[NFC] Implement non-trivial LoopUnswitch with FunctionPass, and integrate it into O3 pipeline

[drcut]

Summary

patch for non-trivial LoopUnswitch implementation

llvm/include/llvm/Transforms/Scalar/FuncSimpleLoopUnswitch.h

+//===- FuncSimpleLoopUnswitch.h - Hoist loop-invariant control flow -*- C++
+//-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_SCALAR_FUNCSIMPLELOOPUNSWITCH_H
+#define LLVM_TRANSFORMS_SCALAR_FUNCSIMPLELOOPUNSWITCH_H
+
+#include "llvm/ADT/STLFunctionalExtras.h"
+#include "llvm/IR/PassManager.h"
+
+namespace llvm {
+
+class Pass;
+class StringRef;
+class raw_ostream;
+
+/// This pass transforms loops that contain branches or switches on loop-
+/// invariant conditions to have multiple loops. For example, it turns the left
+/// into the right code:
+///
+///  for (...)                  if (lic)
+///    A                          for (...)
+///    if (lic)                     A; B; C
+///      B                      else
+///    C                          for (...)
+///                                 A; C
+///
+/// This can increase the size of the code exponentially (doubling it every time
+/// a loop is unswitched) so we only unswitch if the resultant code will be
+/// smaller than a threshold.
+///
+/// This pass expects LICM to be run before it to hoist invariant conditions out
+/// of the loop, to make the unswitching opportunity obvious.
+///
+/// There is a taxonomy of unswitching that we use to classify different forms
+/// of this transformaiton:
+///
+/// - Trival unswitching: this is when the condition can be unswitched without
+///   cloning any code from inside the loop. A non-trivial unswitch requires
+///   code duplication.
+///
+/// - Full unswitching: this is when the branch or switch is completely moved
+///   from inside the loop to outside the loop. Partial unswitching removes the
+///   branch from the clone of the loop but must leave a (somewhat simplified)
+///   branch in the original loop. While theoretically partial unswitching can
+///   be done for switches, the requirements are extreme - we need the loop
+///   invariant input to the switch to be sufficient to collapse to a single
+///   successor in each clone.
+///
+/// This pass always does trivial, full unswitching for both branches and
+/// switches. For branches, it also always does trivial, partial unswitching.
+///
+/// If enabled (via the constructor's `NonTrivial` parameter), this pass will
+/// additionally do non-trivial, full unswitching for branches and switches, and
+/// will do non-trivial, partial unswitching for branches.
+///
+/// Because partial unswitching of switches is extremely unlikely to be possible
+/// in practice and significantly complicates the implementation, this pass does
+/// not currently implement that in any mode.
+class FuncSimpleLoopUnswitchPass
+    : public PassInfoMixin<FuncSimpleLoopUnswitchPass> {
+
+public:
+  FuncSimpleLoopUnswitchPass() {}
+
+  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+
+  void printPipeline(raw_ostream &OS,
+                     function_ref<StringRef(StringRef)> MapClassName2PassName);
+};
+
+} // end namespace llvm
+
+#endif // LLVM_TRANSFORMS_SCALAR_FUNCSIMPLELOOPUNSWITCH_H

llvm/lib/Passes/PassBuilder.cpp

 #include "llvm/Transforms/Scalar/Reg2Mem.h"
 #include "llvm/Transforms/Scalar/RewriteStatepointsForGC.h"
 #include "llvm/Transforms/Scalar/SCCP.h"
 #include "llvm/Transforms/Scalar/SROA.h"
 #include "llvm/Transforms/Scalar/ScalarizeMaskedMemIntrin.h"
 #include "llvm/Transforms/Scalar/Scalarizer.h"
 #include "llvm/Transforms/Scalar/SeparateConstOffsetFromGEP.h"
 #include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
+#include "llvm/Transforms/Scalar/FuncSimpleLoopUnswitch.h"
 #include "llvm/Transforms/Scalar/SimplifyCFG.h"
 #include "llvm/Transforms/Scalar/Sink.h"
 #include "llvm/Transforms/Scalar/SpeculativeExecution.h"
 #include "llvm/Transforms/Scalar/StraightLineStrengthReduce.h"
 #include "llvm/Transforms/Scalar/StructurizeCFG.h"
 #include "llvm/Transforms/Scalar/TLSVariableHoist.h"
 #include "llvm/Transforms/Scalar/TailRecursionElimination.h"
 #include "llvm/Transforms/Scalar/WarnMissedTransforms.h"

llvm/lib/Passes/PassBuilderPipelines.cpp

 #include "llvm/Transforms/Scalar/CallSiteSplitting.h"
 #include "llvm/Transforms/Scalar/ConstraintElimination.h"
 #include "llvm/Transforms/Scalar/CorrelatedValuePropagation.h"
 #include "llvm/Transforms/Scalar/DFAJumpThreading.h"
 #include "llvm/Transforms/Scalar/DeadStoreElimination.h"
 #include "llvm/Transforms/Scalar/DivRemPairs.h"
 #include "llvm/Transforms/Scalar/EarlyCSE.h"
 #include "llvm/Transforms/Scalar/Float2Int.h"
+#include "llvm/Transforms/Scalar/FuncSimpleLoopUnswitch.h"
 #include "llvm/Transforms/Scalar/GVN.h"
 #include "llvm/Transforms/Scalar/IndVarSimplify.h"
 #include "llvm/Transforms/Scalar/InstSimplifyPass.h"
 #include "llvm/Transforms/Scalar/JumpThreading.h"
 #include "llvm/Transforms/Scalar/LICM.h"
 #include "llvm/Transforms/Scalar/LoopDeletion.h"
 #include "llvm/Transforms/Scalar/LoopDistribute.h"
 #include "llvm/Transforms/Scalar/LoopFlatten.h"
static cl::opt<bool> PerformMandatoryInliningsFirst(
     "mandatory-inlining-first", cl::init(true), cl::Hidden,
     cl::desc("Perform mandatory inlinings module-wide, before performing "
              "inlining."));
 
 static cl::opt<bool> EnableO3NonTrivialUnswitching(
     "enable-npm-O3-nontrivial-unswitch", cl::init(true), cl::Hidden,
     cl::desc("Enable non-trivial loop unswitching for -O3"));
 
+static cl::opt<bool> UseFuncNonTrivialUnswitching(
+    "use-FuncPass-nontrivial-unswitch", cl::init(false), cl::Hidden,
+    cl::ZeroOrMore,
+    cl::desc("Use FuncPass implementation for non-trivial loop unswitching"));
+
 static cl::opt<bool> EnableEagerlyInvalidateAnalyses(
     "eagerly-invalidate-analyses", cl::init(true), cl::Hidden,
     cl::desc("Eagerly invalidate more analyses in default pipelines"));
 
 static cl::opt<bool> EnableNoRerunSimplificationPipeline(
     "enable-no-rerun-simplification-pipeline", cl::init(false), cl::Hidden,
     cl::desc(
         "Prevent running the simplification pipeline on a function more "
LPM1.addPass(LICMPass(PTO.LicmMssaOptCap, PTO.LicmMssaNoAccForPromotionCap,
                         /*AllowSpeculation=*/false));
 
   // Disable header duplication in loop rotation at -Oz.
   LPM1.addPass(
       LoopRotatePass(Level != OptimizationLevel::Oz, isLTOPreLink(Phase)));
   // TODO: Investigate promotion cap for O1.
   LPM1.addPass(LICMPass(PTO.LicmMssaOptCap, PTO.LicmMssaNoAccForPromotionCap,
                         /*AllowSpeculation=*/true));
-  LPM1.addPass(
-      SimpleLoopUnswitchPass(/* NonTrivial */ Level == OptimizationLevel::O3 &&
-                             EnableO3NonTrivialUnswitching));
+  bool requireFuncNonTrivialUnswitching = (Level == OptimizationLevel::O3) &&
+                                          EnableO3NonTrivialUnswitching &&
+                                          UseFuncNonTrivialUnswitching;
+  if (!requireFuncNonTrivialUnswitching) {
+    LPM1.addPass(SimpleLoopUnswitchPass(
+        /* NonTrivial */ (Level == OptimizationLevel::O3) &&
     if (EnableLoopFlatten)
       LPM1.addPass(LoopFlattenPass());
+  } else {
+    LPM1.addPass(SimpleLoopUnswitchPass(
+        /* NonTrivial */ false));
+  }
 
   LPM2.addPass(LoopIdiomRecognizePass());
   LPM2.addPass(IndVarSimplifyPass());
 
   for (auto &C : LateLoopOptimizationsEPCallbacks)
     C(LPM2, Level);
 
   LPM2.addPass(LoopDeletionPass());
PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
 
   // We provide the opt remark emitter pass for LICM to use. We only need to do
   // this once as it is immutable.
   FPM.addPass(
       RequireAnalysisPass<OptimizationRemarkEmitterAnalysis, Function>());
   FPM.addPass(createFunctionToLoopPassAdaptor(std::move(LPM1),
                                               /*UseMemorySSA=*/true,
                                               /*UseBlockFrequencyInfo=*/true));
+  if (requireFuncNonTrivialUnswitching) {
+    FPM.addPass(FuncSimpleLoopUnswitchPass());
+    FPM.addPass(createFunctionToLoopPassAdaptor(
+        LoopFlattenPass(),
+        /*UseMemorySSA=*/true, /*UseBlockFrequencyInfo=*/true));
+  }
   FPM.addPass(
       SimplifyCFGPass(SimplifyCFGOptions().convertSwitchRangeToICmp(true)));
   FPM.addPass(InstCombinePass());
   // The loop passes in LPM2 (LoopIdiomRecognizePass, IndVarSimplifyPass,
   // LoopDeletionPass and LoopFullUnrollPass) do not preserve MemorySSA.
   // *All* loop passes must preserve it, in order to be able to use it.
   FPM.addPass(createFunctionToLoopPassAdaptor(std::move(LPM2),
                                               /*UseMemorySSA=*/false,

llvm/lib/Passes/PassRegistry.def

 FUNCTION_PASS("dot-post-dom", PostDomPrinter())
 FUNCTION_PASS("dot-post-dom-only", PostDomOnlyPrinter())
 FUNCTION_PASS("view-dom", DomViewer())
 FUNCTION_PASS("view-dom-only", DomOnlyViewer())
 FUNCTION_PASS("view-post-dom", PostDomViewer())
 FUNCTION_PASS("view-post-dom-only", PostDomOnlyViewer())
 FUNCTION_PASS("fix-irreducible", FixIrreduciblePass())
 FUNCTION_PASS("flattencfg", FlattenCFGPass())
+FUNCTION_PASS("func-simple-loop-unswitch", FuncSimpleLoopUnswitchPass())
 FUNCTION_PASS("make-guards-explicit", MakeGuardsExplicitPass())
 FUNCTION_PASS("gvn-hoist", GVNHoistPass())
 FUNCTION_PASS("gvn-sink", GVNSinkPass())
 FUNCTION_PASS("helloworld", HelloWorldPass())
 FUNCTION_PASS("infer-address-spaces", InferAddressSpacesPass())
 FUNCTION_PASS("instcombine", InstCombinePass())
 FUNCTION_PASS("instcount", InstCountPass())
 FUNCTION_PASS("instsimplify", InstSimplifyPass())

llvm/lib/Transforms/Scalar/CMakeLists.txt

add_llvm_component_library(LLVMScalarOpts
   RewriteStatepointsForGC.cpp
   SCCP.cpp
   SROA.cpp
   Scalar.cpp
   Scalarizer.cpp
   ScalarizeMaskedMemIntrin.cpp
   SeparateConstOffsetFromGEP.cpp
   SimpleLoopUnswitch.cpp
+  FuncSimpleLoopUnswitch.cpp
   SimplifyCFGPass.cpp
   Sink.cpp
   SpeculativeExecution.cpp
   StraightLineStrengthReduce.cpp
   StructurizeCFG.cpp
   TailRecursionElimination.cpp
   TLSVariableHoist.cpp
   WarnMissedTransforms.cpp

