count(L.getHeader()) &&
+           "Header not in loop?");
+    assert(BlocksInLoopAfterFolding.size() <= LiveLoopBlocks.size() &&
+           "All blocks that stay in loop should be live!");
+  }
+
+  /// We need to preserve static reachibility of all loop exit blocks (this is)
+  /// required by loop pass manager. In order to do it, we make the following
+  /// trick:
+  ///
+  ///  preheader:
+  ///    <preheader code>
+  ///    br label %loop_header
+  ///
+  ///  loop_header:
+  ///    ...
+  ///    br i1 false, label %dead_exit, label %loop_block
+  ///    ...
+  ///
+  /// We cannot simply remove edge from the loop to dead exit because in this
+  /// case dead_exit (and its successors) may become unreachable. To avoid that,
+  /// we insert the following fictive preheader:
+  ///
+  ///  preheader:
+  ///    <preheader code>
+  ///    switch i32 0, label %preheader-split,
+  ///                  [i32 1, label %dead_exit_1],
+  ///                  [i32 2, label %dead_exit_2],
+  ///                  ...
+  ///                  [i32 N, label %dead_exit_N],
+  ///
+  ///  preheader-split:
+  ///    br label %loop_header
+  ///
+  ///  loop_header:
+  ///    ...
+  ///    br i1 false, label %dead_exit_N, label %loop_block
+  ///    ...
+  ///
+  /// Doing so, we preserve static reachibility of all dead exits and can later
+  /// remove edges from the loop to these blocks.
+  void handleDeadExits() {
+    // If no dead exits, nothing to do.
+    if (DeadExitBlocks.empty())
+      return;
+
+    // Construct split preheader and the dummy switch to thread edges from it to
+    // dead exits.
+    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+    BasicBlock *Preheader = L.getLoopPreheader();
+    BasicBlock *NewPreheader = Preheader->splitBasicBlock(
+        Preheader->getTerminator(),
+        Twine(Preheader->getName()).concat("-split"));
+    DTU.deleteEdge(Preheader, L.getHeader());
+    DTU.insertEdge(NewPreheader, L.getHeader());
+    DTU.insertEdge(Preheader, NewPreheader);
+    IRBuilder<> Builder(Preheader->getTerminator());
+    SwitchInst *DummySwitch =
+        Builder.CreateSwitch(Builder.getInt32(0), NewPreheader);
+    Preheader->getTerminator()->eraseFromParent();
+
+    unsigned DummyIdx = 1;
+    for (BasicBlock *BB : DeadExitBlocks) {
+      SmallVector<Instruction *, 4> DeadPhis;
+      for (auto &PN : BB->phis())
+        DeadPhis.push_back(&PN);
+
+      // Eliminate all Phis from dead exits.
+      for (Instruction *PN : DeadPhis) {
+        PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
+        PN->eraseFromParent();
+      }
+      assert(DummyIdx != 0 && "Too many dead exits!");
+      DummySwitch->addCase(Builder.getInt32(DummyIdx++), BB);
+      DTU.insertEdge(Preheader, BB);
+      ++NumLoopExitsDeleted;
+    }
+
+    assert(L.getLoopPreheader() == NewPreheader && "Malformed CFG?");
+    if (Loop *OuterLoop = LI.getLoopFor(Preheader)) {
+      OuterLoop->addBasicBlockToLoop(NewPreheader, LI);
+
+      // When we break dead edges, the outer loop may become unreachable from
+      // the current loop. We need to fix loop info accordingly. For this, we
+      // find the most nested loop that still contains L and remove L from all
+      // loops that are inside of it.
+      Loop *StillReachable = nullptr;
+      for (BasicBlock *BB : LiveExitBlocks) {
+        Loop *BBL = LI.getLoopFor(BB);
+        if (BBL && BBL->contains(L.getHeader()))
+          if (!StillReachable ||
+              BBL->getLoopDepth() > StillReachable->getLoopDepth())
+            StillReachable = BBL;
+      }
+
+      // Okay, our loop is no longer in the outer loop (and maybe not in some of
+      // its parents as well). Make the fixup.
