diff --git a/lib/CodeGen/MachineCopyPropagation.cpp b/lib/CodeGen/MachineCopyPropagation.cpp
index 96111225db5..43c80b7c21b 100644
--- a/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/lib/CodeGen/MachineCopyPropagation.cpp
@@ -252,7 +252,11 @@ bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
         report_fatal_error("MachineCopyPropagation should be run after"
                            " register allocation!");
 
-      if (MO.isDef()) {
+      // Treat undef use like defs.
+      // 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.isDef() || MO.isUndef()) {
         Defs.push_back(Reg);
         continue;
       }
diff --git a/test/CodeGen/X86/copy-propagation.ll b/test/CodeGen/X86/copy-propagation.ll
new file mode 100644
index 00000000000..19421a06fa8
--- /dev/null
+++ b/test/CodeGen/X86/copy-propagation.ll
@@ -0,0 +1,44 @@
+; RUN: llc %s -mattr=+avx -o - | FileCheck %s
+; 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.
+;
+; 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.
+; CHECK-NEXT: 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) {
+top:
+  %tmp = call double inttoptr (i64 339752784 to double (double, double)*)(double 1.000000e+00, double 0.000000e+00)
+  %tmp1 = sitofp i64 %arg to double
+  call void inttoptr (i64 339772768 to void (double, double)*)(double %tmp, double %tmp1)
+  %tmp3 = fadd double %tmp1, %tmp
+  ret double %tmp3
+}