llvm/lib/Transforms/Scalar/FuncSimpleLoopUnswitch.cpp

+///===- FuncSimpleLoopUnswitch.cpp - Hoist loop-invariant control flow
+///---------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar/FuncSimpleLoopUnswitch.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/Sequence.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/CFG.h"
+#include "llvm/Analysis/CodeMetrics.h"
+#include "llvm/Analysis/GuardUtils.h"
+#include "llvm/Analysis/LoopAnalysisManager.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopIterator.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/MemorySSA.h"
+#include "llvm/Analysis/MemorySSAUpdater.h"
+#include "llvm/Analysis/MustExecute.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constant.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/InstrTypes.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/IR/Use.h"
+#include "llvm/IR/Value.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/GenericDomTree.h"
+#include "llvm/Support/InstructionCost.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Scalar/LoopPassManager.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
+#include <algorithm>
+#include <cassert>
+#include <iterator>
+#include <numeric>
+#include <utility>
+
+#define DEBUG_TYPE "func-simple-loop-unswitch"
+
+using namespace llvm;
+using namespace llvm::PatternMatch;
+
+STATISTIC(NumBranches, "Number of branches unswitched");
+STATISTIC(NumSwitches, "Number of switches unswitched");
+STATISTIC(NumGuards, "Number of guards turned into branches for unswitching");
+STATISTIC(
+    NumCostMultiplierSkipped,
+    "Number of unswitch candidates that had their cost multiplier skipped");
+
+static cl::opt<int>
+    UnswitchThreshold("func-unswitch-threshold", cl::init(50), cl::Hidden,
+                      cl::ZeroOrMore,
+                      cl::desc("The cost threshold for unswitching a loop."));
+
+static cl::opt<bool> EnableUnswitchCostMultiplier(
+    "func-enable-unswitch-cost-multiplier", cl::init(true), cl::Hidden,
+    cl::desc("Enable unswitch cost multiplier that prohibits exponential "
+             "explosion in nontrivial unswitch."));
+static cl::opt<int> UnswitchSiblingsToplevelDiv(
+    "func-unswitch-siblings-toplevel-div", cl::init(2), cl::Hidden,
+    cl::desc("Toplevel siblings divisor for cost multiplier."));
+static cl::opt<int> UnswitchNumInitialUnscaledCandidates(
+    "func-unswitch-num-initial-unscaled-candidates", cl::init(8), cl::Hidden,
+    cl::desc("Number of unswitch candidates that are ignored when calculating "
+             "cost multiplier."));
+static cl::opt<bool> UnswitchGuards(
+    "func-simple-loop-unswitch-guards", cl::init(true), cl::Hidden,
+    cl::desc("If enabled, simple loop unswitching will also consider "
+             "llvm.experimental.guard intrinsics as unswitch candidates."));
+static cl::opt<bool> DropNonTrivialImplicitNullChecks(
+    "func-simple-loop-unswitch-drop-non-trivial-implicit-null-checks",
+    cl::init(false), cl::Hidden,
+    cl::desc("If enabled, drop make.implicit metadata in unswitched implicit "
+             "null checks to save time analyzing if we can keep it."));
+static cl::opt<unsigned>
+    MSSAThreshold("func-simple-loop-unswitch-memoryssa-threshold",
+                  cl::desc("Max number of memory uses to explore during "
+                           "partial unswitching analysis"),
+                  cl::init(100), cl::Hidden);
+static cl::opt<bool> FreezeLoopUnswitchCond(
+    "func-freeze-loop-unswitch-cond", cl::init(true), cl::Hidden,
+    cl::desc("If enabled, the freeze instruction will be added to condition "
+             "of loop unswitch to prevent miscompilation."));
+
+// Helper to skip (select x, true, false), which matches both a logical AND and
+// OR and can confuse code that tries to determine if \p Cond is either a
+// logical AND or OR but not both.
+static Value *skipTrivialSelect(Value *Cond) {
+  Value *CondNext;
+  while (match(Cond, m_Select(m_Value(CondNext), m_One(), m_Zero())))
+    Cond = CondNext;
+  return Cond;
+}
+
+/// Collect all of the loop invariant input values transitively used by the
+/// homogeneous instruction graph from a given root.
+///
+/// This essentially walks from a root recursively through loop variant operands
+/// which have perform the same logical operation (AND or OR) and finds all
+/// inputs which are loop invariant. For some operations these can be
+/// re-associated and unswitched out of the loop entirely.
+static TinyPtrVector<Value *>
+collectHomogenousInstGraphLoopInvariants(Loop &L, Instruction &Root,
+                                         LoopInfo &LI) {
+  assert(!L.isLoopInvariant(&Root) &&
+         "Only need to walk the graph if root itself is not invariant.");
+  TinyPtrVector<Value *> Invariants;
+
+  bool IsRootAnd = match(&Root, m_LogicalAnd());
+  bool IsRootOr = match(&Root, m_LogicalOr());
+
+  // Build a Worklist and recurse through operators collecting invariants.
+  SmallVector<Instruction *, 4> Worklist;
+  SmallPtrSet<Instruction *, 8> Visited;
+  Worklist.push_back(&Root);
+  Visited.insert(&Root);
+  do {
+    Instruction &I = *Worklist.pop_back_val();
+    for (Value *OpV : I.operand_values()) {
+      // Skip constants as unswitching isn't interesting for them.
+      if (isa<Constant>(OpV))
+        continue;
+
+      // Add it to our result if loop invariant.
+      if (L.isLoopInvariant(OpV)) {
+        Invariants.push_back(OpV);
+        continue;
+      }
+
+      // If not an instruction with the same opcode, nothing we can do.
+      Instruction *OpI = dyn_cast<Instruction>(skipTrivialSelect(OpV));
+
+      if (OpI && ((IsRootAnd && match(OpI, m_LogicalAnd())) ||
+                  (IsRootOr && match(OpI, m_LogicalOr())))) {
+        // Visit this operand.
+        if (Visited.insert(OpI).second)
+          Worklist.push_back(OpI);
+      }
+    }
+  } while (!Worklist.empty());
+
+  return Invariants;
+}
+
+static void replaceLoopInvariantUses(Loop &L, Value *Invariant,
+                                     Constant &Replacement) {
+  assert(!isa<Constant>(Invariant) && "Why are we unswitching on a constant?");
+
+  // Replace uses of LIC in the loop with the given constant.
+  // We use make_early_inc_range as set invalidates the iterator.
+  for (Use &U : llvm::make_early_inc_range(Invariant->uses())) {
+    Instruction *UserI = dyn_cast<Instruction>(U.getUser());
+
+    // Replace this use within the loop body.
+    if (UserI && L.contains(UserI))
+      U.set(&Replacement);
+  }
+}
+
+/// Check that all the LCSSA PHI nodes in the loop exit block have trivial
+/// incoming values along this edge.
+static bool areLoopExitPHIsLoopInvariant(Loop &L, BasicBlock &ExitingBB,
+                                         BasicBlock &ExitBB) {
+  for (Instruction &I : ExitBB) {
+    auto *PN = dyn_cast<PHINode>(&I);
+    if (!PN)
+      // No more PHIs to check.
+      return true;
+
+    // If the incoming value for this edge isn't loop invariant the unswitch
+    // won't be trivial.
+    if (!L.isLoopInvariant(PN->getIncomingValueForBlock(&ExitingBB)))
+      return false;
+  }
+  llvm_unreachable("Basic blocks should never be empty!");
+}
+
+/// Copy a set of loop invariant values \p ToDuplicate and insert them at the
+/// end of \p BB and conditionally branch on the copied condition. We only
+/// branch on a single value.
+static void buildPartialUnswitchConditionalBranch(
+    BasicBlock &BB, ArrayRef<Value *> Invariants, bool Direction,
+    BasicBlock &UnswitchedSucc, BasicBlock &NormalSucc, bool InsertFreeze,
+    Instruction *I, AssumptionCache *AC, DominatorTree &DT) {
+  IRBuilder<> IRB(&BB);
+
+  SmallVector<Value *> FrozenInvariants;
+  for (Value *Inv : Invariants) {
+    if (InsertFreeze && !isGuaranteedNotToBeUndefOrPoison(Inv, AC, I, &DT))
+      Inv = IRB.CreateFreeze(Inv, Inv->getName() + ".fr");
+    FrozenInvariants.push_back(Inv);
+  }
+
+  Value *Cond = Direction ? IRB.CreateOr(FrozenInvariants)
+                          : IRB.CreateAnd(FrozenInvariants);
+  IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc,
+                   Direction ? &NormalSucc : &UnswitchedSucc);
+}
+
+/// Copy a set of loop invariant values, and conditionally branch on them.
+static void buildPartialInvariantUnswitchConditionalBranch(
+    BasicBlock &BB, ArrayRef<Value *> ToDuplicate, bool Direction,
+    BasicBlock &UnswitchedSucc, BasicBlock &NormalSucc, Loop &L,
+    MemorySSAUpdater *MSSAU) {
+  ValueToValueMapTy VMap;
+  for (auto *Val : reverse(ToDuplicate)) {
+    Instruction *Inst = cast<Instruction>(Val);
+    Instruction *NewInst = Inst->clone();
+    BB.getInstList().insert(BB.end(), NewInst);
+    RemapInstruction(NewInst, VMap,
+                     RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
+    VMap[Val] = NewInst;
+
+    if (!MSSAU)
+      continue;
+
+    MemorySSA *MSSA = MSSAU->getMemorySSA();
+    if (auto *MemUse =
+            dyn_cast_or_null<MemoryUse>(MSSA->getMemoryAccess(Inst))) {
+      auto *DefiningAccess = MemUse->getDefiningAccess();
+      // Get the first defining access before the loop.
+      while (L.contains(DefiningAccess->getBlock())) {
+        // If the defining access is a MemoryPhi, get the incoming
+        // value for the pre-header as defining access.
+        if (auto *MemPhi = dyn_cast<MemoryPhi>(DefiningAccess))
+          DefiningAccess =
+              MemPhi->getIncomingValueForBlock(L.getLoopPreheader());
+        else
+          DefiningAccess = cast<MemoryDef>(DefiningAccess)->getDefiningAccess();
+      }
+      MSSAU->createMemoryAccessInBB(NewInst, DefiningAccess,
+                                    NewInst->getParent(),
+                                    MemorySSA::BeforeTerminator);
+    }
+  }
+
+  IRBuilder<> IRB(&BB);
+  Value *Cond = VMap[ToDuplicate[0]];
+  IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc,
+                   Direction ? &NormalSucc : &UnswitchedSucc);
+}
+
+/// Rewrite the PHI nodes in an unswitched loop exit basic block.
+///
+/// Requires that the loop exit and unswitched basic block are the same, and
+/// that the exiting block was a unique predecessor of that block. Rewrites the
+/// PHI nodes in that block such that what were LCSSA PHI nodes become trivial
+/// PHI nodes from the old preheader that now contains the unswitched
+/// terminator.
+static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB,
+                                                  BasicBlock &OldExitingBB,
+                                                  BasicBlock &OldPH) {
+  for (PHINode &PN : UnswitchedBB.phis()) {
+    // When the loop exit is directly unswitched we just need to update the
+    // incoming basic block. We loop to handle weird cases with repeated
+    // incoming blocks, but expect to typically only have one operand here.
+    for (auto i : seq<int>(0, PN.getNumOperands())) {
+      assert(PN.getIncomingBlock(i) == &OldExitingBB &&
+             "Found incoming block different from unique predecessor!");
+      PN.setIncomingBlock(i, &OldPH);
+    }
+  }
+}
+
+/// Rewrite the PHI nodes in the loop exit basic block and the split off
+/// unswitched block.
+///
+/// Because the exit block remains an exit from the loop, this rewrites the
+/// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI
+/// nodes into the unswitched basic block to select between the value in the
+/// old preheader and the loop exit.
+static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB,
+                                                      BasicBlock &UnswitchedBB,
+                                                      BasicBlock &OldExitingBB,
+                                                      BasicBlock &OldPH,
+                                                      bool FullUnswitch) {
+  assert(&ExitBB != &UnswitchedBB &&
+         "Must have different loop exit and unswitched blocks!");
+  Instruction *InsertPt = &*UnswitchedBB.begin();
+  for (PHINode &PN : ExitBB.phis()) {
+    auto *NewPN = PHINode::Create(PN.getType(), /*NumReservedValues*/ 2,
+                                  PN.getName() + ".split", InsertPt);
+
+    // Walk backwards over the old PHI node's inputs to minimize the cost of
+    // removing each one. We have to do this weird loop manually so that we
+    // create the same number of new incoming edges in the new PHI as we expect
+    // each case-based edge to be included in the unswitched switch in some
+    // cases.
+    // FIXME: This is really, really gross. It would be much cleaner if LLVM
+    // allowed us to create a single entry for a predecessor block without
+    // having separate entries for each "edge" even though these edges are
+    // required to produce identical results.
+    for (int i = PN.getNumIncomingValues() - 1; i >= 0; --i) {
+      if (PN.getIncomingBlock(i) != &OldExitingBB)
+        continue;
+
+      Value *Incoming = PN.getIncomingValue(i);
+      if (FullUnswitch)
+        // No more edge from the old exiting block to the exit block.
+        PN.removeIncomingValue(i);
+
+      NewPN->addIncoming(Incoming, &OldPH);
+    }
+
+    // Now replace the old PHI with the new one and wire the old one in as an
+    // input to the new one.
+    PN.replaceAllUsesWith(NewPN);
+    NewPN->addIncoming(&PN, &ExitBB);
+  }
+}
+
+/// Hoist the current loop up to the innermost loop containing a remaining exit.
+///
+/// Because we've removed an exit from the loop, we may have changed the set of
+/// loops reachable and need to move the current loop up the loop nest or even
+/// to an entirely separate nest.
+static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
+                                 DominatorTree &DT, LoopInfo &LI,
+                                 MemorySSAUpdater *MSSAU, ScalarEvolution *SE) {
+  // If the loop is already at the top level, we can't hoist it anywhere.
+  Loop *OldParentL = L.getParentLoop();
+  if (!OldParentL)
+    return;
+
+  SmallVector<BasicBlock *, 4> Exits;
+  L.getExitBlocks(Exits);
+  Loop *NewParentL = nullptr;
+  for (auto *ExitBB : Exits)
+    if (Loop *ExitL = LI.getLoopFor(ExitBB))
+      if (!NewParentL || NewParentL->contains(ExitL))
+        NewParentL = ExitL;
+
+  if (NewParentL == OldParentL)
+    return;
+
+  // The new parent loop (if different) should always contain the old one.
+  if (NewParentL)
+    assert(NewParentL->contains(OldParentL) &&
+           "Can only hoist this loop up the nest!");
+
+  // The preheader will need to move with the body of this loop. However,
+  // because it isn't in this loop we also need to update the primary loop map.
+  assert(OldParentL == LI.getLoopFor(&Preheader) &&
+         "Parent loop of this loop should contain this loop's preheader!");
+  LI.changeLoopFor(&Preheader, NewParentL);
+
+  // Remove this loop from its old parent.
+  OldParentL->removeChildLoop(&L);
+
+  // Add the loop either to the new parent or as a top-level loop.
+  if (NewParentL)
+    NewParentL->addChildLoop(&L);
+  else
+    LI.addTopLevelLoop(&L);
+
+  // Remove this loops blocks from the old parent and every other loop up the
+  // nest until reaching the new parent. Also update all of these
+  // no-longer-containing loops to reflect the nesting change.
+  for (Loop *OldContainingL = OldParentL; OldContainingL != NewParentL;
+       OldContainingL = OldContainingL->getParentLoop()) {
