Index: llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp
===================================================================
--- llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp
+++ llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp
@@ -376,8 +376,6 @@
                   EndIdx = MI.getNumOperands();
          Idx < EndIdx; ++Idx) {
       MachineOperand &MO = MI.getOperand(Idx);
-      // Leave `undef` operands as is, StackMaps will rewrite them
-      // into a constant.
       if (!MO.isReg() || MO.isImplicit() || MO.isUndef())
         continue;
       Register Reg = MO.getReg();
Index: llvm/lib/CodeGen/StackMaps.cpp
===================================================================
--- llvm/lib/CodeGen/StackMaps.cpp
+++ llvm/lib/CodeGen/StackMaps.cpp
@@ -234,12 +234,6 @@
     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());
Index: llvm/test/CodeGen/X86/stackmap-undef-operand-anyregcc.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/stackmap-undef-operand-anyregcc.mir
@@ -0,0 +1,61 @@
+# RUN: llc -mtriple=x86_64-apple-darwin -start-after=virtregrewriter -o - %s | FileCheck %s
+
+# Check there's no assertion for anyregcc with an undef operand to a stackmap.
+
+# CHECK: __LLVM_StackMaps:
+# CHECK-NEXT: .byte	3
+# CHECK-NEXT: .byte	0
+# CHECK-NEXT: .short	0
+# CHECK-NEXT: .long	1
+# CHECK-NEXT: .long	0
+# CHECK-NEXT: .long	1
+# CHECK-NEXT: .quad	_undef_anyregcc_patchpoint
+# CHECK-NEXT: .quad	8
+# CHECK-NEXT: .quad	1
+# CHECK-NEXT: .quad	12
+# CHECK-NEXT: .long	Ltmp0-_undef_anyregcc_patchpoint
+# CHECK-NEXT: .short	0
+# CHECK-NEXT: .short	2
+# CHECK-NEXT: .byte	1
+# CHECK-NEXT: .byte	0
+# CHECK-NEXT: .short	8
+# CHECK-NEXT: .short	0
+# CHECK-NEXT: .short	0
+# CHECK-NEXT: .long	0
+# CHECK-NEXT: .byte	1
+# CHECK-NEXT: .byte	0
+# CHECK-NEXT: .short	8
+# CHECK-NEXT: .short	0
+# CHECK-NEXT: .short	0
+# CHECK-NEXT: .long	0
+# CHECK-NEXT: .p2align	3
+# CHECK-NEXT: .short	0
+# CHECK-NEXT: .short	2
+# CHECK-NEXT: .short	0
+# CHECK-NEXT: .byte	0
+# CHECK-NEXT: .byte	8
+# CHECK-NEXT: .short	7
+# CHECK-NEXT: .byte	0
+# CHECK-NEXT: .byte	8
+# CHECK-NEXT: .p2align	3
+---
+name:  undef_anyregcc_patchpoint
+tracksRegLiveness: true
+frameInfo:
+  hasPatchPoint:   true
+  hasCalls:        true
+fixedStack:
+  - { id: 0, type: default, offset: 72, size: 8, alignment: 8, stack-id: default,
+      isImmutable: true, isAliased: false, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body:             |
+  bb.0:
+    liveins: $rcx, $rdi, $rdx, $rsi, $r8, $r9
+
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    dead renamable $rax = MOV64rm %fixed-stack.0, 1, $noreg, 0, $noreg :: (load (s64) from %fixed-stack.0)
+    renamable $rax = PATCHPOINT 12, 15, 0, 1, 13, undef renamable $rax, csr_64_allregs, implicit-def dead early-clobber $r11, implicit-def $rsp, implicit-def $ssp
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    RET 0, $rax
+
+...
Index: llvm/test/CodeGen/X86/statepoint-fixup-undef.mir
===================================================================
--- llvm/test/CodeGen/X86/statepoint-fixup-undef.mir
+++ llvm/test/CodeGen/X86/statepoint-fixup-undef.mir
@@ -124,12 +124,11 @@
   ; STACKMAP-NEXT:	.byte	0
   ; STACKMAP-NEXT:	.short	0
   ; STACKMAP-NEXT:	.long	1
-  ; STACKMAP-NEXT:	.long	1
+  ; STACKMAP-NEXT:	.long	0
   ; STACKMAP-NEXT:	.long	1
   ; STACKMAP-NEXT:	.quad	test_undef
   ; STACKMAP-NEXT:	.quad	88
   ; STACKMAP-NEXT:	.quad	1
-  ; STACKMAP-NEXT:	.quad	4278124286
   ; STACKMAP-NEXT:	.quad	2
   ; STACKMAP-NEXT:	.long	.Ltmp0-test_undef
   ; STACKMAP-NEXT:	.short	0
@@ -182,10 +181,10 @@
   ; STACKMAP-NEXT:	.short	3
   ; STACKMAP-NEXT:	.short	0
   ; STACKMAP-NEXT:	.long	0
-  ;      This is entry we're looking for, reference to constant pool entry 0xFEFEFEFE
-  ; STACKMAP-NEXT:	.byte	5
+  ;      This is entry we're looking for
+  ; STACKMAP-NEXT:	.byte	1
   ; STACKMAP-NEXT:	.byte	0
-  ; STACKMAP-NEXT:	.short	8
+  ; STACKMAP-NEXT:	.short	4
   ; STACKMAP-NEXT:	.short	0
   ; STACKMAP-NEXT:	.short	0
   ; STACKMAP-NEXT:	.long	0