Index: llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp
+++ llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -210,7 +210,7 @@
 
     bool runOnLoop(Loop *L, LPPassManager &LPM) override;
     bool processCurrentLoop();
-
+    bool isUnreachableDueToPreviousUnswitching(BasicBlock *);
     /// This transformation requires natural loop information & requires that
     /// loop preheaders be inserted into the CFG.
     ///
@@ -483,6 +483,35 @@
   return Changed;
 }
 
+// Return true if the BasicBlock BB is unreachable from the loop header.
+// Return false, otherwise.
+bool LoopUnswitch::isUnreachableDueToPreviousUnswitching(BasicBlock *BB) {
+  auto *Node = DT->getNode(BB)->getIDom();
+  BasicBlock *DomBB = Node->getBlock();
+  while (currentLoop->contains(DomBB)) {
+    BranchInst *BInst = dyn_cast<BranchInst>(DomBB->getTerminator());
+
+    Node = DT->getNode(DomBB)->getIDom();
+    DomBB = Node->getBlock();
+
+    if (!BInst || !BInst->isConditional())
+      continue;
+
+    Value *Cond = BInst->getCondition();
+    if (!isa<ConstantInt>(Cond))
+      continue;
+
+    BasicBlock *UnreachableSucc =
+        Cond == ConstantInt::getTrue(Cond->getContext())
+            ? BInst->getSuccessor(1)
+            : BInst->getSuccessor(0);
+
+    if (DT->dominates(UnreachableSucc, BB))
+      return true;
+  }
+  return false;
+}
+
 /// Do actual work and unswitch loop if possible and profitable.
 bool LoopUnswitch::processCurrentLoop() {
   bool Changed = false;
@@ -593,6 +622,12 @@
       continue;
 
     if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
+      // Some branches may be rendered unreachable because of previous
+      // unswitching.
+      // Unswitch only those branches that are reachable.
+      if (isUnreachableDueToPreviousUnswitching(*I))
+        continue;
+ 
       // If this isn't branching on an invariant condition, we can't unswitch
       // it.
       if (BI->isConditional()) {
Index: llvm/trunk/test/Transforms/LoopUnswitch/elseif-non-exponential-behavior.ll
===================================================================
--- llvm/trunk/test/Transforms/LoopUnswitch/elseif-non-exponential-behavior.ll
+++ llvm/trunk/test/Transforms/LoopUnswitch/elseif-non-exponential-behavior.ll
@@ -0,0 +1,62 @@
+; RUN: opt -loop-unswitch -S - < %s | FileCheck %s
+
+;CHECK-LABEL: @b
+;CHECK: [[Loop1:for\.end.*]]:                              ; preds = %for.cond.us
+;CHECK-NEXT:  %[[PhiVar1:pdt.*]] = phi i32 [ %pdt.0.us, %for.cond.us ]
+;CHECK: [[Loop2:for\.end.*]]:                     ; preds = %for.cond.us1
+;CHECK-NEXT:  %[[PhiVar2:pdt.*]] = phi i32 [ %pdt.0.us2, %for.cond.us1 ]
+;CHECK: [[Loop3:for\.end.*]]:                        ; preds = %for.cond
+;CHECK-NEXT:  %[[PhiVar3:pdt.*]] = phi i32 [ %pdt.0, %for.cond ]
+;CHECK: [[Join1:for\.end.*]]:                                 ; preds = %[[Loop2]], %[[Loop3]]
+;CHECK-NEXT:  %[[PhiRes1:pdt.*]] = phi i32 [ %[[PhiVar3]], %[[Loop3]] ], [ %[[PhiVar2]], %[[Loop2]] ]
+;CHECK: for.end:                                          ; preds = %[[Loop1]], %[[Join1]]
+;CHECK-NEXT:  %[[PhiRes2:pdt.*]] = phi i32 [ %[[PhiRes1]], %[[Join1]] ], [ %[[PhiVar1]], %[[Loop1]] ]
+;CHECK-NEXT:  ret i32 %[[PhiRes2]]
+
+; Function Attrs: nounwind uwtable
+define i32 @b(i32 %x, i32 %y) #0 {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %pdt.0 = phi i32 [ 1, %entry ], [ %pdt.2, %for.inc ]
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 100
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %tobool = icmp ne i32 %x, 0
+  br i1 %tobool, label %if.then, label %if.else
+
+if.then:                                          ; preds = %for.body
+  %mul = mul nsw i32 %pdt.0, 2
+  br label %if.end6
+
+if.else:                                          ; preds = %for.body
+  %tobool1 = icmp ne i32 %y, 0
+  br i1 %tobool1, label %if.then2, label %if.else4
+
+if.then2:                                         ; preds = %if.else
+  %mul3 = mul nsw i32 %pdt.0, 3
+  br label %if.end
+
+if.else4:                                         ; preds = %if.else
+  %mul5 = mul nsw i32 %pdt.0, 4
+  br label %if.end
+
+if.end:                                           ; preds = %if.else4, %if.then2
+  %pdt.1 = phi i32 [ %mul3, %if.then2 ], [ %mul5, %if.else4 ]
+  br label %if.end6
+
+if.end6:                                          ; preds = %if.end, %if.then
+  %pdt.2 = phi i32 [ %mul, %if.then ], [ %pdt.1, %if.end ]
+  br label %for.inc
+
+for.inc:                                          ; preds = %if.end6
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret i32 %pdt.0
+}
+