+    llvm::erase_if(OldContainingL->getBlocksVector(),
+                   [&](const BasicBlock *BB) {
+                     return BB == &Preheader || L.contains(BB);
+                   });
+
+    OldContainingL->getBlocksSet().erase(&Preheader);
+    for (BasicBlock *BB : L.blocks())
+      OldContainingL->getBlocksSet().erase(BB);
+
+    // Because we just hoisted a loop out of this one, we have essentially
+    // created new exit paths from it. That means we need to form LCSSA PHI
+    // nodes for values used in the no-longer-nested loop.
+    formLCSSA(*OldContainingL, DT, &LI, SE);
+
+    // We shouldn't need to form dedicated exits because the exit introduced
+    // here is the (just split by unswitching) preheader. However, after trivial
+    // unswitching it is possible to get new non-dedicated exits out of parent
+    // loop so let's conservatively form dedicated exit blocks and figure out
+    // if we can optimize later.
+    formDedicatedExitBlocks(OldContainingL, &DT, &LI, MSSAU,
+                            /*PreserveLCSSA*/ true);
+  }
+}
+
+/// Build the cloned blocks for an unswitched copy of the given loop.
+///
+/// The cloned blocks are inserted before the loop preheader (`LoopPH`) and
+/// after the split block (`SplitBB`) that will be used to select between the
+/// cloned and original loop.
+///
+/// This routine handles cloning all of the necessary loop blocks and exit
+/// blocks including rewriting their instructions and the relevant PHI nodes.
+/// Any loop blocks or exit blocks which are dominated by a different successor
+/// than the one for this clone of the loop blocks can be trivially skipped. We
+/// use the `DominatingSucc` map to determine whether a block satisfies that
+/// property with a simple map lookup.
+///
+/// It also correctly creates the unconditional branch in the cloned
+/// unswitched parent block to only point at the unswitched successor.
+///
+/// This does not handle most of the necessary updates to `LoopInfo`. Only exit
+/// block splitting is correctly reflected in `LoopInfo`, essentially all of
+/// the cloned blocks (and their loops) are left without full `LoopInfo`
+/// updates. This also doesn't fully update `DominatorTree`. It adds the cloned
+/// blocks to them but doesn't create the cloned `DominatorTree` structure and
+/// instead the caller must recompute an accurate DT. It *does* correctly
+/// update the `AssumptionCache` provided in `AC`.
+static BasicBlock *buildClonedLoopBlocks(
+    Loop &L, BasicBlock *LoopPH, BasicBlock *SplitBB,
+    ArrayRef<BasicBlock *> ExitBlocks, BasicBlock *ParentBB,
+    BasicBlock *UnswitchedSuccBB, BasicBlock *ContinueSuccBB,
+    const SmallDenseMap<BasicBlock *, BasicBlock *, 16> &DominatingSucc,
+    ValueToValueMapTy &VMap,
+    SmallVectorImpl<DominatorTree::UpdateType> &DTUpdates, AssumptionCache &AC,
+    DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
+  SmallVector<BasicBlock *, 4> NewBlocks;
+  NewBlocks.reserve(L.getNumBlocks() + ExitBlocks.size());
+
+  // We will need to clone a bunch of blocks, wrap up the clone operation in
+  // a helper.
+  auto CloneBlock = [&](BasicBlock *OldBB) {
+    // Clone the basic block and insert it before the new preheader.
+    BasicBlock *NewBB = CloneBasicBlock(OldBB, VMap, ".us", OldBB->getParent());
+    NewBB->moveBefore(LoopPH);
+
+    // Record this block and the mapping.
+    NewBlocks.push_back(NewBB);
+    VMap[OldBB] = NewBB;
+
+    return NewBB;
+  };
+
+  // We skip cloning blocks when they have a dominating succ that is not the
+  // succ we are cloning for.
+  auto SkipBlock = [&](BasicBlock *BB) {
+    auto It = DominatingSucc.find(BB);
+    return It != DominatingSucc.end() && It->second != UnswitchedSuccBB;
+  };
+
+  // First, clone the preheader.
+  auto *ClonedPH = CloneBlock(LoopPH);
+
+  // Then clone all the loop blocks, skipping the ones that aren't necessary.
+  for (auto *LoopBB : L.blocks())
+    if (!SkipBlock(LoopBB))
+      CloneBlock(LoopBB);
+
+  // Split all the loop exit edges so that when we clone the exit blocks, if
+  // any of the exit blocks are *also* a preheader for some other loop, we
+  // don't create multiple predecessors entering the loop header.
+  for (auto *ExitBB : ExitBlocks) {
+    if (SkipBlock(ExitBB))
+      continue;
+
+    // When we are going to clone an exit, we don't need to clone all the
+    // instructions in the exit block and we want to ensure we have an easy
+    // place to merge the CFG, so split the exit first. This is always safe to
+    // do because there cannot be any non-loop predecessors of a loop exit in
+    // loop simplified form.
+    auto *MergeBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI, MSSAU);
+
+    // Rearrange the names to make it easier to write test cases by having the
+    // exit block carry the suffix rather than the merge block carrying the
+    // suffix.
+    MergeBB->takeName(ExitBB);
+    ExitBB->setName(Twine(MergeBB->getName()) + ".split");
+
+    // Now clone the original exit block.
+    auto *ClonedExitBB = CloneBlock(ExitBB);
+    assert(ClonedExitBB->getTerminator()->getNumSuccessors() == 1 &&
+           "Exit block should have been split to have one successor!");
+    assert(ClonedExitBB->getTerminator()->getSuccessor(0) == MergeBB &&
+           "Cloned exit block has the wrong successor!");
+
+    // Remap any cloned instructions and create a merge phi node for them.
+    for (auto ZippedInsts : llvm::zip_first(
+             llvm::make_range(ExitBB->begin(), std::prev(ExitBB->end())),
+             llvm::make_range(ClonedExitBB->begin(),
+                              std::prev(ClonedExitBB->end())))) {
+      Instruction &I = std::get<0>(ZippedInsts);
+      Instruction &ClonedI = std::get<1>(ZippedInsts);
+
+      // The only instructions in the exit block should be PHI nodes and
+      // potentially a landing pad.
+      assert(
+          (isa<PHINode>(I) || isa<LandingPadInst>(I) || isa<CatchPadInst>(I)) &&
+          "Bad instruction in exit block!");
+      // We should have a value map between the instruction and its clone.
+      assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!");
+
+      auto *MergePN =
+          PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi",
+                          &*MergeBB->getFirstInsertionPt());
+      I.replaceAllUsesWith(MergePN);
+      MergePN->addIncoming(&I, ExitBB);
+      MergePN->addIncoming(&ClonedI, ClonedExitBB);
+    }
+  }
+
+  // Rewrite the instructions in the cloned blocks to refer to the instructions
+  // in the cloned blocks. We have to do this as a second pass so that we have
+  // everything available. Also, we have inserted new instructions which may
+  // include assume intrinsics, so we update the assumption cache while
+  // processing this.
+  for (auto *ClonedBB : NewBlocks)
+    for (Instruction &I : *ClonedBB) {
+      RemapInstruction(&I, VMap,
+                       RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
+      if (auto *II = dyn_cast<AssumeInst>(&I))
+        AC.registerAssumption(II);
+    }
+
+  // Update any PHI nodes in the cloned successors of the skipped blocks to not
+  // have spurious incoming values.
+  for (auto *LoopBB : L.blocks())
+    if (SkipBlock(LoopBB))
+      for (auto *SuccBB : successors(LoopBB))
+        if (auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB)))
+          for (PHINode &PN : ClonedSuccBB->phis())
+            PN.removeIncomingValue(LoopBB, /*DeletePHIIfEmpty*/ false);
+
+  // Remove the cloned parent as a predecessor of any successor we ended up
+  // cloning other than the unswitched one.
+  auto *ClonedParentBB = cast<BasicBlock>(VMap.lookup(ParentBB));
+  for (auto *SuccBB : successors(ParentBB)) {
+    if (SuccBB == UnswitchedSuccBB)
+      continue;
+
+    auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB));
+    if (!ClonedSuccBB)
+      continue;
+
+    ClonedSuccBB->removePredecessor(ClonedParentBB,
+                                    /*KeepOneInputPHIs*/ true);
+  }
+
+  // Replace the cloned branch with an unconditional branch to the cloned
+  // unswitched successor.
+  auto *ClonedSuccBB = cast<BasicBlock>(VMap.lookup(UnswitchedSuccBB));
+  Instruction *ClonedTerminator = ClonedParentBB->getTerminator();
+  // Trivial Simplification. If Terminator is a conditional branch and
+  // condition becomes dead - erase it.
+  Value *ClonedConditionToErase = nullptr;
+  if (auto *BI = dyn_cast<BranchInst>(ClonedTerminator))
+    ClonedConditionToErase = BI->getCondition();
+  else if (auto *SI = dyn_cast<SwitchInst>(ClonedTerminator))
+    ClonedConditionToErase = SI->getCondition();
+
+  ClonedTerminator->eraseFromParent();
+  BranchInst::Create(ClonedSuccBB, ClonedParentBB);
+
+  if (ClonedConditionToErase)
+    RecursivelyDeleteTriviallyDeadInstructions(ClonedConditionToErase, nullptr,
+                                               MSSAU);
+
+  // If there are duplicate entries in the PHI nodes because of multiple edges
+  // to the unswitched successor, we need to nuke all but one as we replaced it
+  // with a direct branch.
+  for (PHINode &PN : ClonedSuccBB->phis()) {
+    bool Found = false;
+    // Loop over the incoming operands backwards so we can easily delete as we
+    // go without invalidating the index.
+    for (int i = PN.getNumOperands() - 1; i >= 0; --i) {
+      if (PN.getIncomingBlock(i) != ClonedParentBB)
+        continue;
+      if (!Found) {
+        Found = true;
+        continue;
+      }
+      PN.removeIncomingValue(i, /*DeletePHIIfEmpty*/ false);
+    }
+  }
+
+  // Record the domtree updates for the new blocks.
+  SmallPtrSet<BasicBlock *, 4> SuccSet;
+  for (auto *ClonedBB : NewBlocks) {
+    for (auto *SuccBB : successors(ClonedBB))
+      if (SuccSet.insert(SuccBB).second)
+        DTUpdates.push_back({DominatorTree::Insert, ClonedBB, SuccBB});
+    SuccSet.clear();
+  }
+
+  return ClonedPH;
+}
+
+/// Recursively clone the specified loop and all of its children.
+///
+/// The target parent loop for the clone should be provided, or can be null if
+/// the clone is a top-level loop. While cloning, all the blocks are mapped
+/// with the provided value map. The entire original loop must be present in
+/// the value map. The cloned loop is returned.
+static Loop *cloneLoopNest(Loop &OrigRootL, Loop *RootParentL,
+                           const ValueToValueMapTy &VMap, LoopInfo &LI) {
+  auto AddClonedBlocksToLoop = [&](Loop &OrigL, Loop &ClonedL) {
+    assert(ClonedL.getBlocks().empty() && "Must start with an empty loop!");
+    ClonedL.reserveBlocks(OrigL.getNumBlocks());
+    for (auto *BB : OrigL.blocks()) {
+      auto *ClonedBB = cast<BasicBlock>(VMap.lookup(BB));
+      ClonedL.addBlockEntry(ClonedBB);
+      if (LI.getLoopFor(BB) == &OrigL)
+        LI.changeLoopFor(ClonedBB, &ClonedL);
+    }
+  };
+
+  // We specially handle the first loop because it may get cloned into
+  // a different parent and because we most commonly are cloning leaf loops.
+  Loop *ClonedRootL = LI.AllocateLoop();
+  if (RootParentL)
+    RootParentL->addChildLoop(ClonedRootL);
+  else
+    LI.addTopLevelLoop(ClonedRootL);
+  AddClonedBlocksToLoop(OrigRootL, *ClonedRootL);
+
+  if (OrigRootL.isInnermost())
+    return ClonedRootL;
+
+  // If we have a nest, we can quickly clone the entire loop nest using an
+  // iterative approach because it is a tree. We keep the cloned parent in the
+  // data structure to avoid repeatedly querying through a map to find it.
+  SmallVector<std::pair<Loop *, Loop *>, 16> LoopsToClone;
+  // Build up the loops to clone in reverse order as we'll clone them from the
+  // back.
+  for (Loop *ChildL : llvm::reverse(OrigRootL))
+    LoopsToClone.push_back({ClonedRootL, ChildL});
+  do {
+    Loop *ClonedParentL, *L;
+    std::tie(ClonedParentL, L) = LoopsToClone.pop_back_val();
+    Loop *ClonedL = LI.AllocateLoop();
+    ClonedParentL->addChildLoop(ClonedL);
+    AddClonedBlocksToLoop(*L, *ClonedL);
+    for (Loop *ChildL : llvm::reverse(*L))
+      LoopsToClone.push_back({ClonedL, ChildL});
+  } while (!LoopsToClone.empty());
+
+  return ClonedRootL;
+}
+
+/// Build the cloned loops of an original loop from unswitching.
+///
+/// Because unswitching simplifies the CFG of the loop, this isn't a trivial
+/// operation. We need to re-verify that there even is a loop (as the backedge
+/// may not have been cloned), and even if there are remaining backedges the
+/// backedge set may be different. However, we know that each child loop is
+/// undisturbed, we only need to find where to place each child loop within
+/// either any parent loop or within a cloned version of the original loop.
+///
+/// Because child loops may end up cloned outside of any cloned version of the
+/// original loop, multiple cloned sibling loops may be created. All of them
+/// are returned so that the newly introduced loop nest roots can be
+/// identified.
+static void buildClonedLoops(Loop &OrigL, ArrayRef<BasicBlock *> ExitBlocks,
+                             const ValueToValueMapTy &VMap, LoopInfo &LI,
+                             SmallVectorImpl<Loop *> &NonChildClonedLoops) {
+  Loop *ClonedL = nullptr;
+
+  auto *OrigPH = OrigL.getLoopPreheader();
+  auto *OrigHeader = OrigL.getHeader();
+
+  auto *ClonedPH = cast<BasicBlock>(VMap.lookup(OrigPH));
+  auto *ClonedHeader = cast<BasicBlock>(VMap.lookup(OrigHeader));
+
+  // We need to know the loops of the cloned exit blocks to even compute the
+  // accurate parent loop. If we only clone exits to some parent of the
+  // original parent, we want to clone into that outer loop. We also keep track
+  // of the loops that our cloned exit blocks participate in.
+  Loop *ParentL = nullptr;
+  SmallVector<BasicBlock *, 4> ClonedExitsInLoops;
+  SmallDenseMap<BasicBlock *, Loop *, 16> ExitLoopMap;
+  ClonedExitsInLoops.reserve(ExitBlocks.size());
+  for (auto *ExitBB : ExitBlocks)
+    if (auto *ClonedExitBB = cast_or_null<BasicBlock>(VMap.lookup(ExitBB)))
+      if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
+        ExitLoopMap[ClonedExitBB] = ExitL;
+        ClonedExitsInLoops.push_back(ClonedExitBB);
+        if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
+          ParentL = ExitL;
+      }
+  assert((!ParentL || ParentL == OrigL.getParentLoop() ||
+          ParentL->contains(OrigL.getParentLoop())) &&
+         "The computed parent loop should always contain (or be) the parent of "
+         "the original loop.");
+
+  // We build the set of blocks dominated by the cloned header from the set of
+  // cloned blocks out of the original loop. While not all of these will
+  // necessarily be in the cloned loop, it is enough to establish that they
+  // aren't in unreachable cycles, etc.
+  SmallSetVector<BasicBlock *, 16> ClonedLoopBlocks;
+  for (auto *BB : OrigL.blocks())
+    if (auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB)))
+      ClonedLoopBlocks.insert(ClonedBB);
+
+  // Rebuild the set of blocks that will end up in the cloned loop. We may have
+  // skipped cloning some region of this loop which can in turn skip some of
+  // the backedges so we have to rebuild the blocks in the loop based on the
+  // backedges that remain after cloning.
+  SmallVector<BasicBlock *, 16> Worklist;
+  SmallPtrSet<BasicBlock *, 16> BlocksInClonedLoop;
