Index: llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
===================================================================
--- llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
+++ llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
@@ -201,6 +201,14 @@
   SmallSet<int, 16> ProtectedObjs;
   if (MFI.hasStackProtectorIndex()) {
     int StackProtectorFI = MFI.getStackProtectorIndex();
+
+    // We need to make sure we didn't pre-allocate the stack protector when
+    // doing this.
+    // If we already have a stack protector, this will re-assign it to a slot
+    // that is **not** covering the protected objects.
+    assert(!MFI.isObjectPreAllocated(StackProtectorFI) &&
+           "Stack protector pre-allocated in LocalStackSlotAllocation");
+
     StackObjSet LargeArrayObjs;
     StackObjSet SmallArrayObjs;
     StackObjSet AddrOfObjs;
Index: llvm/lib/CodeGen/PrologEpilogInserter.cpp
===================================================================
--- llvm/lib/CodeGen/PrologEpilogInserter.cpp
+++ llvm/lib/CodeGen/PrologEpilogInserter.cpp
@@ -933,8 +933,16 @@
     StackObjSet SmallArrayObjs;
     StackObjSet AddrOfObjs;
 
-    AdjustStackOffset(MFI, StackProtectorFI, StackGrowsDown, Offset, MaxAlign,
-                      Skew);
+    // If we need a stack protector, we need to make sure that
+    // LocalStackSlotPass didn't already allocate a slot for it.
+    // If we are told to use the LocalStackAllocationBlock, the stack protector
+    // is expected to be already pre-allocated.
+    if (!MFI.getUseLocalStackAllocationBlock())
+      AdjustStackOffset(MFI, StackProtectorFI, StackGrowsDown, Offset, MaxAlign,
+                        Skew);
+    else if (!MFI.isObjectPreAllocated(MFI.getStackProtectorIndex()))
+      llvm_unreachable(
+          "Stack protector not pre-allocated by LocalStackSlotPass.");
 
     // Assign large stack objects first.
     for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
@@ -968,6 +976,15 @@
       llvm_unreachable("Unexpected SSPLayoutKind.");
     }
 
+    // We expect **all** the protected stack objects to be pre-allocated by
+    // LocalStackSlotPass. If it turns out that PEI still has to allocate some
+    // of them, we may end up messing up the expected order of the objects.
+    if (MFI.getUseLocalStackAllocationBlock() &&
+        !(LargeArrayObjs.empty() && SmallArrayObjs.empty() &&
+          AddrOfObjs.empty()))
+      llvm_unreachable("Found protected stack objects not pre-allocated by "
+                       "LocalStackSlotPass.");
+
     AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
                           Offset, MaxAlign, Skew);
     AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
Index: test/CodeGen/AArch64/stack-guard-reassign.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/stack-guard-reassign.mir
@@ -0,0 +1,34 @@
+# RUN: llc -mtriple=arm64-apple-ios -start-before=localstackalloc -stop-after=prologepilog -o - %s | FileCheck %s
+
+--- |
+  @__stack_chk_guard = external global i8*
+  define i32 @main(i32, i8**) {
+    %StackGuardSlot = alloca i8*
+    unreachable
+  }
+...
+---
+name:            main
+tracksRegLiveness: true
+frameInfo:
+# CHECK: stackSize: 544
+  stackProtector:  '%stack.0.StackGuardSlot'
+stack:
+  - { id: 0, name: StackGuardSlot, size: 8, alignment: 8, stack-id: default }
+# Verify that the offset assigned to the stack protector is at the top of the
+# frame, covering the locals.
+# CHECK:       - { id: 0, name: StackGuardSlot, type: default, offset: -24, size: 8,
+# CHECK-NEXT:      alignment: 8, stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+# CHECK-NEXT:      local-offset: -8, debug-info-variable: '', debug-info-expression: '',
+# CHECK-NEXT:      debug-info-location: '' }
+  - { id: 1, size: 512, alignment: 1, stack-id: default }
+  - { id: 2, size: 4, alignment: 4, stack-id: default }
+body:             |
+  bb.0:
+    %25:gpr64common = LOAD_STACK_GUARD :: (dereferenceable invariant load 8 from @__stack_chk_guard)
+    STRXui killed %25, %stack.0.StackGuardSlot, 0 :: (volatile store 8 into %stack.0.StackGuardSlot)
+    %28:gpr64 = LDRXui %stack.0.StackGuardSlot, 0 :: (volatile load 8 from %stack.0.StackGuardSlot)
+    %29:gpr64common = LOAD_STACK_GUARD :: (dereferenceable invariant load 8 from @__stack_chk_guard)
+    RET_ReallyLR implicit undef $w0, implicit killed %28, implicit killed %29
+
+...