Index: lib/CodeGen/ExpandPostRAPseudos.cpp
===================================================================
--- lib/CodeGen/ExpandPostRAPseudos.cpp
+++ lib/CodeGen/ExpandPostRAPseudos.cpp
@@ -142,8 +142,9 @@
   MachineOperand &DstMO = MI->getOperand(0);
   MachineOperand &SrcMO = MI->getOperand(1);
 
-  if (SrcMO.getReg() == DstMO.getReg()) {
-    DEBUG(dbgs() << "identity copy: " << *MI);
+  bool IdentityCopy = (SrcMO.getReg() == DstMO.getReg());
+  if (IdentityCopy || SrcMO.isUndef()) {
+    DEBUG(dbgs() << (IdentityCopy ? "identity copy: " : "undef copy:    ") << *MI);
     // No need to insert an identity copy instruction, but replace with a KILL
     // if liveness is changed.
     if (SrcMO.isUndef() || MI->getNumOperands() > 2) {
Index: test/CodeGen/SystemZ/lower-copy-undef-src.mir
===================================================================
--- /dev/null
+++ test/CodeGen/SystemZ/lower-copy-undef-src.mir
@@ -0,0 +1,14 @@
+# RUN: llc -mtriple=s390x-linux-gnu -mcpu=z13 -run-pass=postrapseudos -o - %s | FileCheck %s
+#
+# Test that a COPY with an undef source operand gets handled like an identity
+# copy rather than lowered into a target instruction with the undef flag
+# dropped.
+---
+# CHECK-LABEL: name: undef_copy
+# CHECK: %r13d = KILL undef %r0d, implicit killed %r12q, implicit-def %r12q
+name: undef_copy
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: %r12q
+    %r13d = COPY undef %r0d, implicit killed %r12q, implicit-def %r12q