Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -811,9 +811,11 @@
         dbgs() << "\n";
         if (SuccBB) {
           dbgs() << "  with successor BB#" << SuccBB->getNumber() << '\n';
-          if (PredBB)
+          if (PredBB) {
             dbgs() << "  which has fall-through from BB#"
                    << PredBB->getNumber() << "\n";
+            assert(PredBB->isSuccessor(SuccBB) && "Merging non-cfg successor.");
+          }
         }
         dbgs() << "Looking for common tails of at least "
                << minCommonTailLength << " instruction"
@@ -964,6 +966,8 @@
     SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
     MachineBasicBlock *IBB = &*I;
     MachineBasicBlock *PredBB = &*std::prev(I);
+    if (PredBB && !PredBB->isSuccessor(IBB))
+      PredBB = nullptr;
     MergePotentials.clear();
     for (MachineBasicBlock *PBB : I->predecessors()) {
       if (MergePotentials.size() == TailMergeThreshold)
@@ -1212,7 +1216,7 @@
       // where a BB jumps to more than one landing pad.
       // TODO: Is it ever worth rewriting predecessors which don't already
       // jump to a landing pad, and so can safely jump to the fallthrough?
-    } else {
+    } else if (MBB->isSuccessor(&*FallThrough)) {
       // Rewrite all predecessors of the old block to go to the fallthrough
       // instead.
       while (!MBB->pred_empty()) {
@@ -1340,7 +1344,7 @@
       bool DoTransform = true;
 
       // We have to be careful that the succs of PredBB aren't both no-successor
-      // blocks.  If neither have successors and if PredBB is the second from
+      // blocks.  If neither have successors and if MBB is the second from
       // last block in the function, we'd just keep swapping the two blocks for
       // last.  Only do the swap if one is clearly better to fall through than
       // the other.
Index: test/CodeGen/Mips/eh.ll
===================================================================
--- test/CodeGen/Mips/eh.ll
+++ test/CodeGen/Mips/eh.ll
@@ -24,7 +24,7 @@
 
 lpad:                                             ; preds = %entry
 ; CHECK-EL:  # %lpad
-; CHECK-EL:  beq $5
+; CHECK-EL:  bne $5
 
   %exn.val = landingpad { i8*, i32 }
            cleanup
Index: test/CodeGen/SPARC/missinglabel.ll
===================================================================
--- test/CodeGen/SPARC/missinglabel.ll
+++ test/CodeGen/SPARC/missinglabel.ll
@@ -4,14 +4,14 @@
 
 define void @f() align 2 {
 entry:
-; CHECK: %xcc, .LBB0_1
+; CHECK: %xcc, .LBB0_2
   %cmp = icmp eq i64 undef, 0
   br i1 %cmp, label %targetblock, label %cond.false
 
 cond.false:
   unreachable
 
-; CHECK: .LBB0_1: ! %targetblock
+; CHECK: .LBB0_2: ! %targetblock
 targetblock:
   br i1 undef, label %cond.false.i83, label %exit.i85
 
Index: test/CodeGen/Thumb2/thumb2-cbnz.ll
===================================================================
--- test/CodeGen/Thumb2/thumb2-cbnz.ll
+++ test/CodeGen/Thumb2/thumb2-cbnz.ll
@@ -5,6 +5,7 @@
 
 define void @t(i32 %c, double %b) {
 entry:
+; CHECK:      cmp	r0, #0
   %cmp1 = icmp ne i32 %c, 0
   br i1 %cmp1, label %bb3, label %bb1
 
@@ -23,8 +24,7 @@
   br i1 %cmp3, label %bb11, label %bb9
 
 bb9:                                              ; preds = %bb7
-; CHECK:      cmp	r0, #0
-; CHECK-NEXT:      cbnz
+; CHECK:      cbnz
   %0 = tail call  double @foo(double %b) nounwind readnone ; <double> [#uses=0]
   br label %bb11
 
Index: test/CodeGen/X86/br-fold.ll
===================================================================
--- test/CodeGen/X86/br-fold.ll
+++ test/CodeGen/X86/br-fold.ll
@@ -5,9 +5,11 @@
 ; RUN: llc -mtriple=x86_64-scei-ps4 < %s | FileCheck -check-prefix=PS4 %s
 
 ; X64_DARWIN: orq
+; X64_DARWIN-NEXT: jne
 ; X64_DARWIN-NEXT: %bb8.i329
 
 ; X64_LINUX: orq %rax, %rcx
+; X64_LINUX-NEXT: jne
 ; X64_LINUX-NEXT: %bb8.i329
 
 ; X64_WINDOWS: orq %rax, %rcx
Index: test/CodeGen/X86/tail-merge-unreachable.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/tail-merge-unreachable.ll
@@ -0,0 +1,34 @@
+; RUN: llc -mtriple=x86_64-linux-gnu %s -o - -verify-machineinstrs | FileCheck %s
+
+define i32 @tail_merge_unreachable(i32 %i) {
+entry:
+  br i1 undef, label %sw, label %end
+sw:
+  switch i32 %i, label %end [
+    i32 99,  label %sw.bb
+    i32 98,  label %sw.bb
+    i32 101, label %sw.bb
+    i32 97,  label %sw.bb2
+    i32 96,  label %sw.bb2
+    i32 100, label %sw.bb2
+  ]
+sw.bb:
+  unreachable
+sw.bb2:
+  unreachable
+end:
+  %p = phi i32 [ 1, %sw ], [ 0, %entry ]
+  ret i32 %p
+
+; CHECK-LABEL: tail_merge_unreachable:
+; Range Check
+; CHECK: addl $-96
+; CHECK: cmpl $5
+; CHECK: jbe [[JUMP_TABLE_BLOCK:[.][A-Za-z0-9_]+]]
+; CHECK: retq
+; CHECK: [[JUMP_TABLE_BLOCK]]:
+; CHECK: btl
+; CHECK: jae [[UNREACHABLE_BLOCK:[.][A-Za-z0-9_]+]]
+; CHECK [[UNREACHABLE_BLOCK]]:
+; CHECK: .Lfunc_end0
+}