Index: lib/CodeGen/MachineCopyPropagation.cpp
===================================================================
--- lib/CodeGen/MachineCopyPropagation.cpp
+++ lib/CodeGen/MachineCopyPropagation.cpp
@@ -280,14 +280,6 @@
           MaybeDeadCopies.remove(CI->second);
         }
       }
-      // Treat undef use like defs for copy propagation but not for
-      // dead copy. We would need to do a liveness check to be sure the copy
-      // is dead for undef uses.
-      // The backends are allowed to do whatever they want with undef value
-      // and we cannot be sure this register will not be rewritten to break
-      // some false dependencies for the hardware for instance.
-      if (MO.isUndef())
-        Defs.push_back(Reg);
     }
 
     // The instruction has a register mask operand which means that it clobbers
Index: test/CodeGen/ARM/machine-copyprop.mir
===================================================================
--- /dev/null
+++ test/CodeGen/ARM/machine-copyprop.mir
@@ -0,0 +1,22 @@
+# RUN: llc -o - %s -mtriple=armv7s-- -run-pass=machine-cp | FileCheck %s
+---
+# Test that machine copy prop recognizes the implicit-def operands on a COPY
+# as clobbering the register.
+# CHECK-LABEL: name: func
+# CHECK: %d2 = VMOVv2i32 2, 14, _
+# CHECK: %s5 = COPY %s0, implicit %q1, implicit-def %q1
+# CHECK: VST1q32 %r0, 0, %q1, 14, _
+# The following two COPYs must not be removed
+# CHECK: %s4 = COPY %s20, implicit-def %q1
+# CHECK: %s5 = COPY %s0, implicit killed %d0, implicit %q1, implicit-def %q1
+# CHECK: VST1q32 %r2, 0, %q1, 14, _
+name: func
+body: |
+  bb.0:
+    %d2 = VMOVv2i32 2, 14, _
+    %s5 = COPY %s0, implicit %q1, implicit-def %q1
+    VST1q32 %r0, 0, %q1, 14, _
+    %s4 = COPY %s20, implicit-def %q1
+    %s5 = COPY %s0, implicit killed %d0, implicit %q1, implicit-def %q1
+    VST1q32 %r2, 0, %q1, 14, _
+...
Index: test/CodeGen/X86/copy-propagation.ll
===================================================================
--- test/CodeGen/X86/copy-propagation.ll
+++ test/CodeGen/X86/copy-propagation.ll
@@ -1,37 +1,20 @@
 ; RUN: llc %s -mattr=+avx -o - | FileCheck %s
-; PR21743.
+; Originally from http://llvm.org/PR21743.
 
 target triple = "x86_64-pc-win32-elf"
 
-; Check that copy propagation conservatively assumes that undef register
-; can be rewritten by the backend to break false dependencies for the
-; hardware.
-; In this function we are in this situation:
-; reg1 = copy reg2
-; = inst reg2<undef>
-; reg2 = copy reg1
-; Copy propagation used to remove the last copy.
-; This is incorrect because the undef flag on reg2 in inst, allows next
-; passes to put whatever trashed value in reg2 that may help.
-; In practice we end up with this code:
-; reg1 = copy reg2
-; reg2 = 0
-; = inst reg2<undef>
-; reg2 = copy reg1
-; Therefore, removing the last copy is wrong.
+; Copy propagation may remove COPYs if the result is only used by undef
+; operands.
 ;
 ; CHECK-LABEL: foo:
 ; CHECK: movl	$339752784, %e[[INDIRECT_CALL1:[a-z]+]]
 ; CHECK: callq *%r[[INDIRECT_CALL1]]
 ; Copy the result in a temporary.
-; Note: Technically the regalloc could have been smarter and this move not required,
-; which would have hidden the bug.
+; Note: Technically the regalloc could have been smarter and this move not
+; required, which would have hidden the bug.
 ; CHECK: vmovapd	%xmm0, [[TMP:%xmm[0-9]+]]
-; Crush xmm0.
-; CHECK-NEXT: vxorps %xmm0, %xmm0, %xmm0
 ; CHECK: movl	$339772768, %e[[INDIRECT_CALL2:[a-z]+]]
 ; Set TMP in the first argument of the second call.
-; CHECK-NEXT: vmovapd	[[TMP]], %xmm0
 ; CHECK: callq *%r[[INDIRECT_CALL2]]
 ; CHECK: retq
 define double @foo(i64 %arg) {