diff --git a/llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp b/llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp
--- a/llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp
+++ b/llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp
@@ -380,7 +380,9 @@
                   EndIdx = MI.getNumOperands();
          Idx < EndIdx; ++Idx) {
       MachineOperand &MO = MI.getOperand(Idx);
-      if (!MO.isReg() || MO.isImplicit())
+      // Leave `undef` operands as is, StackMaps will rewrite them
+      // into a constant.
+      if (!MO.isReg() || MO.isImplicit() || MO.isUndef())
         continue;
       Register Reg = MO.getReg();
       assert(Reg.isPhysical() && "Only physical regs are expected");
diff --git a/llvm/lib/CodeGen/StackMaps.cpp b/llvm/lib/CodeGen/StackMaps.cpp
--- a/llvm/lib/CodeGen/StackMaps.cpp
+++ b/llvm/lib/CodeGen/StackMaps.cpp
@@ -234,6 +234,12 @@
     if (MOI->isImplicit())
       return ++MOI;
 
+    if (MOI->isUndef()) {
+      // Record `undef` register as constant. Use same value as ISel uses.
+      Locs.emplace_back(Location::Constant, sizeof(int64_t), 0, 0xFEFEFEFE);
+      return ++MOI;
+    }
+
     assert(Register::isPhysicalRegister(MOI->getReg()) &&
            "Virtreg operands should have been rewritten before now.");
     const TargetRegisterClass *RC = TRI->getMinimalPhysRegClass(MOI->getReg());