Index: lib/Target/AArch64/AArch64InstrInfo.cpp
===================================================================
--- lib/Target/AArch64/AArch64InstrInfo.cpp
+++ lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -261,8 +261,9 @@
     BuildMI(&MBB, DL, get(AArch64::Bcc)).addImm(Cond[0].getImm()).addMBB(TBB);
   } else {
     // Folded compare-and-branch
+    // Note that we use addOperand instead of addReg to keep the flags.
     const MachineInstrBuilder MIB =
-        BuildMI(&MBB, DL, get(Cond[1].getImm())).addReg(Cond[2].getReg());
+        BuildMI(&MBB, DL, get(Cond[1].getImm())).addOperand(Cond[2]);
     if (Cond.size() > 3)
       MIB.addImm(Cond[3].getImm());
     MIB.addMBB(TBB);
Index: test/CodeGen/AArch64/br-undef-cond.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/br-undef-cond.ll
@@ -0,0 +1,26 @@
+; RUN: llc < %s -verify-machineinstrs
+
+; Make sure we don't end up with a CBNZ of an undef v-/phys-reg.
+
+target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+target triple = "arm64-apple-ios"
+
+declare void @bar(i8*)
+
+define void @foo(i8* %m, i32 %off0) {
+.thread1653:
+  br i1 undef, label %0, label %.thread1880
+
+  %1 = icmp eq i32 undef, 0
+  %.not = xor i1 %1, true
+  %brmerge = or i1 %.not, undef
+  br i1 %brmerge, label %.thread1880, label %.thread1705
+
+.thread1705:
+  ret void
+
+.thread1880:
+  %m1652.ph = phi i8* [ %m, %0 ], [ null, %.thread1653 ]
+  call void @bar(i8* %m1652.ph)
+  ret void
+}