Index: lib/Transforms/Scalar/LoopUnswitch.cpp
===================================================================
--- lib/Transforms/Scalar/LoopUnswitch.cpp
+++ lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -37,6 +37,8 @@
 #include "llvm/Analysis/InstructionSimplify.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/TargetTransformInfo.h"
 #include "llvm/Analysis/Utils/Local.h"
@@ -65,6 +67,7 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Scalar/LoopPassManager.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Cloning.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
@@ -180,6 +183,8 @@
 
     Loop *currentLoop = nullptr;
     DominatorTree *DT = nullptr;
+    MemorySSA *MSSA = nullptr;
+    std::unique_ptr<MemorySSAUpdater> MSSAUpdater;
     BasicBlock *loopHeader = nullptr;
     BasicBlock *loopPreheader = nullptr;
 
@@ -214,6 +219,8 @@
     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.addRequired<AssumptionCacheTracker>();
       AU.addRequired<TargetTransformInfoWrapperPass>();
+      if (EnableMSSALoopDependency)
+        AU.addRequired<MemorySSAWrapperPass>();
       if (hasBranchDivergence)
         AU.addRequired<DivergenceAnalysis>();
       getLoopAnalysisUsage(AU);
@@ -383,6 +390,7 @@
 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 INITIALIZE_PASS_DEPENDENCY(LoopPass)
 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(DivergenceAnalysis)
 INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
                       false, false)
@@ -515,6 +523,10 @@
   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
   LPM = &LPM_Ref;
   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+  if (EnableMSSALoopDependency) {
+    MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
+    MSSAUpdater = make_unique<MemorySSAUpdater>(MSSA);
+  }
   currentLoop = L;
   Function *F = currentLoop->getHeader()->getParent();
 
@@ -522,6 +534,9 @@
   if (SanitizeMemory)
     computeLoopSafetyInfo(&SafetyInfo, L);
 
+  if (MSSA)
+    MSSA->verifyMemorySSA();
+
   bool Changed = false;
   do {
     assert(currentLoop->isLCSSAForm(*DT));
@@ -529,6 +544,14 @@
     Changed |= processCurrentLoop();
   } while(redoLoop);
 
+  if (MSSA) {
+    // TODO: Compare performance of update vs rebuild MSSA. Hack:
+    // auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
+    // MSSA->~MemorySSA();
+    // new (MSSA) MemorySSA(*F, &AA, DT);
+    MSSA->verifyMemorySSA();
+  }
+
   return Changed;
 }
 
@@ -979,7 +1002,8 @@
   // First step, split the preheader, so that we know that there is a safe place
   // to insert the conditional branch.  We will change loopPreheader to have a
   // conditional branch on Cond.
-  BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, DT, LI);
+  BasicBlock *NewPH =
+      SplitEdge(loopPreheader, loopHeader, DT, LI, MSSA, MSSAUpdater.get());
 
   // Now that we have a place to insert the conditional branch, create a place
   // to branch to: this is the exit block out of the loop that we should
@@ -990,7 +1014,8 @@
   // without actually branching to it (the exit block should be dominated by the
   // loop header, not the preheader).
   assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
-  BasicBlock *NewExit = SplitBlock(ExitBlock, &ExitBlock->front(), DT, LI);
+  BasicBlock *NewExit = SplitBlock(ExitBlock, &ExitBlock->front(), DT, LI, MSSA,
+                                   MSSAUpdater.get());
 
   // Okay, now we have a position to branch from and a position to branch to,
   // insert the new conditional branch.
@@ -1003,6 +1028,9 @@
   // Delete it, as it is no longer needed.
   delete OldBranch;
 
+  if (MSSA)
+    MSSAUpdater->updatePhisWhenAddingBBPredecessors(NewExit, ExitBlock, DT);
+
   // We need to reprocess this loop, it could be unswitched again.
   redoLoop = true;
 
@@ -1185,8 +1213,8 @@
 
     // Although SplitBlockPredecessors doesn't preserve loop-simplify in
     // general, if we call it on all predecessors of all exits then it does.
-    SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", DT, LI, nullptr,
-                           nullptr, /*PreserveLCSSA*/ true);
+    SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", DT, LI, MSSA,
+                           MSSAUpdater.get(), /*PreserveLCSSA*/ true);
   }
 }
 
@@ -1209,7 +1237,8 @@
 
   // First step, split the preheader and exit blocks, and add these blocks to
   // the LoopBlocks list.
-  BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, DT, LI);
+  BasicBlock *NewPreheader =
+      SplitEdge(loopPreheader, loopHeader, DT, LI, MSSA, MSSAUpdater.get());
   LoopBlocks.push_back(NewPreheader);
 
   // We want the loop to come after the preheader, but before the exit blocks.
@@ -1330,6 +1359,13 @@
   // iteration.
   WeakTrackingVH LICHandle(LIC);
 
+  if (MSSA) {
+    // Update MemorySSA after cloning, and before splitting to unreachables,
+    // since that invalidates the 1:1 mapping of clones in VMap.
+    MSSAUpdater->updateForClonedLoop(loopHeader, ExitBlocks, VMap, DT);
+    MSSA->verifyMemorySSA();
+  }
+
   // Now we rewrite the original code to know that the condition is true and the
   // new code to know that the condition is false.
   RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
@@ -1340,6 +1376,10 @@
   if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
       LICHandle && !isa<Constant>(LICHandle))
     RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
+
+  if (MSSA) {
+    MSSA->verifyMemorySSA();
+  }
 }
 
 /// Remove all instances of I from the worklist vector specified.
@@ -1478,7 +1518,7 @@
     // and hooked up so as to preserve the loop structure, because
     // trying to update it is complicated.  So instead we preserve the
     // loop structure and put the block on a dead code path.
-    SplitEdge(Switch, SISucc, DT, LI);
+    SplitEdge(Switch, SISucc, DT, LI, MSSA, MSSAUpdater.get());
     // Compute the successors instead of relying on the return value
     // of SplitEdge, since it may have split the switch successor
     // after PHI nodes.
@@ -1532,6 +1572,9 @@
           Worklist.push_back(Use);
       LPM->deleteSimpleAnalysisValue(I, L);
       RemoveFromWorklist(I, Worklist);
+      if (MSSA)
+        if (MemoryAccess *MA = MSSA->getMemoryAccess(I))
+          MSSAUpdater->removeMemoryAccess(MA);
       I->eraseFromParent();
       ++NumSimplify;
       continue;
@@ -1571,6 +1614,8 @@
         // Move all of the successor contents from Succ to Pred.
         Pred->getInstList().splice(BI->getIterator(), Succ->getInstList(),
                                    Succ->begin(), Succ->end());
+        if (MSSA)
+          MSSAUpdater->moveAllAfterMergeBlocks(Succ, Pred, BI);
         LPM->deleteSimpleAnalysisValue(BI, L);
         RemoveFromWorklist(BI, Worklist);
         BI->eraseFromParent();