+  for (auto *Pred : predecessors(ClonedHeader)) {
+    // The only possible non-loop header predecessor is the preheader because
+    // we know we cloned the loop in simplified form.
+    if (Pred == ClonedPH)
+      continue;
+
+    // Because the loop was in simplified form, the only non-loop predecessor
+    // should be the preheader.
+    assert(ClonedLoopBlocks.count(Pred) && "Found a predecessor of the loop "
+                                           "header other than the preheader "
+                                           "that is not part of the loop!");
+
+    // Insert this block into the loop set and on the first visit (and if it
+    // isn't the header we're currently walking) put it into the Worklist to
+    // recurse through.
+    if (BlocksInClonedLoop.insert(Pred).second && Pred != ClonedHeader)
+      Worklist.push_back(Pred);
+  }
+
+  // If we had any backedges then there *is* a cloned loop. Put the header into
+  // the loop set and then walk the Worklist backwards to find all the blocks
+  // that remain within the loop after cloning.
+  if (!BlocksInClonedLoop.empty()) {
+    BlocksInClonedLoop.insert(ClonedHeader);
+
+    while (!Worklist.empty()) {
+      BasicBlock *BB = Worklist.pop_back_val();
+      assert(BlocksInClonedLoop.count(BB) &&
+             "Didn't put block into the loop set!");
+
+      // Insert any predecessors that are in the possible set into the cloned
+      // set, and if the insert is successful, add them to the Worklist. Note
+      // that we filter on the blocks that are definitely reachable via the
+      // backedge to the loop header so we may prune out dead code within the
+      // cloned loop.
+      for (auto *Pred : predecessors(BB))
+        if (ClonedLoopBlocks.count(Pred) &&
+            BlocksInClonedLoop.insert(Pred).second)
+          Worklist.push_back(Pred);
+    }
+
+    ClonedL = LI.AllocateLoop();
+    if (ParentL) {
+      ParentL->addBasicBlockToLoop(ClonedPH, LI);
+      ParentL->addChildLoop(ClonedL);
+    } else {
+      LI.addTopLevelLoop(ClonedL);
+    }
+    NonChildClonedLoops.push_back(ClonedL);
+
+    ClonedL->reserveBlocks(BlocksInClonedLoop.size());
+    // We don't want to just add the cloned loop blocks based on how we
+    // discovered them. The original order of blocks was carefully built in
+    // a way that doesn't rely on predecessor ordering. Rather than re-invent
+    // that logic, we just re-walk the original blocks (and those of the child
+    // loops) and filter them as we add them into the cloned loop.
+    for (auto *BB : OrigL.blocks()) {
+      auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB));
+      if (!ClonedBB || !BlocksInClonedLoop.count(ClonedBB))
+        continue;
+
+      // Directly add the blocks that are only in this loop.
+      if (LI.getLoopFor(BB) == &OrigL) {
+        ClonedL->addBasicBlockToLoop(ClonedBB, LI);
+        continue;
+      }
+
+      // We want to manually add it to this loop and parents.
+      // Registering it with LoopInfo will happen when we clone the top
+      // loop for this block.
+      for (Loop *PL = ClonedL; PL; PL = PL->getParentLoop())
+        PL->addBlockEntry(ClonedBB);
+    }
+
+    // Now add each child loop whose header remains within the cloned loop. All
+    // of the blocks within the loop must satisfy the same constraints as the
+    // header so once we pass the header checks we can just clone the entire
+    // child loop nest.
+    for (Loop *ChildL : OrigL) {
+      auto *ClonedChildHeader =
+          cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader()));
+      if (!ClonedChildHeader || !BlocksInClonedLoop.count(ClonedChildHeader))
+        continue;
+
+#ifndef NDEBUG
+      // We should never have a cloned child loop header but fail to have
+      // all of the blocks for that child loop.
+      for (auto *ChildLoopBB : ChildL->blocks())
+        assert(BlocksInClonedLoop.count(
+                   cast<BasicBlock>(VMap.lookup(ChildLoopBB))) &&
+               "Child cloned loop has a header within the cloned outer "
+               "loop but not all of its blocks!");
+#endif
+
+      cloneLoopNest(*ChildL, ClonedL, VMap, LI);
+    }
+  }
+
+  // Now that we've handled all the components of the original loop that were
+  // cloned into a new loop, we still need to handle anything from the original
+  // loop that wasn't in a cloned loop.
+
+  // Figure out what blocks are left to place within any loop nest containing
+  // the unswitched loop. If we never formed a loop, the cloned PH is one of
+  // them.
+  SmallPtrSet<BasicBlock *, 16> UnloopedBlockSet;
+  if (BlocksInClonedLoop.empty())
+    UnloopedBlockSet.insert(ClonedPH);
+  for (auto *ClonedBB : ClonedLoopBlocks)
+    if (!BlocksInClonedLoop.count(ClonedBB))
+      UnloopedBlockSet.insert(ClonedBB);
+
+  // Copy the cloned exits and sort them in ascending loop depth, we'll work
+  // backwards across these to process them inside out. The order shouldn't
+  // matter as we're just trying to build up the map from inside-out; we use
+  // the map in a more stably ordered way below.
+  auto OrderedClonedExitsInLoops = ClonedExitsInLoops;
+  llvm::sort(OrderedClonedExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
+    return ExitLoopMap.lookup(LHS)->getLoopDepth() <
+           ExitLoopMap.lookup(RHS)->getLoopDepth();
+  });
+
+  // Populate the existing ExitLoopMap with everything reachable from each
+  // exit, starting from the inner most exit.
+  while (!UnloopedBlockSet.empty() && !OrderedClonedExitsInLoops.empty()) {
+    assert(Worklist.empty() && "Didn't clear Worklist!");
+
+    BasicBlock *ExitBB = OrderedClonedExitsInLoops.pop_back_val();
+    Loop *ExitL = ExitLoopMap.lookup(ExitBB);
+
+    // Walk the CFG back until we hit the cloned PH adding everything reachable
+    // and in the unlooped set to this exit block's loop.
+    Worklist.push_back(ExitBB);
+    do {
+      BasicBlock *BB = Worklist.pop_back_val();
+      // We can stop recursing at the cloned preheader (if we get there).
+      if (BB == ClonedPH)
+        continue;
+
+      for (BasicBlock *PredBB : predecessors(BB)) {
+        // If this pred has already been moved to our set or is part of some
+        // (inner) loop, no update needed.
+        if (!UnloopedBlockSet.erase(PredBB)) {
+          assert(
+              (BlocksInClonedLoop.count(PredBB) || ExitLoopMap.count(PredBB)) &&
+              "Predecessor not mapped to a loop!");
+          continue;
+        }
+
+        // We just insert into the loop set here. We'll add these blocks to the
+        // exit loop after we build up the set in an order that doesn't rely on
+        // predecessor order (which in turn relies on use list order).
+        bool Inserted = ExitLoopMap.insert({PredBB, ExitL}).second;
+        (void)Inserted;
+        assert(Inserted && "Should only visit an unlooped block once!");
+
+        // And recurse through to its predecessors.
+        Worklist.push_back(PredBB);
+      }
+    } while (!Worklist.empty());
+  }
+
+  // Now that the ExitLoopMap gives as  mapping for all the non-looping cloned
+  // blocks to their outer loops, walk the cloned blocks and the cloned exits
+  // in their original order adding them to the correct loop.
+
+  // We need a stable insertion order. We use the order of the original loop
+  // order and map into the correct parent loop.
+  for (auto *BB : llvm::concat<BasicBlock *const>(
+           makeArrayRef(ClonedPH), ClonedLoopBlocks, ClonedExitsInLoops))
+    if (Loop *OuterL = ExitLoopMap.lookup(BB))
+      OuterL->addBasicBlockToLoop(BB, LI);
+
+#ifndef NDEBUG
+  for (auto &BBAndL : ExitLoopMap) {
+    auto *BB = BBAndL.first;
+    auto *OuterL = BBAndL.second;
+    assert(LI.getLoopFor(BB) == OuterL &&
+           "Failed to put all blocks into outer loops!");
+  }
+#endif
+
+  // Now that all the blocks are placed into the correct containing loop in the
+  // absence of child loops, find all the potentially cloned child loops and
+  // clone them into whatever outer loop we placed their header into.
+  for (Loop *ChildL : OrigL) {
+    auto *ClonedChildHeader =
+        cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader()));
+    if (!ClonedChildHeader || BlocksInClonedLoop.count(ClonedChildHeader))
+      continue;
+
+#ifndef NDEBUG
+    for (auto *ChildLoopBB : ChildL->blocks())
+      assert(VMap.count(ChildLoopBB) &&
+             "Cloned a child loop header but not all of that loops blocks!");
+#endif
+
+    NonChildClonedLoops.push_back(cloneLoopNest(
+        *ChildL, ExitLoopMap.lookup(ClonedChildHeader), VMap, LI));
+  }
+}
+
+static void
+deleteDeadClonedBlocks(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
+                       ArrayRef<std::unique_ptr<ValueToValueMapTy>> VMaps,
+                       DominatorTree &DT, MemorySSAUpdater *MSSAU) {
+  // Find all the dead clones, and remove them from their successors.
+  SmallVector<BasicBlock *, 16> DeadBlocks;
+  for (BasicBlock *BB : llvm::concat<BasicBlock *const>(L.blocks(), ExitBlocks))
+    for (auto &VMap : VMaps)
+      if (BasicBlock *ClonedBB = cast_or_null<BasicBlock>(VMap->lookup(BB)))
+        if (!DT.isReachableFromEntry(ClonedBB)) {
+          for (BasicBlock *SuccBB : successors(ClonedBB))
+            SuccBB->removePredecessor(ClonedBB);
+          DeadBlocks.push_back(ClonedBB);
+        }
+
+  // Remove all MemorySSA in the dead blocks
+  if (MSSAU) {
+    SmallSetVector<BasicBlock *, 8> DeadBlockSet(DeadBlocks.begin(),
+                                                 DeadBlocks.end());
+    MSSAU->removeBlocks(DeadBlockSet);
+  }
+
+  // Drop any remaining references to break cycles.
+  for (BasicBlock *BB : DeadBlocks)
+    BB->dropAllReferences();
+  // Erase them from the IR.
+  for (BasicBlock *BB : DeadBlocks)
+    BB->eraseFromParent();
+}
+
+static void
+deleteDeadBlocksFromLoop(Loop &L, SmallVectorImpl<BasicBlock *> &ExitBlocks,
+                         DominatorTree &DT, LoopInfo &LI,
+                         MemorySSAUpdater *MSSAU,
+                         function_ref<void(Loop &, StringRef)> DestroyLoopCB) {
+  // Find all the dead blocks tied to this loop, and remove them from their
+  // successors.
+  SmallSetVector<BasicBlock *, 8> DeadBlockSet;
+
+  // Start with loop/exit blocks and get a transitive closure of reachable dead
+  // blocks.
+  SmallVector<BasicBlock *, 16> DeathCandidates(ExitBlocks.begin(),
+                                                ExitBlocks.end());
+  DeathCandidates.append(L.blocks().begin(), L.blocks().end());
+  while (!DeathCandidates.empty()) {
+    auto *BB = DeathCandidates.pop_back_val();
+    if (!DeadBlockSet.count(BB) && !DT.isReachableFromEntry(BB)) {
+      for (BasicBlock *SuccBB : successors(BB)) {
+        SuccBB->removePredecessor(BB);
+        DeathCandidates.push_back(SuccBB);
+      }
+      DeadBlockSet.insert(BB);
+    }
+  }
+
+  // Remove all MemorySSA in the dead blocks
+  if (MSSAU)
+    MSSAU->removeBlocks(DeadBlockSet);
+
+  // Filter out the dead blocks from the exit blocks list so that it can be
+  // used in the caller.
+  llvm::erase_if(ExitBlocks,
+                 [&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
+
+  // Walk from this loop up through its parents removing all of the dead blocks.
+  for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) {
+    for (auto *BB : DeadBlockSet)
+      ParentL->getBlocksSet().erase(BB);
+    llvm::erase_if(ParentL->getBlocksVector(),
+                   [&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
+  }
+
+  // Now delete the dead child loops. This raw delete will clear them
+  // recursively.
+  llvm::erase_if(L.getSubLoopsVector(), [&](Loop *ChildL) {
+    if (!DeadBlockSet.count(ChildL->getHeader()))
+      return false;
+
+    assert(llvm::all_of(ChildL->blocks(),
+                        [&](BasicBlock *ChildBB) {
+                          return DeadBlockSet.count(ChildBB);
+                        }) &&
+           "If the child loop header is dead all blocks in the child loop must "
+           "be dead as well!");
+    DestroyLoopCB(*ChildL, ChildL->getName());
+    LI.destroy(ChildL);
+    return true;
+  });
+
+  // Remove the loop mappings for the dead blocks and drop all the references
+  // from these blocks to others to handle cyclic references as we start
+  // deleting the blocks themselves.
+  for (auto *BB : DeadBlockSet) {
+    // Check that the dominator tree has already been updated.
+    assert(!DT.getNode(BB) && "Should already have cleared domtree!");
+    LI.changeLoopFor(BB, nullptr);
+    // Drop all uses of the instructions to make sure we won't have dangling
+    // uses in other blocks.
+    for (auto &I : *BB)
+      if (!I.use_empty())
+        I.replaceAllUsesWith(UndefValue::get(I.getType()));
+    BB->dropAllReferences();
+  }
+
+  // Actually delete the blocks now that they've been fully unhooked from the
+  // IR.
+  for (auto *BB : DeadBlockSet)
+    BB->eraseFromParent();
+}
+
+/// Recompute the set of blocks in a loop after unswitching.
+///
+/// This walks from the original headers predecessors to rebuild the loop. We
+/// take advantage of the fact that new blocks can't have been added, and so we
+/// filter by the original loop's blocks. This also handles potentially
+/// unreachable code that we don't want to explore but might be found examining
+/// the predecessors of the header.
+///
+/// If the original loop is no longer a loop, this will return an empty set. If
+/// it remains a loop, all the blocks within it will be added to the set
+/// (including those blocks in inner loops).
+static SmallPtrSet<const BasicBlock *, 16> recomputeLoopBlockSet(Loop &L,
+                                                                 LoopInfo &LI) {
+  SmallPtrSet<const BasicBlock *, 16> LoopBlockSet;
+
+  auto *PH = L.getLoopPreheader();
+  auto *Header = L.getHeader();
+
+  // A Worklist to use while walking backwards from the header.
+  SmallVector<BasicBlock *, 16> Worklist;
+
+  // First walk the predecessors of the header to find the backedges. This will
+  // form the basis of our walk.
+  for (auto *Pred : predecessors(Header)) {
+    // Skip the preheader.
+    if (Pred == PH)
+      continue;
+
+    // Because the loop was in simplified form, the only non-loop predecessor
+    // is the preheader.
+    assert(L.contains(Pred) && "Found a predecessor of the loop header other "
+                               "than the preheader that is not part of the "
+                               "loop!");
+
+    // Insert this block into the loop set and on the first visit and, if it
+    // isn't the header we're currently walking, put it into the Worklist to
+    // recurse through.
+    if (LoopBlockSet.insert(Pred).second && Pred != Header)
+      Worklist.push_back(Pred);
+  }
+
+  // If no backedges were found, we're done.
+  if (LoopBlockSet.empty())
+    return LoopBlockSet;
+
+  // We found backedges, recurse through them to identify the loop blocks.
+  while (!Worklist.empty()) {
+    BasicBlock *BB = Worklist.pop_back_val();
+    assert(LoopBlockSet.count(BB) && "Didn't put block into the loop set!");
+
+    // No need to walk past the header.
+    if (BB == Header)
+      continue;
+
+    // Because we know the inner loop structure remains valid we can use the