+      if (StillReachable != OuterLoop) {
+        LI.changeLoopFor(NewPreheader, StillReachable);
+        for (Loop *NotContaining = OuterLoop; NotContaining != StillReachable;
+             NotContaining = NotContaining->getParentLoop()) {
+          NotContaining->removeBlockFromLoop(NewPreheader);
+          for (auto *BB : L.blocks())
+            NotContaining->removeBlockFromLoop(BB);
+        }
+        OuterLoop->removeChildLoop(&L);
+        if (StillReachable)
+          StillReachable->addChildLoop(&L);
+        else
+          LI.addTopLevelLoop(&L);
+      }
+    }
+  }
+
+  /// Delete loop blocks that have become unreachable after folding. Make all
+  /// relevant updates to DT and LI.
+  void deleteDeadLoopBlocks() {
+    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+    if (MSSAU) {
+      SmallPtrSet<BasicBlock *, 8> DeadLoopBlocksSet(DeadLoopBlocks.begin(),
+                                                     DeadLoopBlocks.end());
+      MSSAU->removeBlocks(DeadLoopBlocksSet);
+    }
+    for (auto *BB : DeadLoopBlocks) {
+      assert(BB != L.getHeader() &&
+             "Header of the current loop cannot be dead!");
+      LLVM_DEBUG(dbgs() << "Deleting dead loop block " << BB->getName()
+                        << "\n");
+      if (LI.isLoopHeader(BB)) {
+        assert(LI.getLoopFor(BB) != &L && "Attempt to remove current loop!");
+        LI.erase(LI.getLoopFor(BB));
+      }
+      LI.removeBlock(BB);
+      DeleteDeadBlock(BB, &DTU);
+      ++NumLoopBlocksDeleted;
+    }
+  }
+
+  /// Constant-fold terminators of blocks acculumated in FoldCandidates into the
+  /// unconditional branches.
+  void foldTerminators() {
+    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+
+    for (BasicBlock *BB : FoldCandidates) {
+      assert(LI.getLoopFor(BB) == &L && "Should be a loop block!");
+      BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(BB);
+      assert(TheOnlySucc && "Should have one live successor!");
+
+      LLVM_DEBUG(dbgs() << "Replacing terminator of " << BB->getName()
+                        << " with an unconditional branch to the block "
+                        << TheOnlySucc->getName() << "\n");
+
+      SmallPtrSet<BasicBlock *, 2> DeadSuccessors;
+      // Remove all BB's successors except for the live one.
+      unsigned TheOnlySuccDuplicates = 0;
+      for (auto *Succ : successors(BB))
+        if (Succ != TheOnlySucc) {
+          DeadSuccessors.insert(Succ);
+          // If our successor lies in a different loop, we don't want to remove
+          // the one-input Phi because it is a LCSSA Phi.
+          bool PreserveLCSSAPhi = !L.contains(Succ);
+          Succ->removePredecessor(BB, PreserveLCSSAPhi);
+          if (MSSAU)
+            MSSAU->removeEdge(BB, Succ);
+        } else
+          ++TheOnlySuccDuplicates;
+
+      assert(TheOnlySuccDuplicates > 0 && "Should be!");
+      // If TheOnlySucc was BB's successor more than once, after transform it
+      // will be its successor only once. Remove redundant inputs from
+      // TheOnlySucc's Phis.
