diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5243,7 +5243,7 @@
     assert(VT.isInteger() && VT == Operand.getValueType() &&
            "Invalid ABS!");
     if (OpOpcode == ISD::UNDEF)
-      return getUNDEF(VT);
+      return getConstant(0, DL, VT);
     break;
   case ISD::BSWAP:
     assert(VT.isInteger() && VT == Operand.getValueType() &&
diff --git a/llvm/test/CodeGen/X86/pr55271.ll b/llvm/test/CodeGen/X86/pr55271.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/X86/pr55271.ll
@@ -0,0 +1,15 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+; abs(undef) should fold to 0 not undef.
+
+declare i32 @llvm.abs.i32(i32, i1 immarg) #0
+
+define i32 @abs(i32 %0) {
+; CHECK-LABEL: abs:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    retq
+  %2 = call i32 @llvm.abs.i32(i32 undef, i1 false)
+  ret i32 %2
+}