+    // loop structure to jump immediately across the entire nested loop.
+    // Further, because it is in loop simplified form, we can directly jump
+    // to its preheader afterward.
+    if (Loop *InnerL = LI.getLoopFor(BB))
+      if (InnerL != &L) {
+        assert(L.contains(InnerL) &&
+               "Should not reach a loop *outside* this loop!");
+        // The preheader is the only possible predecessor of the loop so
+        // insert it into the set and check whether it was already handled.
+        auto *InnerPH = InnerL->getLoopPreheader();
+        assert(L.contains(InnerPH) && "Cannot contain an inner loop block "
+                                      "but not contain the inner loop "
+                                      "preheader!");
+        if (!LoopBlockSet.insert(InnerPH).second)
+          // The only way to reach the preheader is through the loop body
+          // itself so if it has been visited the loop is already handled.
+          continue;
+
+        // Insert all of the blocks (other than those already present) into
+        // the loop set. We expect at least the block that led us to find the
+        // inner loop to be in the block set, but we may also have other loop
+        // blocks if they were already enqueued as predecessors of some other
+        // outer loop block.
+        for (auto *InnerBB : InnerL->blocks()) {
+          if (InnerBB == BB) {
+            assert(LoopBlockSet.count(InnerBB) &&
+                   "Block should already be in the set!");
+            continue;
+          }
+
+          LoopBlockSet.insert(InnerBB);
+        }
+
+        // Add the preheader to the Worklist so we will continue past the
+        // loop body.
+        Worklist.push_back(InnerPH);
+        continue;
+      }
+
+    // Insert any predecessors that were in the original loop into the new
+    // set, and if the insert is successful, add them to the Worklist.
+    for (auto *Pred : predecessors(BB))
+      if (L.contains(Pred) && LoopBlockSet.insert(Pred).second)
+        Worklist.push_back(Pred);
+  }
+
+  assert(LoopBlockSet.count(Header) && "Cannot fail to add the header!");
+
+  // We've found all the blocks participating in the loop, return our completed
+  // set.
+  return LoopBlockSet;
+}
+
+/// Rebuild a loop after unswitching removes some subset of blocks and edges.
+///
+/// The removal may have removed some child loops entirely but cannot have
+/// disturbed any remaining child loops. However, they may need to be hoisted
+/// to the parent loop (or to be top-level loops). The original loop may be
+/// completely removed.
+///
+/// The sibling loops resulting from this update are returned. If the original
+/// loop remains a valid loop, it will be the first entry in this list with all
+/// of the newly sibling loops following it.
+///
+/// Returns true if the loop remains a loop after unswitching, and false if it
+/// is no longer a loop after unswitching (and should not continue to be
+/// referenced).
+static bool rebuildLoopAfterUnswitch(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
+                                     LoopInfo &LI,
+                                     SmallVectorImpl<Loop *> &HoistedLoops) {
+  auto *PH = L.getLoopPreheader();
+
+  // Compute the actual parent loop from the exit blocks. Because we may have
+  // pruned some exits the loop may be different from the original parent.
+  Loop *ParentL = nullptr;
+  SmallVector<Loop *, 4> ExitLoops;
+  SmallVector<BasicBlock *, 4> ExitsInLoops;
+  ExitsInLoops.reserve(ExitBlocks.size());
+  for (auto *ExitBB : ExitBlocks)
+    if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
+      ExitLoops.push_back(ExitL);
+      ExitsInLoops.push_back(ExitBB);
+      if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
+        ParentL = ExitL;
+    }
+
+  // Recompute the blocks participating in this loop. This may be empty if it
+  // is no longer a loop.
+  auto LoopBlockSet = recomputeLoopBlockSet(L, LI);
+
+  // If we still have a loop, we need to re-set the loop's parent as the exit
+  // block set changing may have moved it within the loop nest. Note that this
+  // can only happen when this loop has a parent as it can only hoist the loop
+  // *up* the nest.
+  if (!LoopBlockSet.empty() && L.getParentLoop() != ParentL) {
+    // Remove this loop's (original) blocks from all of the intervening loops.
+    for (Loop *IL = L.getParentLoop(); IL != ParentL;
+         IL = IL->getParentLoop()) {
+      IL->getBlocksSet().erase(PH);
+      for (auto *BB : L.blocks())
+        IL->getBlocksSet().erase(BB);
+      llvm::erase_if(IL->getBlocksVector(), [&](BasicBlock *BB) {
+        return BB == PH || L.contains(BB);
+      });
+    }
+
+    LI.changeLoopFor(PH, ParentL);
+    L.getParentLoop()->removeChildLoop(&L);
+    if (ParentL)
+      ParentL->addChildLoop(&L);
+    else
+      LI.addTopLevelLoop(&L);
+  }
+
+  // Now we update all the blocks which are no longer within the loop.
+  auto &Blocks = L.getBlocksVector();
+  auto BlocksSplitI =
+      LoopBlockSet.empty()
+          ? Blocks.begin()
+          : std::stable_partition(
+                Blocks.begin(), Blocks.end(),
+                [&](BasicBlock *BB) { return LoopBlockSet.count(BB); });
+
+  // Before we erase the list of unlooped blocks, build a set of them.
+  SmallPtrSet<BasicBlock *, 16> UnloopedBlocks(BlocksSplitI, Blocks.end());
+  if (LoopBlockSet.empty())
+    UnloopedBlocks.insert(PH);
+
+  // Now erase these blocks from the loop.
+  for (auto *BB : make_range(BlocksSplitI, Blocks.end()))
+    L.getBlocksSet().erase(BB);
+  Blocks.erase(BlocksSplitI, Blocks.end());
+
+  // Sort the exits in ascending loop depth, we'll work backwards across these
+  // to process them inside out.
+  llvm::stable_sort(ExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
+    return LI.getLoopDepth(LHS) < LI.getLoopDepth(RHS);
+  });
+
+  // We'll build up a set for each exit loop.
+  SmallPtrSet<BasicBlock *, 16> NewExitLoopBlocks;
+  Loop *PrevExitL = L.getParentLoop(); // The deepest possible exit loop.
+
+  auto RemoveUnloopedBlocksFromLoop =
+      [](Loop &L, SmallPtrSetImpl<BasicBlock *> &UnloopedBlocks) {
+        for (auto *BB : UnloopedBlocks)
+          L.getBlocksSet().erase(BB);
+        llvm::erase_if(L.getBlocksVector(), [&](BasicBlock *BB) {
+          return UnloopedBlocks.count(BB);
+        });
+      };
+
+  SmallVector<BasicBlock *, 16> Worklist;
+  while (!UnloopedBlocks.empty() && !ExitsInLoops.empty()) {
+    assert(Worklist.empty() && "Didn't clear Worklist!");
+    assert(NewExitLoopBlocks.empty() && "Didn't clear loop set!");
+
+    // Grab the next exit block, in decreasing loop depth order.
+    BasicBlock *ExitBB = ExitsInLoops.pop_back_val();
+    Loop &ExitL = *LI.getLoopFor(ExitBB);
+    assert(ExitL.contains(&L) && "Exit loop must contain the inner loop!");
+
+    // Erase all of the unlooped blocks from the loops between the previous
+    // exit loop and this exit loop. This works because the ExitInLoops list is
+    // sorted in increasing order of loop depth and thus we visit loops in
+    // decreasing order of loop depth.
+    for (; PrevExitL != &ExitL; PrevExitL = PrevExitL->getParentLoop())
+      RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
+
+    // Walk the CFG back until we hit the cloned PH adding everything reachable
+    // and in the unlooped set to this exit block's loop.
+    Worklist.push_back(ExitBB);
+    do {
+      BasicBlock *BB = Worklist.pop_back_val();
+      // We can stop recursing at the cloned preheader (if we get there).
+      if (BB == PH)
+        continue;
+
+      for (BasicBlock *PredBB : predecessors(BB)) {
+        // If this pred has already been moved to our set or is part of some
+        // (inner) loop, no update needed.
+        if (!UnloopedBlocks.erase(PredBB)) {
+          assert((NewExitLoopBlocks.count(PredBB) ||
+                  ExitL.contains(LI.getLoopFor(PredBB))) &&
+                 "Predecessor not in a nested loop (or already visited)!");
+          continue;
+        }
+
+        // We just insert into the loop set here. We'll add these blocks to the
+        // exit loop after we build up the set in a deterministic order rather
+        // than the predecessor-influenced visit order.
+        bool Inserted = NewExitLoopBlocks.insert(PredBB).second;
+        (void)Inserted;
+        assert(Inserted && "Should only visit an unlooped block once!");
+
+        // And recurse through to its predecessors.
+        Worklist.push_back(PredBB);
+      }
+    } while (!Worklist.empty());
+
+    // If blocks in this exit loop were directly part of the original loop (as
+    // opposed to a child loop) update the map to point to this exit loop. This
+    // just updates a map and so the fact that the order is unstable is fine.
+    for (auto *BB : NewExitLoopBlocks)
+      if (Loop *BBL = LI.getLoopFor(BB))
+        if (BBL == &L || !L.contains(BBL))
+          LI.changeLoopFor(BB, &ExitL);
+
+    // We will remove the remaining unlooped blocks from this loop in the next
+    // iteration or below.
+    NewExitLoopBlocks.clear();
+  }
+
+  // Any remaining unlooped blocks are no longer part of any loop unless they
+  // are part of some child loop.
+  for (; PrevExitL; PrevExitL = PrevExitL->getParentLoop())
+    RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
+  for (auto *BB : UnloopedBlocks)
+    if (Loop *BBL = LI.getLoopFor(BB))
+      if (BBL == &L || !L.contains(BBL))
+        LI.changeLoopFor(BB, nullptr);
+
+  // Sink all the child loops whose headers are no longer in the loop set to
+  // the parent (or to be top level loops). We reach into the loop and directly
+  // update its subloop vector to make this batch update efficient.
+  auto &SubLoops = L.getSubLoopsVector();
+  auto SubLoopsSplitI =
+      LoopBlockSet.empty()
+          ? SubLoops.begin()
+          : std::stable_partition(
+                SubLoops.begin(), SubLoops.end(), [&](Loop *SubL) {
+                  return LoopBlockSet.count(SubL->getHeader());
+                });
+  for (auto *HoistedL : make_range(SubLoopsSplitI, SubLoops.end())) {
+    HoistedLoops.push_back(HoistedL);
+    HoistedL->setParentLoop(nullptr);
+
+    // To compute the new parent of this hoisted loop we look at where we
+    // placed the preheader above. We can't lookup the header itself because we
+    // retained the mapping from the header to the hoisted loop. But the
+    // preheader and header should have the exact same new parent computed
+    // based on the set of exit blocks from the original loop as the preheader
+    // is a predecessor of the header and so reached in the reverse walk. And
+    // because the loops were all in simplified form the preheader of the
+    // hoisted loop can't be part of some *other* loop.
+    if (auto *NewParentL = LI.getLoopFor(HoistedL->getLoopPreheader()))
+      NewParentL->addChildLoop(HoistedL);
+    else
+      LI.addTopLevelLoop(HoistedL);
+  }
+  SubLoops.erase(SubLoopsSplitI, SubLoops.end());
+
+  // Actually delete the loop if nothing remained within it.
+  if (Blocks.empty()) {
+    assert(SubLoops.empty() &&
+           "Failed to remove all subloops from the original loop!");
+    if (Loop *ParentL = L.getParentLoop())
+      ParentL->removeChildLoop(llvm::find(*ParentL, &L));
+    else
+      LI.removeLoop(llvm::find(LI, &L));
+    // markLoopAsDeleted for L should be triggered by the caller (it is
+    // typically done by using the UnswitchCB callback).
+    LI.destroy(&L);
+    return false;
+  }
+
+  return true;
+}
+
+/// Helper to visit a dominator subtree, invoking a callable on each node.
+///
+/// Returning false at any point will stop walking past that node of the tree.
+template <typename CallableT>
+void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) {
+  SmallVector<DomTreeNode *, 4> DomWorklist;
+  DomWorklist.push_back(DT[BB]);
+#ifndef NDEBUG
+  SmallPtrSet<DomTreeNode *, 4> Visited;
+  Visited.insert(DT[BB]);
+#endif
+  do {
+    DomTreeNode *N = DomWorklist.pop_back_val();
+
+    // Visit this node.
+    if (!Callable(N->getBlock()))
+      continue;
+
+    // Accumulate the child nodes.
+    for (DomTreeNode *ChildN : *N) {
+      assert(Visited.insert(ChildN).second &&
+             "Cannot visit a node twice when walking a tree!");
+      DomWorklist.push_back(ChildN);
+    }
+  } while (!DomWorklist.empty());
+}
+
+static void unswitchNontrivialInvariants(
+    Loop &L, Instruction &TI, ArrayRef<Value *> Invariants,
+    SmallVectorImpl<BasicBlock *> &ExitBlocks, IVConditionInfo &PartialIVInfo,
+    DominatorTree &DT, LoopInfo &LI, AssumptionCache &AC,
+    function_ref<void(bool, bool, ArrayRef<Loop *>)> UnswitchCB,
+    ScalarEvolution *SE, MemorySSAUpdater *MSSAU,
+    function_ref<void(Loop &, StringRef)> DestroyLoopCB) {
+  auto *ParentBB = TI.getParent();
+  BranchInst *BI = dyn_cast<BranchInst>(&TI);
+  SwitchInst *SI = BI ? nullptr : cast<SwitchInst>(&TI);
+
+  // We can only unswitch switches, conditional branches with an invariant
+  // condition, or combining invariant conditions with an instruction or
+  // partially invariant instructions.
+  assert((SI || (BI && BI->isConditional())) &&
+         "Can only unswitch switches and conditional branch!");
+  bool PartiallyInvariant = !PartialIVInfo.InstToDuplicate.empty();
+  bool FullUnswitch =
+      SI || (skipTrivialSelect(BI->getCondition()) == Invariants[0] &&
+             !PartiallyInvariant);
+  if (FullUnswitch)
+    assert(Invariants.size() == 1 &&
+           "Cannot have other invariants with full unswitching!");
+  else
+    assert(isa<Instruction>(skipTrivialSelect(BI->getCondition())) &&
+           "Partial unswitching requires an instruction as the condition!");
+
+  if (MSSAU && VerifyMemorySSA)
+    MSSAU->getMemorySSA()->verifyMemorySSA();
+
+  // Constant and BBs tracking the cloned and continuing successor. When we are
+  // unswitching the entire condition, this can just be trivially chosen to
+  // unswitch towards `true`. However, when we are unswitching a set of
+  // invariants combined with `and` or `or` or partially invariant instructions,
+  // the combining operation determines the best direction to unswitch: we want
+  // to unswitch the direction that will collapse the branch.
+  bool Direction = true;
+  int ClonedSucc = 0;
+  if (!FullUnswitch) {
+    Value *Cond = skipTrivialSelect(BI->getCondition());
+    (void)Cond;
+    assert(((match(Cond, m_LogicalAnd()) ^ match(Cond, m_LogicalOr())) ||
+            PartiallyInvariant) &&
+           "Only `or`, `and`, an `select`, partially invariant instructions "
+           "can combine invariants being unswitched.");
+    if (!match(Cond, m_LogicalOr())) {
+      if (match(Cond, m_LogicalAnd()) ||
+          (PartiallyInvariant && !PartialIVInfo.KnownValue->isOneValue())) {
+        Direction = false;
+        ClonedSucc = 1;
+      }
+    }
+  }
+
+  BasicBlock *RetainedSuccBB =
+      BI ? BI->getSuccessor(1 - ClonedSucc) : SI->getDefaultDest();
+  SmallSetVector<BasicBlock *, 4> UnswitchedSuccBBs;
+  if (BI)
+    UnswitchedSuccBBs.insert(BI->getSuccessor(ClonedSucc));
+  else
+    for (auto Case : SI->cases())
+      if (Case.getCaseSuccessor() != RetainedSuccBB)
+        UnswitchedSuccBBs.insert(Case.getCaseSuccessor());
+
+  assert(!UnswitchedSuccBBs.count(RetainedSuccBB) &&