+      bool PreserveLCSSAPhi = !L.contains(TheOnlySucc);
+      for (unsigned Dup = 1; Dup < TheOnlySuccDuplicates; ++Dup)
+        TheOnlySucc->removePredecessor(BB, PreserveLCSSAPhi);
+      if (MSSAU && TheOnlySuccDuplicates > 1)
+        MSSAU->removeDuplicatePhiEdgesBetween(BB, TheOnlySucc);
+
+      IRBuilder<> Builder(BB->getContext());
+      Instruction *Term = BB->getTerminator();
+      Builder.SetInsertPoint(Term);
+      Builder.CreateBr(TheOnlySucc);
+      Term->eraseFromParent();
+
+      for (auto *DeadSucc : DeadSuccessors)
+        DTU.deleteEdge(BB, DeadSucc);
+
+      ++NumTerminatorsFolded;
+    }
+  }
+
+public:
+  ConstantTerminatorFoldingImpl(Loop &L, LoopInfo &LI, DominatorTree &DT,
+                                ScalarEvolution &SE,
+                                MemorySSAUpdater *MSSAU)
+      : L(L), LI(LI), DT(DT), SE(SE), MSSAU(MSSAU) {}
+  bool run() {
+    assert(L.getLoopLatch() && "Should be single latch!");
+
+    // Collect all available information about status of blocks after constant
+    // folding.
+    analyze();
+
+    LLVM_DEBUG(dbgs() << "In function " << L.getHeader()->getParent()->getName()
+                      << ": ");
+
+    if (HasIrreducibleCFG) {
+      LLVM_DEBUG(dbgs() << "Loops with irreducible CFG are not supported!\n");
+      return false;
+    }
+
+    // Nothing to constant-fold.
+    if (FoldCandidates.empty()) {
+      LLVM_DEBUG(
+          dbgs() << "No constant terminator folding candidates found in loop "
+                 << L.getHeader()->getName() << "\n");
+      return false;
+    }
+
+    // TODO: Support deletion of the current loop.
+    if (DeleteCurrentLoop) {
+      LLVM_DEBUG(
+          dbgs()
+          << "Give up constant terminator folding in loop "
+          << L.getHeader()->getName()
+          << ": we don't currently support deletion of the current loop.\n");
+      return false;
+    }
+
+    // TODO: Support blocks that are not dead, but also not in loop after the
+    // folding.
+    if (BlocksInLoopAfterFolding.size() + DeadLoopBlocks.size() !=
+        L.getNumBlocks()) {
+      LLVM_DEBUG(
+          dbgs() << "Give up constant terminator folding in loop "
+                 << L.getHeader()->getName()
+                 << ": we don't currently"
+                    " support blocks that are not dead, but will stop "
+                    "being a part of the loop after constant-folding.\n");
+      return false;
+    }
+
+    SE.forgetTopmostLoop(&L);
+    // Dump analysis results.
+    LLVM_DEBUG(dump());
+
+    LLVM_DEBUG(dbgs() << "Constant-folding " << FoldCandidates.size()
+                      << " terminators in loop " << L.getHeader()->getName()
+                      << "\n");
+
+    // Make the actual transforms.
+    handleDeadExits();
+    foldTerminators();
+
+    if (!DeadLoopBlocks.empty()) {
+      LLVM_DEBUG(dbgs() << "Deleting " << DeadLoopBlocks.size()
+                    << " dead blocks in loop " << L.getHeader()->getName()
+                    << "\n");
+      deleteDeadLoopBlocks();
+    }
+
+#ifndef NDEBUG
+    // Make sure that we have preserved all data structures after the transform.
+    DT.verify();
+    assert(DT.isReachableFromEntry(L.getHeader()));
+    LI.verify(DT);
+#endif
+
+    return true;
+  }
+};
+} // namespace
+
+/// Turn branches and switches with known constant conditions into unconditional
+/// branches.
+static bool constantFoldTerminators(Loop &L, DominatorTree &DT, LoopInfo &LI,
+                                    ScalarEvolution &SE,
+                                    MemorySSAUpdater *MSSAU) {
+  if (!EnableTermFolding)
+    return false;
+
+  // To keep things simple, only process loops with single latch. We
+  // canonicalize most loops to this form. We can support multi-latch if needed.
