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diff --git a/llvm/lib/CodeGen/UnreachableBlockElim.cpp b/llvm/lib/CodeGen/UnreachableBlockElim.cpp
new file mode 100644
index 000000000000..3289eff71336
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+++ b/llvm/lib/CodeGen/UnreachableBlockElim.cpp
@@ -0,0 +1,211 @@
+//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass is an extremely simple version of the SimplifyCFG pass.  Its sole
+// job is to delete LLVM basic blocks that are not reachable from the entry
+// node.  To do this, it performs a simple depth first traversal of the CFG,
+// then deletes any unvisited nodes.
+//
+// Note that this pass is really a hack.  In particular, the instruction
+// selectors for various targets should just not generate code for unreachable
+// blocks.  Until LLVM has a more systematic way of defining instruction
+// selectors, however, we cannot really expect them to handle additional
+// complexity.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/UnreachableBlockElim.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/TargetInstrInfo.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/Constant.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Type.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+using namespace llvm;
+
+namespace {
+class UnreachableBlockElimLegacyPass : public FunctionPass {
+  bool runOnFunction(Function &F) override {
+    return llvm::EliminateUnreachableBlocks(F);
+  }
+
+public:
+  static char ID; // Pass identification, replacement for typeid
+  UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
+    initializeUnreachableBlockElimLegacyPassPass(
+        *PassRegistry::getPassRegistry());
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addPreserved<DominatorTreeWrapperPass>();
+  }
+};
+}
+char UnreachableBlockElimLegacyPass::ID = 0;
+INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
+                "Remove unreachable blocks from the CFG", false, false)
+
+FunctionPass *llvm::createUnreachableBlockEliminationPass() {
+  return new UnreachableBlockElimLegacyPass();
+}
+
+PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
+                                                FunctionAnalysisManager &AM) {
+  bool Changed = llvm::EliminateUnreachableBlocks(F);
+  if (!Changed)
+    return PreservedAnalyses::all();
+  PreservedAnalyses PA;
+  PA.preserve<DominatorTreeAnalysis>();
+  return PA;
+}
+
+namespace {
+  class UnreachableMachineBlockElim : public MachineFunctionPass {
+    bool runOnMachineFunction(MachineFunction &F) override;
+    void getAnalysisUsage(AnalysisUsage &AU) const override;
+    MachineModuleInfo *MMI;
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
+  };
+}
+char UnreachableMachineBlockElim::ID = 0;
+
+INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
+  "Remove unreachable machine basic blocks", false, false)
+
+char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
+
+void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addPreserved<MachineLoopInfo>();
+  AU.addPreserved<MachineDominatorTree>();
+  MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
+  df_iterator_default_set<MachineBasicBlock*> Reachable;
+  bool ModifiedPHI = false;
+
+  auto *MMIWP = getAnalysisIfAvailable<MachineModuleInfoWrapperPass>();
+  MMI = MMIWP ? &MMIWP->getMMI() : nullptr;
+  MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
+  MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
+
+  // Mark all reachable blocks.
+  for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
+    (void)BB/* Mark all reachable blocks */;
+
+  // Loop over all dead blocks, remembering them and deleting all instructions
+  // in them.
+  std::vector<MachineBasicBlock*> DeadBlocks;
+  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
+    MachineBasicBlock *BB = &*I;
+
+    // Test for deadness.
+    if (!Reachable.count(BB)) {
+      DeadBlocks.push_back(BB);
+
+      // Update dominator and loop info.
+      if (MLI) MLI->removeBlock(BB);
+      if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
+
+      while (BB->succ_begin() != BB->succ_end()) {
+        MachineBasicBlock* succ = *BB->succ_begin();
+
+        MachineBasicBlock::iterator start = succ->begin();
+        while (start != succ->end() && start->isPHI()) {
+          for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
+            if (start->getOperand(i).isMBB() &&
+                start->getOperand(i).getMBB() == BB) {
+              start->RemoveOperand(i);
+              start->RemoveOperand(i-1);
+            }
+
+          start++;
+        }
+
+        BB->removeSuccessor(BB->succ_begin());
+      }
+    }
+  }
+
+  // Actually remove the blocks now.
+  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
+    // Remove any call site information for calls in the block.
+    for (auto &I : DeadBlocks[i]->instrs())
+      if (I.isCall(MachineInstr::IgnoreBundle))
+        DeadBlocks[i]->getParent()->eraseCallSiteInfo(&I);
+
+    DeadBlocks[i]->eraseFromParent();
+  }
+
+  // Cleanup PHI nodes.
+  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
+    MachineBasicBlock *BB = &*I;
+    // Prune unneeded PHI entries.
+    SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
+                                             BB->pred_end());
+    MachineBasicBlock::iterator phi = BB->begin();
+    while (phi != BB->end() && phi->isPHI()) {
+      for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
+        if (!preds.count(phi->getOperand(i).getMBB())) {
+          phi->RemoveOperand(i);
+          phi->RemoveOperand(i-1);
+          ModifiedPHI = true;
+        }
+
+      if (phi->getNumOperands() == 3) {
+        const MachineOperand &Input = phi->getOperand(1);
+        const MachineOperand &Output = phi->getOperand(0);
+        Register InputReg = Input.getReg();
+        Register OutputReg = Output.getReg();
+        assert(Output.getSubReg() == 0 && "Cannot have output subregister");
+        ModifiedPHI = true;
+
+        if (InputReg != OutputReg) {
+          MachineRegisterInfo &MRI = F.getRegInfo();
+          unsigned InputSub = Input.getSubReg();
+          if (InputSub == 0 &&
+              MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
+              !Input.isUndef()) {
+            MRI.replaceRegWith(OutputReg, InputReg);
+          } else {
+            // The input register to the PHI has a subregister or it can't be
+            // constrained to the proper register class or it is undef:
+            // insert a COPY instead of simply replacing the output
+            // with the input.
+            const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
+            BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
+                    TII->get(TargetOpcode::COPY), OutputReg)
+                .addReg(InputReg, getRegState(Input), InputSub);
+          }
+          phi++->eraseFromParent();
+        }
+        continue;
+      }
+
+      ++phi;
+    }
+  }
+
+  F.RenumberBlocks();
+
+  return (!DeadBlocks.empty() || ModifiedPHI);
+}