Index: include/llvm/Transforms/Utils/Local.h
===================================================================
--- include/llvm/Transforms/Utils/Local.h
+++ include/llvm/Transforms/Utils/Local.h
@@ -345,6 +345,9 @@
 /// Returns true if any basic block was removed.
 bool removeUnreachableBlocks(Function &F, LazyValueInfo *LVI = nullptr);
 
+bool removeUnreachable(Function &F, DominatorTree &DT,
+                       LazyValueInfo *LVI = nullptr);
+
 /// Combine the metadata of two instructions so that K can replace J
 ///
 /// Metadata not listed as known via KnownIDs is removed
Index: lib/Transforms/IPO/GlobalOpt.cpp
===================================================================
--- lib/Transforms/IPO/GlobalOpt.cpp
+++ lib/Transforms/IPO/GlobalOpt.cpp
@@ -2034,14 +2034,10 @@
     // So, remove unreachable blocks from the function, because a) there's
     // no point in analyzing them and b) GlobalOpt should otherwise grow
     // some more complicated logic to break these cycles.
-    // Removing unreachable blocks might invalidate the dominator so we
-    // recalculate it.
     if (!F->isDeclaration()) {
-      if (removeUnreachableBlocks(*F)) {
-        auto &DT = LookupDomTree(*F);
-        DT.recalculate(*F);
+      auto &DT = LookupDomTree(*F);
+      if (removeUnreachable(*F, DT))
         Changed = true;
-      }
     }
 
     Changed |= processGlobal(*F, TLI, LookupDomTree);
Index: lib/Transforms/Utils/Local.cpp
===================================================================
--- lib/Transforms/Utils/Local.cpp
+++ lib/Transforms/Utils/Local.cpp
@@ -16,6 +16,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/Hashing.h"
+#include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
@@ -1700,6 +1701,51 @@
   return true;
 }
 
+bool llvm::removeUnreachable(Function &F, DominatorTree &DT,
+                             LazyValueInfo *LVI) {
+  ReversePostOrderTraversal<Function *> RPOT(&F);
+  SmallPtrSet<BasicBlock *, 8> Live;
+  SmallPtrSet<BasicBlock *, 8> Processed;
+
+  for (BasicBlock *BB : RPOT)
+    Live.insert(BB);
+
+  if (Live.size() == F.size())
+    return false;
+
+  SmallVector<BasicBlock *, 8> Worklist;
+  for (BasicBlock &BB : F) {
+    if (!Live.count(&BB))
+      Worklist.push_back(&BB);
+  }
+
+  while (!Worklist.empty()) {
+    BasicBlock *BB = Worklist.pop_back_val();
+    if (Processed.count(BB))
+      continue;
+    auto *N = DT.getNode(BB);
+    if (!N) {
+      if (LVI)
+        LVI->eraseBlock(BB);
+      BB->dropAllReferences();
+      BB->eraseFromParent();
+      continue;
+    }
+    for (auto *DTN : post_order(N)) {
+      BasicBlock *DTBlock = DTN->getBlock();
+      Processed.insert(DTBlock);
+      for (BasicBlock *Succ : successors(DTBlock))
+        Succ->removePredecessor(DTBlock);
+      if (LVI)
+        LVI->eraseBlock(DTBlock);
+      DT.eraseNode(DTBlock);
+      DTBlock->dropAllReferences();
+      DTBlock->eraseFromParent();
+    }
+  }
+  return true;
+}
+
 void llvm::combineMetadata(Instruction *K, const Instruction *J,
                            ArrayRef<unsigned> KnownIDs) {
   SmallVector<std::pair<unsigned, MDNode *>, 4> Metadata;
Index: test/Other/new-pm-lto-defaults.ll
===================================================================
--- test/Other/new-pm-lto-defaults.ll
+++ test/Other/new-pm-lto-defaults.ll
@@ -40,8 +40,8 @@
 ; CHECK-O-NEXT: Running pass: GlobalSplitPass
 ; CHECK-O-NEXT: Running pass: WholeProgramDevirtPass
 ; CHECK-O2-NEXT: Running pass: GlobalOptPass
-; CHECK-O2-NEXT: Running pass: ModuleToFunctionPassAdaptor<{{.*}}PromotePass>
 ; CHECK-O2-NEXT: Running analysis: DominatorTreeAnalysis
+; CHECK-O2-NEXT: Running pass: ModuleToFunctionPassAdaptor<{{.*}}PromotePass>
 ; CHECK-O2-NEXT: Running analysis: AssumptionAnalysis
 ; CHECK-O2-NEXT: Running pass: ConstantMergePass
 ; CHECK-O2-NEXT: Running pass: DeadArgumentEliminationPass