+  if (!L.getLoopLatch())
+    return false;
+
+  ConstantTerminatorFoldingImpl BranchFolder(L, LI, DT, SE, MSSAU);
+  return BranchFolder.run();
+}
+
+static bool mergeBlocksIntoPredecessors(Loop &L, DominatorTree &DT,
+                                        LoopInfo &LI, MemorySSAUpdater *MSSAU) {
   bool Changed = false;
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
   // Copy blocks into a temporary array to avoid iterator invalidation issues
   // as we remove them.
   SmallVector<WeakTrackingVH, 16> Blocks(L.blocks());
@@ -57,19 +578,38 @@ static bool simplifyLoopCFG(Loop &L, DominatorTree &DT, LoopInfo &LI,
       continue;
 
     // Merge Succ into Pred and delete it.
-    MergeBlockIntoPredecessor(Succ, &DT, &LI);
+    MergeBlockIntoPredecessor(Succ, &DTU, &LI, MSSAU);
 
-    SE.forgetLoop(&L);
     Changed = true;
   }
 
   return Changed;
 }
 
+static bool simplifyLoopCFG(Loop &L, DominatorTree &DT, LoopInfo &LI,
+                            ScalarEvolution &SE, MemorySSAUpdater *MSSAU) {
+  bool Changed = false;
+
+  // Constant-fold terminators with known constant conditions.
+  Changed |= constantFoldTerminators(L, DT, LI, SE, MSSAU);
+
+  // Eliminate unconditional branches by merging blocks into their predecessors.
+  Changed |= mergeBlocksIntoPredecessors(L, DT, LI, MSSAU);
+
+  if (Changed)
+    SE.forgetTopmostLoop(&L);
+
+  return Changed;
+}
+
 PreservedAnalyses LoopSimplifyCFGPass::run(Loop &L, LoopAnalysisManager &AM,
                                            LoopStandardAnalysisResults &AR,
                                            LPMUpdater &) {
-  if (!simplifyLoopCFG(L, AR.DT, AR.LI, AR.SE))
+  Optional<MemorySSAUpdater> MSSAU;
+  if (EnableMSSALoopDependency && AR.MSSA)
+    MSSAU = MemorySSAUpdater(AR.MSSA);
+  if (!simplifyLoopCFG(L, AR.DT, AR.LI, AR.SE,
+                       MSSAU.hasValue() ? MSSAU.getPointer() : nullptr))
     return PreservedAnalyses::all();
 
   return getLoopPassPreservedAnalyses();
@@ -90,10 +630,22 @@ public:
     DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
     ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
-    return simplifyLoopCFG(*L, DT, LI, SE);
+    Optional<MemorySSAUpdater> MSSAU;
+    if (EnableMSSALoopDependency) {
+      MemorySSA *MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
+      MSSAU = MemorySSAUpdater(MSSA);
+      if (VerifyMemorySSA)
+        MSSA->verifyMemorySSA();
+    }
+    return simplifyLoopCFG(*L, DT, LI, SE,
+                           MSSAU.hasValue() ? MSSAU.getPointer() : nullptr);
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
+    if (EnableMSSALoopDependency) {
+      AU.addRequired<MemorySSAWrapperPass>();
+      AU.addPreserved<MemorySSAWrapperPass>();
+    }
     AU.addPreserved<DependenceAnalysisWrapperPass>();
     getLoopAnalysisUsage(AU);
   }
@@ -104,6 +656,7 @@ char LoopSimplifyCFGLegacyPass::ID = 0;
 INITIALIZE_PASS_BEGIN(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
                       "Simplify loop CFG", false, false)
 INITIALIZE_PASS_DEPENDENCY(LoopPass)
+INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
 INITIALIZE_PASS_END(LoopSimplifyCFGLegacyPass, "loop-simplifycfg",
                     "Simplify loop CFG", false, false)