+         "Should not unswitch the same successor we are retaining!");
+
+  // The branch should be in this exact loop. Any inner loop's invariant branch
+  // should be handled by unswitching that inner loop. The caller of this
+  // routine should filter out any candidates that remain (but were skipped for
+  // whatever reason).
+  assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!");
+
+  // Compute the parent loop now before we start hacking on things.
+  Loop *ParentL = L.getParentLoop();
+  // Get blocks in RPO order for MSSA update, before changing the CFG.
+  LoopBlocksRPO LBRPO(&L);
+  if (MSSAU)
+    LBRPO.perform(&LI);
+
+  // Compute the outer-most loop containing one of our exit blocks. This is the
+  // furthest up our loopnest which can be mutated, which we will use below to
+  // update things.
+  Loop *OuterExitL = &L;
+  for (auto *ExitBB : ExitBlocks) {
+    Loop *NewOuterExitL = LI.getLoopFor(ExitBB);
+    if (!NewOuterExitL) {
+      // We exited the entire nest with this block, so we're done.
+      OuterExitL = nullptr;
+      break;
+    }
+    if (NewOuterExitL != OuterExitL && NewOuterExitL->contains(OuterExitL))
+      OuterExitL = NewOuterExitL;
+  }
+
+  // At this point, we're definitely going to unswitch something so invalidate
+  // any cached information in ScalarEvolution for the outer most loop
+  // containing an exit block and all nested loops.
+  if (SE) {
+    if (OuterExitL)
+      SE->forgetLoop(OuterExitL);
+    else
+      SE->forgetTopmostLoop(&L);
+  }
+
+  bool InsertFreeze = false;
+  if (FreezeLoopUnswitchCond) {
+    ICFLoopSafetyInfo SafetyInfo;
+    SafetyInfo.computeLoopSafetyInfo(&L);
+    InsertFreeze = !SafetyInfo.isGuaranteedToExecute(TI, &DT, &L);
+  }
+
+  // If the edge from this terminator to a successor dominates that successor,
+  // store a map from each block in its dominator subtree to it. This lets us
+  // tell when cloning for a particular successor if a block is dominated by
+  // some *other* successor with a single data structure. We use this to
+  // significantly reduce cloning.
+  SmallDenseMap<BasicBlock *, BasicBlock *, 16> DominatingSucc;
+  for (auto *SuccBB : llvm::concat<BasicBlock *const>(
+           makeArrayRef(RetainedSuccBB), UnswitchedSuccBBs))
+    if (SuccBB->getUniquePredecessor() ||
+        llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
+          return PredBB == ParentBB || DT.dominates(SuccBB, PredBB);
+        }))
+      visitDomSubTree(DT, SuccBB, [&](BasicBlock *BB) {
+        DominatingSucc[BB] = SuccBB;
+        return true;
+      });
+
+  // Split the preheader, so that we know that there is a safe place to insert
+  // the conditional branch. We will change the preheader to have a conditional
+  // branch on LoopCond. The original preheader will become the split point
+  // between the unswitched versions, and we will have a new preheader for the
+  // original loop.
+  BasicBlock *SplitBB = L.getLoopPreheader();
+  BasicBlock *LoopPH = SplitEdge(SplitBB, L.getHeader(), &DT, &LI, MSSAU);
+
+  // Keep track of the dominator tree updates needed.
+  SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
+
+  // Clone the loop for each unswitched successor.
+  SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps;
+  VMaps.reserve(UnswitchedSuccBBs.size());
+  SmallDenseMap<BasicBlock *, BasicBlock *, 4> ClonedPHs;
+  for (auto *SuccBB : UnswitchedSuccBBs) {
+    VMaps.emplace_back(new ValueToValueMapTy());
+    ClonedPHs[SuccBB] = buildClonedLoopBlocks(
+        L, LoopPH, SplitBB, ExitBlocks, ParentBB, SuccBB, RetainedSuccBB,
+        DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU);
+  }
+
+  // Drop metadata if we may break its semantics by moving this instr into the
+  // split block.
+  if (TI.getMetadata(LLVMContext::MD_make_implicit)) {
+    if (DropNonTrivialImplicitNullChecks)
+      // Do not spend time trying to understand if we can keep it, just drop it
+      // to save compile time.
+      TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
+    else {
+      // It is only legal to preserve make.implicit metadata if we are
+      // guaranteed no reach implicit null check after following this branch.
+      ICFLoopSafetyInfo SafetyInfo;
+      SafetyInfo.computeLoopSafetyInfo(&L);
+      if (!SafetyInfo.isGuaranteedToExecute(TI, &DT, &L))
+        TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
+    }
+  }
+
+  // The stitching of the branched code back together depends on whether we're
+  // doing full unswitching or not with the exception that we always want to
+  // nuke the initial terminator placed in the split block.
+  SplitBB->getTerminator()->eraseFromParent();
+  if (FullUnswitch) {
+    // Splice the terminator from the original loop and rewrite its
+    // successors.
+    SplitBB->getInstList().splice(SplitBB->end(), ParentBB->getInstList(), TI);
+
+    // Keep a clone of the terminator for MSSA updates.
+    Instruction *NewTI = TI.clone();
+    ParentBB->getInstList().push_back(NewTI);
+
+    // First wire up the moved terminator to the preheaders.
+    if (BI) {
+      BasicBlock *ClonedPH = ClonedPHs.begin()->second;
+      BI->setSuccessor(ClonedSucc, ClonedPH);
+      BI->setSuccessor(1 - ClonedSucc, LoopPH);
+      if (InsertFreeze) {
+        auto Cond = skipTrivialSelect(BI->getCondition());
+        if (!isGuaranteedNotToBeUndefOrPoison(Cond, &AC, BI, &DT))
+          BI->setCondition(new FreezeInst(Cond, Cond->getName() + ".fr", BI));
+      }
+      DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
+    } else {
+      assert(SI && "Must either be a branch or switch!");
+
+      // Walk the cases and directly update their successors.
+      assert(SI->getDefaultDest() == RetainedSuccBB &&
+             "Not retaining default successor!");
+      SI->setDefaultDest(LoopPH);
+      for (auto &Case : SI->cases())
+        if (Case.getCaseSuccessor() == RetainedSuccBB)
+          Case.setSuccessor(LoopPH);
+        else
+          Case.setSuccessor(ClonedPHs.find(Case.getCaseSuccessor())->second);
+
+      if (InsertFreeze) {
+        auto Cond = SI->getCondition();
+        if (!isGuaranteedNotToBeUndefOrPoison(Cond, &AC, SI, &DT))
+          SI->setCondition(new FreezeInst(Cond, Cond->getName() + ".fr", SI));
+      }
+      // We need to use the set to populate domtree updates as even when there
+      // are multiple cases pointing at the same successor we only want to
+      // remove and insert one edge in the domtree.
+      for (BasicBlock *SuccBB : UnswitchedSuccBBs)
+        DTUpdates.push_back(
+            {DominatorTree::Insert, SplitBB, ClonedPHs.find(SuccBB)->second});
+    }
+
+    if (MSSAU) {
+      DT.applyUpdates(DTUpdates);
+      DTUpdates.clear();
+
+      // Remove all but one edge to the retained block and all unswitched
+      // blocks. This is to avoid having duplicate entries in the cloned Phis,
+      // when we know we only keep a single edge for each case.
+      MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, RetainedSuccBB);
+      for (BasicBlock *SuccBB : UnswitchedSuccBBs)
+        MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, SuccBB);
+
+      for (auto &VMap : VMaps)
+        MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
+                                   /*IgnoreIncomingWithNoClones=*/true);
+      MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
+
+      // Remove all edges to unswitched blocks.
+      for (BasicBlock *SuccBB : UnswitchedSuccBBs)
+        MSSAU->removeEdge(ParentBB, SuccBB);
+    }
+
+    // Now unhook the successor relationship as we'll be replacing
+    // the terminator with a direct branch. This is much simpler for branches
+    // than switches so we handle those first.
+    if (BI) {
+      // Remove the parent as a predecessor of the unswitched successor.
+      assert(UnswitchedSuccBBs.size() == 1 &&
+             "Only one possible unswitched block for a branch!");
+      BasicBlock *UnswitchedSuccBB = *UnswitchedSuccBBs.begin();
+      UnswitchedSuccBB->removePredecessor(ParentBB,
+                                          /*KeepOneInputPHIs*/ true);
+      DTUpdates.push_back({DominatorTree::Delete, ParentBB, UnswitchedSuccBB});
+    } else {
+      // Note that we actually want to remove the parent block as a predecessor
+      // of *every* case successor. The case successor is either unswitched,
+      // completely eliminating an edge from the parent to that successor, or it
+      // is a duplicate edge to the retained successor as the retained successor
+      // is always the default successor and as we'll replace this with a direct
+      // branch we no longer need the duplicate entries in the PHI nodes.
+      SwitchInst *NewSI = cast<SwitchInst>(NewTI);
+      assert(NewSI->getDefaultDest() == RetainedSuccBB &&
+             "Not retaining default successor!");
+      for (auto &Case : NewSI->cases())
+        Case.getCaseSuccessor()->removePredecessor(ParentBB,
+                                                   /*KeepOneInputPHIs*/ true);
+
+      // We need to use the set to populate domtree updates as even when there
+      // are multiple cases pointing at the same successor we only want to
+      // remove and insert one edge in the domtree.
+      for (BasicBlock *SuccBB : UnswitchedSuccBBs)
+        DTUpdates.push_back({DominatorTree::Delete, ParentBB, SuccBB});
+    }
+
+    // After MSSAU update, remove the cloned terminator instruction NewTI.
+    ParentBB->getTerminator()->eraseFromParent();
+
+    // Create a new unconditional branch to the continuing block (as opposed to
+    // the one cloned).
+    BranchInst::Create(RetainedSuccBB, ParentBB);
+  } else {
+    assert(BI && "Only branches have partial unswitching.");
+    assert(UnswitchedSuccBBs.size() == 1 &&
+           "Only one possible unswitched block for a branch!");
+    BasicBlock *ClonedPH = ClonedPHs.begin()->second;
+    // When doing a partial unswitch, we have to do a bit more work to build up
+    // the branch in the split block.
+    if (PartiallyInvariant)
+      buildPartialInvariantUnswitchConditionalBranch(
+          *SplitBB, Invariants, Direction, *ClonedPH, *LoopPH, L, MSSAU);
+    else {
+      buildPartialUnswitchConditionalBranch(
+          *SplitBB, Invariants, Direction, *ClonedPH, *LoopPH,
+          FreezeLoopUnswitchCond, BI, &AC, DT);
+    }
+    DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
+
+    if (MSSAU) {
+      DT.applyUpdates(DTUpdates);
+      DTUpdates.clear();
+
+      // Perform MSSA cloning updates.
+      for (auto &VMap : VMaps)
+        MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
+                                   /*IgnoreIncomingWithNoClones=*/true);
+      MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
+    }
+  }
+
+  // Apply the updates accumulated above to get an up-to-date dominator tree.
+  DT.applyUpdates(DTUpdates);
+
+  // Now that we have an accurate dominator tree, first delete the dead cloned
+  // blocks so that we can accurately build any cloned loops. It is important to
+  // not delete the blocks from the original loop yet because we still want to
+  // reference the original loop to understand the cloned loop's structure.
+  deleteDeadClonedBlocks(L, ExitBlocks, VMaps, DT, MSSAU);
+
+  // Build the cloned loop structure itself. This may be substantially
+  // different from the original structure due to the simplified CFG. This also
+  // handles inserting all the cloned blocks into the correct loops.
+  SmallVector<Loop *, 4> NonChildClonedLoops;
+  for (std::unique_ptr<ValueToValueMapTy> &VMap : VMaps)
+    buildClonedLoops(L, ExitBlocks, *VMap, LI, NonChildClonedLoops);
+
+  // Now that our cloned loops have been built, we can update the original loop.
+  // First we delete the dead blocks from it and then we rebuild the loop
+  // structure taking these deletions into account.
+  deleteDeadBlocksFromLoop(L, ExitBlocks, DT, LI, MSSAU, DestroyLoopCB);
+
+  if (MSSAU && VerifyMemorySSA)
+    MSSAU->getMemorySSA()->verifyMemorySSA();
+
+  SmallVector<Loop *, 4> HoistedLoops;
+  bool IsStillLoop = rebuildLoopAfterUnswitch(L, ExitBlocks, LI, HoistedLoops);
+
+  if (MSSAU && VerifyMemorySSA)
+    MSSAU->getMemorySSA()->verifyMemorySSA();
+
+  // This transformation has a high risk of corrupting the dominator tree, and
+  // the below steps to rebuild loop structures will result in hard to debug
+  // errors in that case so verify that the dominator tree is sane first.
+  // FIXME: Remove this when the bugs stop showing up and rely on existing
+  // verification steps.
+  assert(DT.verify(DominatorTree::VerificationLevel::Fast));
+
+  if (BI && !PartiallyInvariant) {
+    // If we unswitched a branch which collapses the condition to a known
+    // constant we want to replace all the uses of the invariants within both
+    // the original and cloned blocks. We do this here so that we can use the
+    // now updated dominator tree to identify which side the users are on.
+    assert(UnswitchedSuccBBs.size() == 1 &&
+           "Only one possible unswitched block for a branch!");
+    BasicBlock *ClonedPH = ClonedPHs.begin()->second;
+
+    // When considering multiple partially-unswitched invariants
+    // we cant just go replace them with constants in both branches.
+    //
+    // For 'AND' we infer that true branch ("continue") means true
+    // for each invariant operand.
+    // For 'OR' we can infer that false branch ("continue") means false
+    // for each invariant operand.
+    // So it happens that for multiple-partial case we dont replace
+    // in the unswitched branch.
+    bool ReplaceUnswitched =
+        FullUnswitch || (Invariants.size() == 1) || PartiallyInvariant;
+
+    ConstantInt *UnswitchedReplacement =
+        Direction ? ConstantInt::getTrue(BI->getContext())
+                  : ConstantInt::getFalse(BI->getContext());
+    ConstantInt *ContinueReplacement =
+        Direction ? ConstantInt::getFalse(BI->getContext())
+                  : ConstantInt::getTrue(BI->getContext());
+    for (Value *Invariant : Invariants) {
+      assert(!isa<Constant>(Invariant) &&
+             "Should not be replacing constant values!");
+      // Use make_early_inc_range here as set invalidates the iterator.
+      for (Use &U : llvm::make_early_inc_range(Invariant->uses())) {
+        Instruction *UserI = dyn_cast<Instruction>(U.getUser());
+        if (!UserI)
+          continue;
+
+        // Replace it with the 'continue' side if in the main loop body, and the
+        // unswitched if in the cloned blocks.
+        if (DT.dominates(LoopPH, UserI->getParent()))
+          U.set(ContinueReplacement);
+        else if (ReplaceUnswitched &&
+                 DT.dominates(ClonedPH, UserI->getParent()))
+          U.set(UnswitchedReplacement);
+      }
+    }
+  }
+
+  // We can change which blocks are exit blocks of all the cloned sibling
+  // loops, the current loop, and any parent loops which shared exit blocks
+  // with the current loop. As a consequence, we need to re-form LCSSA for
+  // them. But we shouldn't need to re-form LCSSA for any child loops.
+  // FIXME: This could be made more efficient by tracking which exit blocks are
+  // new, and focusing on them, but that isn't likely to be necessary.
+  //
