Index: llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
===================================================================
--- llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
+++ llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
@@ -199,19 +199,26 @@
   // Make sure that the stack protector comes before the local variables on the
   // stack.
   SmallSet<int, 16> ProtectedObjs;
-  if (MFI.getStackProtectorIndex() >= 0) {
+  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;
 
-    AdjustStackOffset(MFI, MFI.getStackProtectorIndex(), Offset,
-                      StackGrowsDown, MaxAlign);
+    AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, MaxAlign);
 
     // Assign large stack objects first.
     for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
       if (MFI.isDeadObjectIndex(i))
         continue;
-      if (MFI.getStackProtectorIndex() == (int)i)
+      if (StackProtectorFI == (int)i)
         continue;
 
       switch (MFI.getObjectSSPLayout(i)) {
Index: llvm/lib/CodeGen/PrologEpilogInserter.cpp
===================================================================
--- llvm/lib/CodeGen/PrologEpilogInserter.cpp
+++ llvm/lib/CodeGen/PrologEpilogInserter.cpp
@@ -927,18 +927,23 @@
   // Make sure that the stack protector comes before the local variables on the
   // stack.
   SmallSet<int, 16> ProtectedObjs;
-  if (MFI.getStackProtectorIndex() >= 0) {
+  // If we have a stack protector index, we need to make sure that
+  // LocalStackSlotPass didn't already allocate a slot for it. If it did, we
+  // need to use that one.
+  if (MFI.hasStackProtectorIndex() &&
+      !(MFI.getUseLocalStackAllocationBlock() &&
+        MFI.isObjectPreAllocated(MFI.getStackProtectorIndex()))) {
+    int StackProtectorFI = MFI.getStackProtectorIndex();
     StackObjSet LargeArrayObjs;
     StackObjSet SmallArrayObjs;
     StackObjSet AddrOfObjs;
 
-    AdjustStackOffset(MFI, MFI.getStackProtectorIndex(), StackGrowsDown,
-                      Offset, MaxAlign, Skew);
+    AdjustStackOffset(MFI, StackProtectorFI, StackGrowsDown, Offset, MaxAlign,
+                      Skew);
 
     // Assign large stack objects first.
     for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
-      if (MFI.isObjectPreAllocated(i) &&
-          MFI.getUseLocalStackAllocationBlock())
+      if (MFI.isObjectPreAllocated(i) && MFI.getUseLocalStackAllocationBlock())
         continue;
       if (i >= MinCSFrameIndex && i <= MaxCSFrameIndex)
         continue;
@@ -946,8 +951,7 @@
         continue;
       if (MFI.isDeadObjectIndex(i))
         continue;
-      if (MFI.getStackProtectorIndex() == (int)i ||
-          EHRegNodeFrameIndex == (int)i)
+      if (StackProtectorFI == (int)i || EHRegNodeFrameIndex == (int)i)
         continue;
       if (MFI.getStackID(i) !=
           TargetStackID::Default) // Only allocate objects on the default stack.
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: 0 }
+# 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: 0, 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: 0 }
+  - { id: 2, size: 4, alignment: 4, stack-id: 0 }
+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
+
+...