Index: llvm/include/llvm/CodeGen/StackMaps.h
===================================================================
--- llvm/include/llvm/CodeGen/StackMaps.h
+++ llvm/include/llvm/CodeGen/StackMaps.h
@@ -267,6 +267,14 @@
     Location() = default;
     Location(LocationType Type, unsigned Size, unsigned Reg, int64_t Offset)
         : Type(Type), Size(Size), Reg(Reg), Offset(Offset) {}
+
+    bool isUndefReg() const {
+      return Type == Constant && Reg == 0 && Offset == 0xFEFEFEFE;
+    }
+
+    static Location getUndefReg() {
+      return Location(Constant, sizeof(int64_t), 0, 0xFEFEFEFE);
+    }
   };
 
   struct LiveOutReg {
Index: llvm/lib/CodeGen/StackMaps.cpp
===================================================================
--- llvm/lib/CodeGen/StackMaps.cpp
+++ llvm/lib/CodeGen/StackMaps.cpp
@@ -236,7 +236,7 @@
 
     if (MOI->isUndef()) {
       // Record `undef` register as constant. Use same value as ISel uses.
-      Locs.emplace_back(Location::Constant, sizeof(int64_t), 0, 0xFEFEFEFE);
+      Locs.emplace_back(Location::getUndefReg());
       return ++MOI;
     }
 
@@ -481,7 +481,8 @@
   for (auto &Loc : Locations) {
     // Constants are encoded as sign-extended integers.
     // -1 is directly encoded as .long 0xFFFFFFFF with no constant pool.
-    if (Loc.Type == Location::Constant && !isInt<32>(Loc.Offset)) {
+    if (Loc.Type == Location::Constant && !isInt<32>(Loc.Offset) &&
+        !Loc.isUndefReg()) {
       Loc.Type = Location::ConstantIndex;
       // ConstPool is intentionally a MapVector of 'uint64_t's (as
       // opposed to 'int64_t's).  We should never be in a situation
@@ -546,7 +547,8 @@
   if (opers.isAnyReg()) {
     unsigned NArgs = opers.getNumCallArgs();
     for (unsigned i = 0, e = (opers.hasDef() ? NArgs + 1 : NArgs); i != e; ++i)
-      assert(Locations[i].Type == Location::Register &&
+      assert((Locations[i].Type == Location::Register ||
+              Locations[i].isUndefReg()) &&
              "anyreg arg must be in reg.");
   }
 #endif
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	4
+# CHECK-NEXT:	.byte	0
+# CHECK-NEXT:	.short	8
+# CHECK-NEXT:	.short	0
+# CHECK-NEXT:	.short	0
+# CHECK-NEXT:	.long	4278124286
+# 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:	.byte	0
   ; STACKMAP:	.short	0
   ; STACKMAP:	.long	1
-  ; STACKMAP:	.long	1
+  ; STACKMAP:	.long	0
   ; STACKMAP:	.long	1
   ; STACKMAP:	.quad	test_undef
   ; STACKMAP:	.quad	88
   ; STACKMAP:	.quad	1
-  ; STACKMAP:	.quad	4278124286
   ; STACKMAP:	.quad	2
   ; STACKMAP:	.long	.Ltmp0-test_undef
   ; STACKMAP:	.short	0
@@ -182,13 +181,13 @@
   ; STACKMAP:	.short	3
   ; STACKMAP:	.short	0
   ; STACKMAP:	.long	0
-  ;      This is entry we're looking for, reference to constant pool entry 0xFEFEFEFE
-  ; STACKMAP:	.byte	5
+  ;      This is entry we're looking for, reference to constant entry 0xFEFEFEFE
+  ; STACKMAP:	.byte	4
   ; STACKMAP:	.byte	0
   ; STACKMAP:	.short	8
   ; STACKMAP:	.short	0
   ; STACKMAP:	.short	0
-  ; STACKMAP:	.long	0
+  ; STACKMAP:	.long	4278124286
   ; STACKMAP:	.byte	4
   ; STACKMAP:	.byte	0
   ; STACKMAP:	.short	8