+  // In order to reasonably rebuild LCSSA we need to walk inside-out across the
+  // loop nest and update every loop that could have had its exits changed. We
+  // also need to cover any intervening loops. We add all of these loops to
+  // a list and sort them by loop depth to achieve this without updating
+  // unnecessary loops.
+  auto UpdateLoop = [&](Loop &UpdateL) {
+#ifndef NDEBUG
+    UpdateL.verifyLoop();
+    for (Loop *ChildL : UpdateL) {
+      ChildL->verifyLoop();
+      assert(ChildL->isRecursivelyLCSSAForm(DT, LI) &&
+             "Perturbed a child loop's LCSSA form!");
+    }
+#endif
+    // First build LCSSA for this loop so that we can preserve it when
+    // forming dedicated exits. We don't want to perturb some other loop's
+    // LCSSA while doing that CFG edit.
+    formLCSSA(UpdateL, DT, &LI, SE);
+
+    // For loops reached by this loop's original exit blocks we may
+    // introduced new, non-dedicated exits. At least try to re-form dedicated
+    // exits for these loops. This may fail if they couldn't have dedicated
+    // exits to start with.
+    formDedicatedExitBlocks(&UpdateL, &DT, &LI, MSSAU, /*PreserveLCSSA*/ true);
+  };
+
+  // For non-child cloned loops and hoisted loops, we just need to update LCSSA
+  // and we can do it in any order as they don't nest relative to each other.
+  //
+  // Also check if any of the loops we have updated have become top-level loops
+  // as that will necessitate widening the outer loop scope.
+  for (Loop *UpdatedL :
+       llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops)) {
+    UpdateLoop(*UpdatedL);
+    if (UpdatedL->isOutermost())
+      OuterExitL = nullptr;
+  }
+  if (IsStillLoop) {
+    UpdateLoop(L);
+    if (L.isOutermost())
+      OuterExitL = nullptr;
+  }
+
+  // If the original loop had exit blocks, walk up through the outer most loop
+  // of those exit blocks to update LCSSA and form updated dedicated exits.
+  if (OuterExitL != &L)
+    for (Loop *OuterL = ParentL; OuterL != OuterExitL;
+         OuterL = OuterL->getParentLoop())
+      UpdateLoop(*OuterL);
+
+#ifndef NDEBUG
+  // Verify the entire loop structure to catch any incorrect updates before we
+  // progress in the pass pipeline.
+  LI.verify(DT);
+#endif
+
+  // Now that we've unswitched something, make callbacks to report the changes.
+  // For that we need to merge together the updated loops and the cloned loops
+  // and check whether the original loop survived.
+  SmallVector<Loop *, 4> SibLoops;
+  for (Loop *UpdatedL : llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops))
+    if (UpdatedL->getParentLoop() == ParentL)
+      SibLoops.push_back(UpdatedL);
+  UnswitchCB(IsStillLoop, PartiallyInvariant, SibLoops);
+
+  if (MSSAU && VerifyMemorySSA)
+    MSSAU->getMemorySSA()->verifyMemorySSA();
+
+  if (BI)
+    ++NumBranches;
+  else
+    ++NumSwitches;
+}
+
+/// Recursively compute the cost of a dominator subtree based on the per-block
+/// cost map provided.
+///
+/// The recursive computation is memozied into the provided DT-indexed cost map
+/// to allow querying it for most nodes in the domtree without it becoming
+/// quadratic.
+static InstructionCost computeDomSubtreeCost(
+    DomTreeNode &N,
+    const SmallDenseMap<BasicBlock *, InstructionCost, 4> &BBCostMap,
+    SmallDenseMap<DomTreeNode *, InstructionCost, 4> &DTCostMap) {
+  // Don't accumulate cost (or recurse through) blocks not in our block cost
+  // map and thus not part of the duplication cost being considered.
+  auto BBCostIt = BBCostMap.find(N.getBlock());
+  if (BBCostIt == BBCostMap.end())
+    return 0;
+
+  // Lookup this node to see if we already computed its cost.
+  auto DTCostIt = DTCostMap.find(&N);
+  if (DTCostIt != DTCostMap.end())
+    return DTCostIt->second;
+
+  // If not, we have to compute it. We can't use insert above and update
+  // because computing the cost may insert more things into the map.
+  InstructionCost Cost = std::accumulate(
+      N.begin(), N.end(), BBCostIt->second,
+      [&](InstructionCost Sum, DomTreeNode *ChildN) -> InstructionCost {
+        return Sum + computeDomSubtreeCost(*ChildN, BBCostMap, DTCostMap);
+      });
+  bool Inserted = DTCostMap.insert({&N, Cost}).second;
+  (void)Inserted;
+  assert(Inserted && "Should not insert a node while visiting children!");
+  return Cost;
+}
+
+/// Turns a llvm.experimental.guard intrinsic into implicit control flow branch,
+/// making the following replacement:
+///
+///   --code before guard--
+///   call void (i1, ...) @llvm.experimental.guard(i1 %cond) [ "deopt"() ]
+///   --code after guard--
+///
+/// into
+///
+///   --code before guard--
+///   br i1 %cond, label %guarded, label %deopt
+///
+/// guarded:
+///   --code after guard--
+///
+/// deopt:
+///   call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+///   unreachable
+///
+/// It also makes all relevant DT and LI updates, so that all structures are in
+/// valid state after this transform.
+static BranchInst *
+turnGuardIntoBranch(IntrinsicInst *GI, Loop &L,
+                    SmallVectorImpl<BasicBlock *> &ExitBlocks,
+                    DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
+  SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
+  LLVM_DEBUG(dbgs() << "Turning " << *GI << " into a branch.\n");
+  BasicBlock *CheckBB = GI->getParent();
+
+  if (MSSAU && VerifyMemorySSA)
+    MSSAU->getMemorySSA()->verifyMemorySSA();
+
+  // Remove all CheckBB's successors from DomTree. A block can be seen among
+  // successors more than once, but for DomTree it should be added only once.
+  SmallPtrSet<BasicBlock *, 4> Successors;
+  for (auto *Succ : successors(CheckBB))
+    if (Successors.insert(Succ).second)
+      DTUpdates.push_back({DominatorTree::Delete, CheckBB, Succ});
+
+  Instruction *DeoptBlockTerm =
+      SplitBlockAndInsertIfThen(GI->getArgOperand(0), GI, true);
+  BranchInst *CheckBI = cast<BranchInst>(CheckBB->getTerminator());
+  // SplitBlockAndInsertIfThen inserts control flow that branches to
+  // DeoptBlockTerm if the condition is true.  We want the opposite.
+  CheckBI->swapSuccessors();
+
+  BasicBlock *GuardedBlock = CheckBI->getSuccessor(0);
+  GuardedBlock->setName("guarded");
+  CheckBI->getSuccessor(1)->setName("deopt");
+  BasicBlock *DeoptBlock = CheckBI->getSuccessor(1);
+
+  // We now have a new exit block.
+  ExitBlocks.push_back(CheckBI->getSuccessor(1));
+
+  if (MSSAU)
+    MSSAU->moveAllAfterSpliceBlocks(CheckBB, GuardedBlock, GI);
+
+  GI->moveBefore(DeoptBlockTerm);
+  GI->setArgOperand(0, ConstantInt::getFalse(GI->getContext()));
+
+  // Add new successors of CheckBB into DomTree.
+  for (auto *Succ : successors(CheckBB))
+    DTUpdates.push_back({DominatorTree::Insert, CheckBB, Succ});
+
+  // Now the blocks that used to be CheckBB's successors are GuardedBlock's
+  // successors.
+  for (auto *Succ : Successors)
+    DTUpdates.push_back({DominatorTree::Insert, GuardedBlock, Succ});
+
+  // Make proper changes to DT.
+  DT.applyUpdates(DTUpdates);
+  // Inform LI of a new loop block.
+  L.addBasicBlockToLoop(GuardedBlock, LI);
+
+  if (MSSAU) {
+    MemoryDef *MD = cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(GI));
+    MSSAU->moveToPlace(MD, DeoptBlock, MemorySSA::BeforeTerminator);
+    if (VerifyMemorySSA)
+      MSSAU->getMemorySSA()->verifyMemorySSA();
+  }
+
+  ++NumGuards;
+  return CheckBI;
+}
+
+/// Cost multiplier is a way to limit potentially exponential behavior
+/// of loop-unswitch. Cost is multipied in proportion of 2^number of unswitch
+/// candidates available. Also accounting for the number of "sibling" loops with
+/// the idea to account for previous unswitches that already happened on this
+/// cluster of loops. There was an attempt to keep this formula simple,
+/// just enough to limit the worst case behavior. Even if it is not that simple
+/// now it is still not an attempt to provide a detailed heuristic size
+/// prediction.
+///
+/// TODO: Make a proper accounting of "explosion" effect for all kinds of
+/// unswitch candidates, making adequate predictions instead of wild guesses.
+/// That requires knowing not just the number of "remaining" candidates but
+/// also costs of unswitching for each of these candidates.
+static int CalculateUnswitchCostMultiplier(
+    Instruction &TI, Loop &L, LoopInfo &LI, DominatorTree &DT,
+    ArrayRef<std::pair<Instruction *, TinyPtrVector<Value *>>>
+        UnswitchCandidates) {
+
+  // Guards and other exiting conditions do not contribute to exponential
+  // explosion as soon as they dominate the latch (otherwise there might be
+  // another path to the latch remaining that does not allow to eliminate the
+  // loop copy on unswitch).
+  BasicBlock *Latch = L.getLoopLatch();
+  BasicBlock *CondBlock = TI.getParent();
+  if (DT.dominates(CondBlock, Latch) &&
+      (isGuard(&TI) ||
+       llvm::count_if(successors(&TI), [&L](BasicBlock *SuccBB) {
+         return L.contains(SuccBB);
+       }) <= 1)) {
+    NumCostMultiplierSkipped++;
+    return 1;
+  }
+
+  auto *ParentL = L.getParentLoop();
+  int SiblingsCount = (ParentL ? ParentL->getSubLoopsVector().size()
+                               : std::distance(LI.begin(), LI.end()));
+  // Count amount of clones that all the candidates might cause during
+  // unswitching. Branch/guard counts as 1, switch counts as log2 of its cases.
+  int UnswitchedClones = 0;
+  for (auto Candidate : UnswitchCandidates) {
+    Instruction *CI = Candidate.first;
+    BasicBlock *CondBlock = CI->getParent();
+    bool SkipExitingSuccessors = DT.dominates(CondBlock, Latch);
+    if (isGuard(CI)) {
+      if (!SkipExitingSuccessors)
+        UnswitchedClones++;
+      continue;
+    }
+    int NonExitingSuccessors = llvm::count_if(
+        successors(CondBlock), [SkipExitingSuccessors, &L](BasicBlock *SuccBB) {
+          return !SkipExitingSuccessors || L.contains(SuccBB);
+        });
+    UnswitchedClones += Log2_32(NonExitingSuccessors);
+  }
+
+  // Ignore up to the "unscaled candidates" number of unswitch candidates
+  // when calculating the power-of-two scaling of the cost. The main idea
+  // with this control is to allow a small number of unswitches to happen
+  // and rely more on siblings multiplier (see below) when the number
+  // of candidates is small.
+  unsigned ClonesPower =
+      std::max(UnswitchedClones - (int)UnswitchNumInitialUnscaledCandidates, 0);
+
+  // Allowing top-level loops to spread a bit more than nested ones.
+  int SiblingsMultiplier =
+      std::max((ParentL ? SiblingsCount
+                        : SiblingsCount / (int)UnswitchSiblingsToplevelDiv),
+               1);
+  // Compute the cost multiplier in a way that won't overflow by saturating
+  // at an upper bound.
+  int CostMultiplier;
+  if (ClonesPower > Log2_32(UnswitchThreshold) ||
+      SiblingsMultiplier > UnswitchThreshold)
+    CostMultiplier = UnswitchThreshold;
+  else
+    CostMultiplier = std::min(SiblingsMultiplier * (1 << ClonesPower),
+                              (int)UnswitchThreshold);
+
+  LLVM_DEBUG(dbgs() << "  Computed multiplier  " << CostMultiplier
+                    << " (siblings " << SiblingsMultiplier << " * clones "
+                    << (1 << ClonesPower) << ")"
+                    << " for unswitch candidate: " << TI << "\n");
+  return CostMultiplier;
+}
+
+static bool unswitchBestCondition(
+    Loop &L, DominatorTree &DT, LoopInfo &LI, AssumptionCache &AC,
+    AAResults &AA, TargetTransformInfo &TTI,
+    function_ref<void(bool, bool, ArrayRef<Loop *>)> UnswitchCB,
+    ScalarEvolution *SE, MemorySSAUpdater *MSSAU,
+    function_ref<void(Loop &, StringRef)> DestroyLoopCB) {
+  // Collect all invariant conditions within this loop (as opposed to an inner
+  // loop which would be handled when visiting that inner loop).
+  SmallVector<std::pair<Instruction *, TinyPtrVector<Value *>>, 4>
+      UnswitchCandidates;
+
+  // Whether or not we should also collect guards in the loop.
+  bool CollectGuards = false;
+  if (UnswitchGuards) {
+    auto *GuardDecl = L.getHeader()->getParent()->getParent()->getFunction(
+        Intrinsic::getName(Intrinsic::experimental_guard));
+    if (GuardDecl && !GuardDecl->use_empty())
+      CollectGuards = true;
+  }
+
+  IVConditionInfo PartialIVInfo;
+  for (auto *BB : L.blocks()) {
+    if (LI.getLoopFor(BB) != &L)
+      continue;
+
+    if (CollectGuards)
+      for (auto &I : *BB)
+        if (isGuard(&I)) {
+          auto *Cond = cast<IntrinsicInst>(&I)->getArgOperand(0);
+          // TODO: Support AND, OR conditions and partial unswitching.
+          if (!isa<Constant>(Cond) && L.isLoopInvariant(Cond))
+            UnswitchCandidates.push_back({&I, {Cond}});
+        }
+
+    if (auto *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
+      // We can only consider fully loop-invariant switch conditions as we need
+      // to completely eliminate the switch after unswitching.
+      if (!isa<Constant>(SI->getCondition()) &&
+          L.isLoopInvariant(SI->getCondition()) && !BB->getUniqueSuccessor())
+        UnswitchCandidates.push_back({SI, {SI->getCondition()}});
+      continue;
+    }
+
+    auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
+    if (!BI || !BI->isConditional() || isa<Constant>(BI->getCondition()) ||
+        BI->getSuccessor(0) == BI->getSuccessor(1))
+      continue;
+
+    Value *Cond = skipTrivialSelect(BI->getCondition());
+    if (isa<Constant>(Cond))
+      continue;
+
+    if (L.isLoopInvariant(Cond)) {
+      UnswitchCandidates.push_back({BI, {Cond}});
+      continue;
+    }
+
+    Instruction &CondI = *cast<Instruction>(Cond);
+    if (match(&CondI, m_CombineOr(m_LogicalAnd(), m_LogicalOr()))) {
+      TinyPtrVector<Value *> Invariants =
+          collectHomogenousInstGraphLoopInvariants(L, CondI, LI);
+      if (Invariants.empty())
+        continue;
+
+      UnswitchCandidates.push_back({BI, std::move(Invariants)});
+      continue;
+    }
+  }
+
+  Instruction *PartialIVCondBranch = nullptr;
+  if (MSSAU && !findOptionMDForLoop(&L, "llvm.loop.unswitch.partial.disable") &&
+      !any_of(UnswitchCandidates, [&L](auto &TerminatorAndInvariants) {
+        return TerminatorAndInvariants.first == L.getHeader()->getTerminator();
+      })) {
+    MemorySSA *MSSA = MSSAU->getMemorySSA();
+    if (auto Info = hasPartialIVCondition(L, MSSAThreshold, *MSSA, AA)) {
+      LLVM_DEBUG(
+          dbgs() << "simple-loop-unswitch: Found partially invariant condition "
+                 << *Info->InstToDuplicate[0] << "\n");
+      PartialIVInfo = *Info;
+      PartialIVCondBranch = L.getHeader()->getTerminator();
+      TinyPtrVector<Value *> ValsToDuplicate;
+      for (auto *Inst : Info->InstToDuplicate)
+        ValsToDuplicate.push_back(Inst);
+      UnswitchCandidates.push_back(
+          {L.getHeader()->getTerminator(), std::move(ValsToDuplicate)});
+    }
+  }
+
+  // If we didn't find any candidates, we're done.
+  if (UnswitchCandidates.empty())
+    return false;
+
+  // Check if there are irreducible CFG cycles in this loop. If so, we cannot
+  // easily unswitch non-trivial edges out of the loop. Doing so might turn the
+  // irreducible control flow into reducible control flow and introduce new
+  // loops "out of thin air". If we ever discover important use cases for doing
+  // this, we can add support to loop unswitch, but it is a lot of complexity
