Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -31,6 +31,7 @@
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineJumpTableInfo.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
@@ -372,6 +373,30 @@
   if (UpdateLiveIns) {
     NewDest->clearLiveIns();
     computeLiveIns(LiveRegs, *MRI, *NewDest);
+
+    LivePhysRegs CalcLiveIns;
+    for (MachineBasicBlock *Pred : NewDest->predecessors()) {
+      // Check if all live-ins of NewDest are live-on-exit from Pred.
+      // If not, add an IMPLICIT_DEF.
+      // Normally, this should check if for each live-in to NewDest there
+      // exists a downward-exposed def in Pred. To avoid using stepForward
+      // in LivePhysRegs, scan the block backward instead, and then check
+      // if we would need to add a new live-in to Pred.
+      CalcLiveIns.init(*TRI);
+      CalcLiveIns.addLiveOutsNoPristines(*Pred);
+      for (const MachineInstr &MI : llvm::reverse(*Pred))
+        CalcLiveIns.stepBackward(MI);
+
+      LivePhysRegs LiveIns(*TRI);
+      LiveIns.addLiveIns(*Pred);
+      for (MCPhysReg R : CalcLiveIns) {
+        if (!LiveIns.available(*MRI, R))
+          continue;
+        MachineBasicBlock::iterator At = Pred->getFirstTerminator();
+        const DebugLoc &dl = Pred->findDebugLoc(At);
+        BuildMI(*Pred, At, dl, TII->get(TargetOpcode::IMPLICIT_DEF), R);
+      }
+    }
   }
 
   ++NumTailMerge;
Index: test/CodeGen/Hexagon/branchfolder-insert-impdef.mir
===================================================================
--- /dev/null
+++ test/CodeGen/Hexagon/branchfolder-insert-impdef.mir
@@ -0,0 +1,43 @@
+# RUN: llc -march=hexagon -run-pass branch-folder %s -o - -verify-machineinstrs | FileCheck %s
+
+# Branch folding will perform tail merging of bb.1 and bb.2, and bb.2 will
+# become the common tail. The use of R0 in bb.2 is <undef> while the
+# corresponding use in bb.1 is not. The common tail will have the <undef>
+# flag removed, which will cause R0 to become a live-in to bb.2. The problem
+# is that R0 is not live-out from all predecessors of bb.2, namely is not
+# live-out from bb.0. To remedy that, the branch folder should add an
+# IMPLICIT_DEF to that block.
+
+# CHECK-LABEL: bb.0:
+# CHECK: %r0 = IMPLICIT_DEF
+# CHECK-LABEL: bb.1:
+# CHECK-LABEL: bb.2:
+# CHECK: liveins: %r0
+# CHECK: PS_storerhabs 0, %r0
+# CHECK: PS_jmpret
+
+---
+name: fred
+tracksRegLiveness: true
+
+body: |
+  bb.0:
+    successors: %bb.1, %bb.2
+      J2_jumpt undef %p0, %bb.2, implicit-def %pc
+      J2_jump %bb.1, implicit-def %pc
+
+  bb.1:
+    successors: %bb.3
+      %r0 = L2_loadruh_io undef %r1, 0
+      PS_storerhabs 0, killed %r0
+      J2_jump %bb.3, implicit-def %pc
+
+  bb.2:
+    successors: %bb.3
+      PS_storerhabs 0, undef %r0
+      J2_jump %bb.3, implicit-def %pc
+
+  bb.3:
+      PS_jmpret killed %r31, implicit-def %pc
+...
+
Index: test/CodeGen/Hexagon/livephysregs-lane-masks2.mir
===================================================================
--- test/CodeGen/Hexagon/livephysregs-lane-masks2.mir
+++ test/CodeGen/Hexagon/livephysregs-lane-masks2.mir
@@ -13,13 +13,13 @@
 
 body: |
   bb.0:
-    liveins: %p2, %r0
+    liveins: %p0:0x1, %p2, %r0
     successors: %bb.1, %bb.2
         J2_jumpt killed %p2, %bb.1, implicit-def %pc
         J2_jump %bb.2, implicit-def %pc
 
   bb.1:
-    liveins: %r0, %r19
+    liveins: %p0:0x1, %r0, %r19
     successors: %bb.3
         %r2 = A2_tfrsi 4
         %r1 = COPY %r19
@@ -28,7 +28,7 @@
         J2_jump %bb.3, implicit-def %pc
 
   bb.2:
-    liveins: %r0, %r18
+    liveins: %p0:0x1, %r0, %r18
     successors: %bb.3
         %r2 = A2_tfrsi 5
         %r1 = L2_loadrh_io %r18, 0