Index: lib/Target/AArch64/AArch64RedundantCopyElimination.cpp
===================================================================
--- lib/Target/AArch64/AArch64RedundantCopyElimination.cpp
+++ lib/Target/AArch64/AArch64RedundantCopyElimination.cpp
@@ -119,15 +119,22 @@
   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;) {
     MachineInstr *MI = &*I;
     ++I;
-    if (MI->isCopy() && MI->getOperand(0).isReg() &&
-        MI->getOperand(1).isReg()) {
+    if (MI->isCopy()) {
 
       unsigned DefReg = MI->getOperand(0).getReg();
       unsigned SrcReg = MI->getOperand(1).getReg();
 
       if ((SrcReg == AArch64::XZR || SrcReg == AArch64::WZR) &&
           !MRI->isReserved(DefReg) &&
-          (TargetReg == DefReg || TRI->isSuperRegister(DefReg, TargetReg))) {
+          (TargetReg == DefReg || TRI->isSuperRegister(DefReg, TargetReg)) &&
+          // Don't remove COPYs with implicit defs that aren't known zero.  This
+          // can happen if e.g. this is a W reg COPY and we're depending on it
+          // zeroing out the high 32-bits of the corresponding X reg but we
+          // don't know the full contents of the X reg because the CBZ is on the
+          // W subreg.
+          all_of(MI->implicit_operands(), [TargetReg](MachineOperand &O) {
+            return !O.isDef() || O.getReg() == TargetReg;
+          })) {
         DEBUG(dbgs() << "Remove redundant Copy : ");
         DEBUG((MI)->print(dbgs()));
 
Index: test/CodeGen/AArch64/machine-copy-remove.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/machine-copy-remove.mir
@@ -0,0 +1,69 @@
+# RUN: llc -mtriple=aarch64--linux-gnu -run-pass=aarch64-copyelim %s -verify-machineinstrs -o - 2>&1 | FileCheck %s
+
+--- |
+  define void @f_WWimpdef() { ret void }
+  define void @f_WWimpdef2() { ret void }
+...
+---
+# Make sure we don't remove a COPY that is implicitly defining the top
+# 32-bits of an X reg if we don't know the top 32-bits are zero.
+# CHECK-LABEL: name: f_WWimpdef
+# CHECK: %w0 = COPY %xzr, implicit-def %x0
+name:            f_WWimpdef
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    successors: %bb.1, %bb.2
+    liveins: %w0, %x1
+
+    CBNZW %w0, %bb.2
+
+  bb.1:
+    successors: %bb.3
+
+    %w0 = COPY %xzr, implicit-def %x0
+    B %bb.3
+
+  bb.2:
+    successors: %bb.3
+    liveins: %x1
+
+    %x0 = LDRXui %x1, 0 :: (load 8)
+
+  bb.3:
+    liveins: %x0
+
+    RET_ReallyLR implicit %x0
+
+...
+# Make sure we do remove a COPY that is implicitly defining the top
+# 32-bits of an X reg if we know the top 32-bits are zero.
+# CHECK-LABEL: name: f_WWimpdef2
+# CHECK-NOT: %w0 = COPY %xzr, implicit-def %x0
+name:            f_WWimpdef2
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    successors: %bb.1, %bb.2
+    liveins: %x0, %x1
+
+    CBNZX %x0, %bb.2
+
+  bb.1:
+    successors: %bb.3
+
+    %w0 = COPY %xzr, implicit-def %x0
+    B %bb.3
+
+  bb.2:
+    successors: %bb.3
+    liveins: %x1
+
+    %x0 = LDRXui %x1, 0 :: (load 8)
+
+  bb.3:
+    liveins: %x0
+
+    RET_ReallyLR implicit %x0
+
+...