+  // for what seems little or no real world benefit.
+  LoopBlocksRPO RPOT(&L);
+  RPOT.perform(&LI);
+  if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI))
+    return false;
+
+  SmallVector<BasicBlock *, 4> ExitBlocks;
+  L.getUniqueExitBlocks(ExitBlocks);
+
+  // We cannot unswitch if exit blocks contain a cleanuppad/catchswitch
+  // instruction as we don't know how to split those exit blocks.
+  // FIXME: We should teach SplitBlock to handle this and remove this
+  // restriction.
+  for (auto *ExitBB : ExitBlocks) {
+    auto *I = ExitBB->getFirstNonPHI();
+    if (isa<CleanupPadInst>(I) || isa<CatchSwitchInst>(I)) {
+      LLVM_DEBUG(dbgs() << "Cannot unswitch because of cleanuppad/catchswitch "
+                           "in exit block\n");
+      return false;
+    }
+  }
+
+  LLVM_DEBUG(
+      dbgs() << "Considering " << UnswitchCandidates.size()
+             << " non-trivial loop invariant conditions for unswitching.\n");
+
+  // Given that unswitching these terminators will require duplicating parts of
+  // the loop, so we need to be able to model that cost. Compute the ephemeral
+  // values and set up a data structure to hold per-BB costs. We cache each
+  // block's cost so that we don't recompute this when considering different
+  // subsets of the loop for duplication during unswitching.
+  SmallPtrSet<const Value *, 4> EphValues;
+  CodeMetrics::collectEphemeralValues(&L, &AC, EphValues);
+  SmallDenseMap<BasicBlock *, InstructionCost, 4> BBCostMap;
+
+  // Compute the cost of each block, as well as the total loop cost. Also, bail
+  // out if we see instructions which are incompatible with loop unswitching
+  // (convergent, noduplicate, or cross-basic-block tokens).
+  // FIXME: We might be able to safely handle some of these in non-duplicated
+  // regions.
+  TargetTransformInfo::TargetCostKind CostKind =
+      L.getHeader()->getParent()->hasMinSize()
+          ? TargetTransformInfo::TCK_CodeSize
+          : TargetTransformInfo::TCK_SizeAndLatency;
+  InstructionCost LoopCost = 0;
+  for (auto *BB : L.blocks()) {
+    InstructionCost Cost = 0;
+    for (auto &I : *BB) {
+      if (EphValues.count(&I))
+        continue;
+
+      if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB))
+        return false;
+      if (auto *CB = dyn_cast<CallBase>(&I))
+        if (CB->isConvergent() || CB->cannotDuplicate())
+          return false;
+
+      Cost += TTI.getUserCost(&I, CostKind);
+    }
+    assert(Cost >= 0 && "Must not have negative costs!");
+    LoopCost += Cost;
+    assert(LoopCost >= 0 && "Must not have negative loop costs!");
+    BBCostMap[BB] = Cost;
+  }
+  LLVM_DEBUG(dbgs() << "  Total loop cost: " << LoopCost << "\n");
+
+  // Now we find the best candidate by searching for the one with the following
+  // properties in order:
+  //
+  // 1) An unswitching cost below the threshold
+  // 2) The smallest number of duplicated unswitch candidates (to avoid
+  //    creating redundant subsequent unswitching)
+  // 3) The smallest cost after unswitching.
+  //
+  // We prioritize reducing fanout of unswitch candidates provided the cost
+  // remains below the threshold because this has a multiplicative effect.
+  //
+  // This requires memoizing each dominator subtree to avoid redundant work.
+  //
+  // FIXME: Need to actually do the number of candidates part above.
+  SmallDenseMap<DomTreeNode *, InstructionCost, 4> DTCostMap;
+  // Given a terminator which might be unswitched, computes the non-duplicated
+  // cost for that terminator.
+  auto ComputeUnswitchedCost = [&](Instruction &TI,
+                                   bool FullUnswitch) -> InstructionCost {
+    BasicBlock &BB = *TI.getParent();
+    SmallPtrSet<BasicBlock *, 4> Visited;
+
+    InstructionCost Cost = 0;
+    for (BasicBlock *SuccBB : successors(&BB)) {
+      // Don't count successors more than once.
+      if (!Visited.insert(SuccBB).second)
+        continue;
+
+      // If this is a partial unswitch candidate, then it must be a conditional
+      // branch with a condition of either `or`, `and`, their corresponding
+      // select forms or partially invariant instructions. In that case, one of
+      // the successors is necessarily duplicated, so don't even try to remove
+      // its cost.
+      if (!FullUnswitch) {
+        auto &BI = cast<BranchInst>(TI);
+        Value *Cond = skipTrivialSelect(BI.getCondition());
+        if (match(Cond, m_LogicalAnd())) {
+          if (SuccBB == BI.getSuccessor(1))
+            continue;
+        } else if (match(Cond, m_LogicalOr())) {
+          if (SuccBB == BI.getSuccessor(0))
+            continue;
+        } else if ((PartialIVInfo.KnownValue->isOneValue() &&
+                    SuccBB == BI.getSuccessor(0)) ||
+                   (!PartialIVInfo.KnownValue->isOneValue() &&
+                    SuccBB == BI.getSuccessor(1)))
+          continue;
+      }
+
+      // This successor's domtree will not need to be duplicated after
+      // unswitching if the edge to the successor dominates it (and thus the
+      // entire tree). This essentially means there is no other path into this
+      // subtree and so it will end up live in only one clone of the loop.
+      if (SuccBB->getUniquePredecessor() ||
+          llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
+            return PredBB == &BB || DT.dominates(SuccBB, PredBB);
+          })) {
+        Cost += computeDomSubtreeCost(*DT[SuccBB], BBCostMap, DTCostMap);
+        assert(Cost <= LoopCost &&
+               "Non-duplicated cost should never exceed total loop cost!");
+      }
+    }
+
+    // Now scale the cost by the number of unique successors minus one. We
+    // subtract one because there is already at least one copy of the entire
+    // loop. This is computing the new cost of unswitching a condition.
+    // Note that guards always have 2 unique successors that are implicit and
+    // will be materialized if we decide to unswitch it.
+    int SuccessorsCount = isGuard(&TI) ? 2 : Visited.size();
+    assert(SuccessorsCount > 1 &&
+           "Cannot unswitch a condition without multiple distinct successors!");
+    return (LoopCost - Cost) * (SuccessorsCount - 1);
+  };
+  Instruction *BestUnswitchTI = nullptr;
+  InstructionCost BestUnswitchCost = 0;
+  ArrayRef<Value *> BestUnswitchInvariants;
+  for (auto &TerminatorAndInvariants : UnswitchCandidates) {
+    Instruction &TI = *TerminatorAndInvariants.first;
+    ArrayRef<Value *> Invariants = TerminatorAndInvariants.second;
+    BranchInst *BI = dyn_cast<BranchInst>(&TI);
+    InstructionCost CandidateCost = ComputeUnswitchedCost(
+        TI, /*FullUnswitch*/ !BI ||
+                (Invariants.size() == 1 &&
+                 Invariants[0] == skipTrivialSelect(BI->getCondition())));
+    // Calculate cost multiplier which is a tool to limit potentially
+    // exponential behavior of loop-unswitch.
+    if (EnableUnswitchCostMultiplier) {
+      int CostMultiplier =
+          CalculateUnswitchCostMultiplier(TI, L, LI, DT, UnswitchCandidates);
+      assert(
+          (CostMultiplier > 0 && CostMultiplier <= UnswitchThreshold) &&
+          "cost multiplier needs to be in the range of 1..UnswitchThreshold");
+      CandidateCost *= CostMultiplier;
+      LLVM_DEBUG(dbgs() << "  Computed cost of " << CandidateCost
+                        << " (multiplier: " << CostMultiplier << ")"
+                        << " for unswitch candidate: " << TI << "\n");
+    } else {
+      LLVM_DEBUG(dbgs() << "  Computed cost of " << CandidateCost
+                        << " for unswitch candidate: " << TI << "\n");
+    }
+
+    if (!BestUnswitchTI || CandidateCost < BestUnswitchCost) {
+      BestUnswitchTI = &TI;
+      BestUnswitchCost = CandidateCost;
+      BestUnswitchInvariants = Invariants;
+    }
+  }
+  assert(BestUnswitchTI && "Failed to find loop unswitch candidate");
+
+  if (BestUnswitchCost >= UnswitchThreshold) {
+    LLVM_DEBUG(dbgs() << "Cannot unswitch, lowest cost found: "
+                      << BestUnswitchCost << "\n");
+    return false;
+  }
+
+  if (BestUnswitchTI != PartialIVCondBranch)
+    PartialIVInfo.InstToDuplicate.clear();
+
+  // If the best candidate is a guard, turn it into a branch.
+  if (isGuard(BestUnswitchTI))
+    BestUnswitchTI = turnGuardIntoBranch(cast<IntrinsicInst>(BestUnswitchTI), L,
+                                         ExitBlocks, DT, LI, MSSAU);
+
+  LLVM_DEBUG(dbgs() << "  Unswitching non-trivial (cost = " << BestUnswitchCost
+                    << ") terminator: " << *BestUnswitchTI << "\n");
+  unswitchNontrivialInvariants(L, *BestUnswitchTI, BestUnswitchInvariants,
+                               ExitBlocks, PartialIVInfo, DT, LI, AC,
+                               UnswitchCB, SE, MSSAU, DestroyLoopCB);
+  return true;
+}
+
+/// Unswitch control flow predicated on loop invariant conditions.
+///
+/// This first hoists all branches or switches which are trivial (IE, do not
+/// require duplicating any part of the loop) out of the loop body. It then
+/// looks at other loop invariant control flows and tries to unswitch those as
+/// well by cloning the loop if the result is small enough.
+///
+/// The `DT`, `LI`, `AC`, `AA`, `TTI` parameters are required analyses that are
+/// also updated based on the unswitch. The `MSSA` analysis is also updated if
+/// valid (i.e. its use is enabled).
+///
+/// The `UnswitchCB` callback provided will be run after unswitching is
+/// complete, with the first parameter set to `true` if the provided loop
+/// remains a loop, and a list of new sibling loops created.
+///
+/// If `SE` is non-null, we will update that analysis based on the unswitching
+/// done.
+static bool
+unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI, AssumptionCache &AC,
+             AAResults &AA, TargetTransformInfo &TTI,
+             function_ref<void(bool, bool, ArrayRef<Loop *>)> UnswitchCB,
+             ScalarEvolution *SE, MemorySSAUpdater *MSSAU,
+             function_ref<void(Loop &, StringRef)> DestroyLoopCB) {
+  assert(L.isRecursivelyLCSSAForm(DT, LI) &&
+         "Loops must be in LCSSA form before unswitching.");
+
+  // Must be in loop simplified form: we need a preheader and dedicated exits.
+  if (!L.isLoopSimplifyForm())
+    return false;
+
+  // NonTrivial should be allowed only for targets without branch divergence.
+  //
+  // FIXME: If divergence analysis becomes available to a loop
+  // transform, we should allow unswitching for non-trivial uniform
+  // branches even on targets that have divergence.
+  // https://bugs.llvm.org/show_bug.cgi?id=48819
+  bool ContinueWithNonTrivial = !TTI.hasBranchDivergence();
+  if (!ContinueWithNonTrivial)
+    return false;
+
+  // Skip non-trivial unswitching for optsize functions.
+  if (L.getHeader()->getParent()->hasOptSize())
+    return false;
+
+  // Skip non-trivial unswitching for loops that cannot be cloned.
+  if (!L.isSafeToClone())
+    return false;
+
+  // For non-trivial unswitching, because it often creates new loops, we rely on
+  // the pass manager to iterate on the loops rather than trying to immediately
+  // reach a fixed point. There is no substantial advantage to iterating
+  // internally, and if any of the new loops are simplified enough to contain
+  // trivial unswitching we want to prefer those.
+
+  // Try to unswitch the best invariant condition. We prefer this full unswitch
+  // to a partial unswitch when possible below the threshold.
+  if (unswitchBestCondition(L, DT, LI, AC, AA, TTI, UnswitchCB, SE, MSSAU,
+                            DestroyLoopCB))
+    return true;
+
+  // No other opportunities to unswitch.
+  return false;
+}
+
+PreservedAnalyses FuncSimpleLoopUnswitchPass::run(Function &F,
+                                                  FunctionAnalysisManager &AM) {
+  LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
+  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
+  auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
+  auto MSSA = &AM.getResult<MemorySSAAnalysis>(F).getMSSA();
+  bool Changed = false;
+  for (const auto &L : LI) {
+    Changed |= simplifyLoop(L, &DT, &LI, &SE, nullptr, nullptr,
+                            /*PreserveLCSSA=*/false);
+    Changed |= formLCSSARecursively(*L, DT, &LI, &SE);
+  }
+
+  SmallPriorityWorklist<Loop *, 4> Worklist;
+  appendLoopsToWorklist(LI, Worklist);
+  while (!Worklist.empty()) {
+    Loop &L = *Worklist.pop_back_val();
+    LLVM_DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << L
+                      << "\n");
+
+    // Save the current loop name in a variable so that we can report it even
+    // after it has been deleted.
+    std::string LoopName = std::string(L.getName());
+
+    auto &LAM = AM.getResult<LoopAnalysisManagerFunctionProxy>(F).getManager();
+
+    auto UnswitchCB = [&L, &LAM, &LoopName, &Worklist](
+                          bool CurrentLoopValid, bool PartiallyInvariant,
+                          ArrayRef<Loop *> NewLoops) {
+      // If we did a non-trivial unswitch, we have added new (cloned) loops.
+      if (!NewLoops.empty())
+        appendLoopsToWorklist(NewLoops, Worklist);
+
+      // If the current loop remains valid, we should revisit it to catch any
+      // other unswitch opportunities. Otherwise, we need to mark it as deleted.
+      if (CurrentLoopValid) {
+        if (PartiallyInvariant) {
+          // Mark the new loop as partially unswitched, to avoid unswitching on
+          // the same condition again.
+          auto &Context = L.getHeader()->getContext();
+          MDNode *DisableUnswitchMD = MDNode::get(
+              Context,
+              MDString::get(Context, "llvm.loop.unswitch.partial.disable"));
+          MDNode *NewLoopID = makePostTransformationMetadata(
+              Context, L.getLoopID(), {"llvm.loop.unswitch.partial"},
+              {DisableUnswitchMD});
+          L.setLoopID(NewLoopID);
+        } else
+          Worklist.insert(&L);
+      } else
+        LAM.clear(L, LoopName);
+    };
+
+    auto DestroyLoopCB = [&LAM](Loop &L, StringRef Name) {
+      LAM.clear(L, Name);
+    };
+
+    Optional<MemorySSAUpdater> MSSAU;
+    if (MSSA) {
+      MSSAU = MemorySSAUpdater(MSSA);
+      if (VerifyMemorySSA)
+        MSSA->verifyMemorySSA();
+    }
+    Changed |= unswitchLoop(
+        L, AM.getResult<DominatorTreeAnalysis>(F),
+        AM.getResult<LoopAnalysis>(F), AM.getResult<AssumptionAnalysis>(F),
+        AM.getResult<AAManager>(F), AM.getResult<TargetIRAnalysis>(F),
+        UnswitchCB, &AM.getResult<ScalarEvolutionAnalysis>(F),
+        MSSAU.hasValue() ? MSSAU.getPointer() : nullptr, DestroyLoopCB);
+  }
+  if (!Changed)
+    return PreservedAnalyses::all();
+
+  if (MSSA && VerifyMemorySSA)
+    MSSA->verifyMemorySSA();
+
+  // Historically this pass has had issues with the dominator tree so verify it
+  // in asserts builds.
+  assert(AM.getResult<DominatorTreeAnalysis>(F).verify(
+      DominatorTree::VerificationLevel::Fast));
+
+  auto PA = getLoopPassPreservedAnalyses();
+  if (MSSA)
+    PA.preserve<MemorySSAAnalysis>();
+  return PA;
+}
+
+void FuncSimpleLoopUnswitchPass::printPipeline(
+    raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
+  static_cast<PassInfoMixin<FuncSimpleLoopUnswitchPass> *>(this)->printPipeline(
+      OS, MapClassName2PassName);
+}
 No newline at end of file