diff --git a/llvm/lib/Target/AArch64/AArch64StackTagging.cpp b/llvm/lib/Target/AArch64/AArch64StackTagging.cpp
index d87bb522c99e8..eb44836718da2 100644
--- a/llvm/lib/Target/AArch64/AArch64StackTagging.cpp
+++ b/llvm/lib/Target/AArch64/AArch64StackTagging.cpp
@@ -560,8 +560,19 @@ bool AArch64StackTagging::runOnFunction(Function &Fn) {
     AllocaInst *AI = Info.AI;
     unsigned int Tag = NextTag;
     NextTag = (NextTag + 1) % 16;
+    Instruction *PreTagInstr = AI->getNextNode();
+    // LLDB has Swift memset alloca's to zero (in order to prevent reading
+    // garbage values from uninited vars) - defer tagging until after this.
+    // Note: ensuring that the memset uses the untagged pointer is not enough
+    // on its own, as a later pass will replace it with the tagged pointer;
+    // this does not happen if the tagp occurs after the memset.
+    Instruction *LLDBMemset = nullptr;
+    if (PreTagInstr && PreTagInstr->hasMetadata("Swift.isSwiftLLDBpreinit")) {
+      LLDBMemset = PreTagInstr;
+      PreTagInstr = PreTagInstr->getNextNode();
+    }
+    IRBuilder<> IRB(PreTagInstr);
     // Replace alloca with tagp(alloca).
-    IRBuilder<> IRB(Info.AI->getNextNode());
     Instruction *TagPCall =
         IRB.CreateIntrinsic(Intrinsic::aarch64_tagp, {Info.AI->getType()},
                             {Constant::getNullValue(Info.AI->getType()), Base,
@@ -570,7 +581,12 @@ bool AArch64StackTagging::runOnFunction(Function &Fn) {
       TagPCall->setName(Info.AI->getName() + ".tag");
     // Does not replace metadata, so we don't have to handle DbgVariableRecords.
     Info.AI->replaceUsesWithIf(TagPCall, [&](const Use &U) {
-      return !isa<LifetimeIntrinsic>(U.getUser());
+      if (isa<LifetimeIntrinsic>(U.getUser()))
+        return false;
+      // The Swift LLDB pre-init memset must use the untagged pointer
+      if (U.getUser() == LLDBMemset)
+        return false;
+      return true;
     });
     TagPCall->setOperand(0, Info.AI);
 
diff --git a/llvm/test/CodeGen/AArch64/stack-tagging-swift-preinit.ll b/llvm/test/CodeGen/AArch64/stack-tagging-swift-preinit.ll
new file mode 100644
index 0000000000000..e99b70736bf6f
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/stack-tagging-swift-preinit.ll
@@ -0,0 +1,71 @@
+; RUN: opt < %s -aarch64-stack-tagging -S -o - | FileCheck %s
+
+; Test that Swift preinit memsets use untagged pointers and occur before tagging.
+; In Swift codegen, LLDB uses preinit memsets to zero-initialize memory before 
+; actual initialization, and these memsets must use the untagged (base) pointer
+; to avoid a tag fault.
+
+target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+target triple = "arm64-apple-darwin"
+
+%TSi = type <{ i64 }>
+%TSd = type <{ double }>
+
+; CHECK-LABEL: define swiftcc void @test_swift_preinit_memset
+; Function Attrs: sanitize_memtag
+define swiftcc void @test_swift_preinit_memset() #0 {
+entry:
+  %x = alloca %TSi, align 8
+  call void @llvm.memset.p0.i64(ptr align 8 %x, i8 0, i64 8, i1 false), !Swift.isSwiftLLDBpreinit !17
+
+  ; CHECK: %x = alloca { %TSi, [8 x i8] }, align 16
+  ; Swift preinit memsets should use the untagged pointer and happen before any tagging intrinsics
+  ; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr align 8 %x, i8 0, i64 8, i1 false){{.*}}!Swift.isSwiftLLDBpreinit
+  ; CHECK-NEXT: [[XTAGGED:%[^ ]+]] = call ptr @llvm.aarch64.tagp.{{.*}}(ptr %x
+
+  %y = alloca %TSd, align 8
+  call void @llvm.memset.p0.i64(ptr align 8 %y, i8 0, i64 8, i1 false), !Swift.isSwiftLLDBpreinit !17
+  ; CHECK: %y = alloca { %TSd, [8 x i8] }, align 16
+  ; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr align 8 %y, i8 0, i64 8, i1 false){{.*}}!Swift.isSwiftLLDBpreinit
+  ; CHECK-NEXT: [[YTAGGED:%[^ ]+]] = call ptr @llvm.aarch64.tagp.{{.*}}(ptr %y
+
+  call void @llvm.lifetime.start.p0(i64 8, ptr %x)
+  ; Lifetime intrinsics use the untagged pointers
+  ; CHECK: call void @llvm.lifetime.start.p0(ptr %x)
+
+  %x._value = getelementptr inbounds nuw %TSi, ptr %x, i32 0, i32 0
+  ; CHECK: %x._value = getelementptr inbounds nuw %TSi, ptr [[XTAGGED]], i32 0, i32 0
+  store i64 0, ptr %x._value, align 8
+
+  call void @llvm.lifetime.start.p0(i64 8, ptr %y)
+  ; CHECK: call void @llvm.lifetime.start.p0(ptr %y)
+
+  %y._value = getelementptr inbounds nuw %TSd, ptr %y, i32 0, i32 0
+  ; CHECK: %y._value = getelementptr inbounds nuw %TSd, ptr [[YTAGGED]], i32 0, i32 0
+  store double 42.000000e+00, ptr %y._value, align 8
+
+  call void asm sideeffect "nop", ""()
+
+  call void @llvm.lifetime.end.p0(i64 8, ptr %y)
+  call void @llvm.lifetime.end.p0(i64 8, ptr %x)
+
+  ; CHECK: call void @llvm.lifetime.end.p0(ptr %y)
+  ; CHECK: call void @llvm.lifetime.end.p0(ptr %x)
+
+  ret void
+}
+
+; Function Attrs: nocallback nofree nounwind willreturn memory(argmem: write)
+declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1 immarg) #1
+
+; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
+declare void @llvm.lifetime.start.p0(i64 immarg, ptr nocapture) #2
+
+; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
+declare void @llvm.lifetime.end.p0(i64 immarg, ptr nocapture) #2
+
+attributes #0 = { sanitize_memtag }
+attributes #1 = { nocallback nofree nounwind willreturn memory(argmem: write) }
+attributes #2 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
+
+!17 = !{}