diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -1053,9 +1053,8 @@
     switch (static_cast<Instruction::BinaryOps>(Opcode)) {
     case Instruction::Xor:
       if (isa<UndefValue>(C1) && isa<UndefValue>(C2))
-        // Handle undef ^ undef -> 0 special case. This is a common
-        // idiom (misuse).
-        return Constant::getNullValue(C1->getType());
+        // undef ^ undef -> undef
+        return C1;
       LLVM_FALLTHROUGH;
     case Instruction::Add:
     case Instruction::Sub:
diff --git a/llvm/test/Analysis/ConstantFolding/vscale.ll b/llvm/test/Analysis/ConstantFolding/vscale.ll
--- a/llvm/test/Analysis/ConstantFolding/vscale.ll
+++ b/llvm/test/Analysis/ConstantFolding/vscale.ll
@@ -159,7 +159,7 @@
 
 define <vscale x 4 x i32> @xor() {
 ; CHECK-LABEL: @xor(
-; CHECK-NEXT:    ret <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <vscale x 4 x i32> undef
 ;
   %r = xor <vscale x 4 x i32> undef, undef
   ret <vscale x 4 x i32> %r
diff --git a/llvm/test/Transforms/InstCombine/xor-undef.ll b/llvm/test/Transforms/InstCombine/xor-undef.ll
--- a/llvm/test/Transforms/InstCombine/xor-undef.ll
+++ b/llvm/test/Transforms/InstCombine/xor-undef.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -instcombine -S | grep zeroinitializer
+; RUN: opt < %s -instcombine -S | grep "ret <2 x i64> undef"
 
 define <2 x i64> @f() {
 	%tmp = xor <2 x i64> undef, undef