diff --git a/llvm/lib/Target/AArch64/AArch64CallingConvention.td b/llvm/lib/Target/AArch64/AArch64CallingConvention.td
--- a/llvm/lib/Target/AArch64/AArch64CallingConvention.td
+++ b/llvm/lib/Target/AArch64/AArch64CallingConvention.td
@@ -444,6 +444,15 @@
 def CSR_Darwin_AArch64_AAVPCS : CalleeSavedRegs<(add LR, FP, X19, X20, X21,
                                                  X22, X23, X24, X25, X26, X27,
                                                  X28, (sequence "Q%u", 8, 23))>;
+
+// For Windows calling convention on a non-windows OS, where X18 is treated
+// as reserved, back up X18 when entering non-windows code (marked with the
+// Windows calling convention) and restore when returning regardless of
+// whether the individual function uses it - it might call other functions
+// that clobber it.
+def CSR_Darwin_AArch64_AAPCS_Win64
+    : CalleeSavedRegs<(add CSR_Darwin_AArch64_AAPCS, X18)>;
+
 def CSR_Darwin_AArch64_AAPCS_ThisReturn
     : CalleeSavedRegs<(add CSR_Darwin_AArch64_AAPCS, X0)>;
 
diff --git a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
--- a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
@@ -2522,8 +2522,8 @@
   (void)CC;
   // MachO's compact unwind format relies on all registers being stored in
   // pairs.
-  assert((!produceCompactUnwindFrame(MF) ||
-          CC == CallingConv::PreserveMost || CC == CallingConv::CXX_FAST_TLS ||
+  assert((!produceCompactUnwindFrame(MF) || CC == CallingConv::PreserveMost ||
+          CC == CallingConv::CXX_FAST_TLS || CC == CallingConv::Win64 ||
           (Count & 1) == 0) &&
          "Odd number of callee-saved regs to spill!");
   int ByteOffset = AFI->getCalleeSavedStackSize();
@@ -2608,8 +2608,8 @@
 
     // MachO's compact unwind format relies on all registers being stored in
     // adjacent register pairs.
-    assert((!produceCompactUnwindFrame(MF) ||
-            CC == CallingConv::PreserveMost || CC == CallingConv::CXX_FAST_TLS ||
+    assert((!produceCompactUnwindFrame(MF) || CC == CallingConv::PreserveMost ||
+            CC == CallingConv::CXX_FAST_TLS || CC == CallingConv::Win64 ||
             (RPI.isPaired() &&
              ((RPI.Reg1 == AArch64::LR && RPI.Reg2 == AArch64::FP) ||
               RPI.Reg1 + 1 == RPI.Reg2))) &&
diff --git a/llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp b/llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp
--- a/llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp
@@ -135,6 +135,8 @@
     return CSR_Darwin_AArch64_AAPCS_SwiftTail_SaveList;
   if (MF->getFunction().getCallingConv() == CallingConv::PreserveMost)
     return CSR_Darwin_AArch64_RT_MostRegs_SaveList;
+  if (MF->getFunction().getCallingConv() == CallingConv::Win64)
+    return CSR_Darwin_AArch64_AAPCS_Win64_SaveList;
   return CSR_Darwin_AArch64_AAPCS_SaveList;
 }
 
diff --git a/llvm/test/CodeGen/AArch64/win64cc-darwin-backup-x18.ll b/llvm/test/CodeGen/AArch64/win64cc-darwin-backup-x18.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/win64cc-darwin-backup-x18.ll
@@ -0,0 +1,31 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+
+;; Testing that x18 is backed up and restored, and that x29 (if used) still
+;; points to the x29,x30 pair on the stack.
+
+; RUN: llc < %s -mtriple=arm64-apple-darwin | FileCheck %s
+; RUN: llc < %s -mtriple=arm64-apple-darwin -mattr=+reserve-x18 | FileCheck %s
+
+declare dso_local void @other()
+
+define win64cc void @func(i32 noundef %0) #0 {
+; CHECK-LABEL: func:
+; CHECK:         ; %bb.0: ; %entry
+; CHECK-NEXT:    str x18, [sp, #-32]! ; 8-byte Folded Spill
+; CHECK-NEXT:    stp x29, x30, [sp, #16] ; 16-byte Folded Spill
+; CHECK:         ldp x29, x30, [sp, #16] ; 16-byte Folded Reload
+; CHECK-NEXT:    ldr x18, [sp], #32 ; 8-byte Folded Reload
+; CHECK-NEXT:    ret
+entry:
+  %al = alloca i32, align 4
+  store i32 %0, ptr %al, align 4
+  tail call void @other()
+  ret void
+}
+
+define i32 @main() {
+  call win64cc void @func(i32 noundef 17)
+  ret i32 0
+}
+
+attributes #0 = { nounwind }