diff --git a/lib/IRGen/IRGenSIL.cpp b/lib/IRGen/IRGenSIL.cpp
index c1dabec314a35..e58e77be09dc0 100644
--- a/lib/IRGen/IRGenSIL.cpp
+++ b/lib/IRGen/IRGenSIL.cpp
@@ -746,9 +746,19 @@ class IRGenSILFunction :
                                    getEarliestInsertionPoint()->getIterator());
     // No debug location is how LLVM marks prologue instructions.
     ZeroInitBuilder.SetCurrentDebugLocation(nullptr);
-    ZeroInitBuilder.CreateMemSet(
-        AI, llvm::ConstantInt::get(IGM.Int8Ty, 0),
-        Size, llvm::MaybeAlign(AI->getAlign()));
+    // note that this memset is before lifetime.start which is Undefined
+    // Behaviour in LLVM
+    llvm::CallInst *Memset =
+        ZeroInitBuilder.CreateMemSet(AI, llvm::ConstantInt::get(IGM.Int8Ty, 0),
+                                     Size, llvm::MaybeAlign(AI->getAlign()));
+
+    // memtag-stack tagging needs to use this metadata to determine whether to
+    // delay tagging until after the memset. This can be removed if the memset
+    // is removed or moved after the lifetime.start
+    llvm::LLVMContext *Ctx = &Memset->getContext();
+    llvm::MDNode *Meta =
+        llvm::MDNode::get(*Ctx, llvm::MDString::get(*Ctx, "true"));
+    Memset->setMetadata("Swift.isSwiftLLDBpreinit", Meta);
   }
 
   /// Try to emit an inline assembly gadget which extends the lifetime of
diff --git a/test/DebugInfo/guard-let.swift b/test/DebugInfo/guard-let.swift
index 5b5cf832b5d4c..7547035fefaf3 100644
--- a/test/DebugInfo/guard-let.swift
+++ b/test/DebugInfo/guard-let.swift
@@ -23,7 +23,7 @@ public func f(_ i : Int?)
   // CHECK1: %[[alloca:.*]] = alloca %TSiSg
   // CHECK1: #dbg_declare(ptr %i.debug
   // CHECK1: call void @llvm.memset{{.*}}(ptr align {{(4|8)}} %[[alloca]],
-  // CHECK1-SAME:                         i8 0, i64 {{(5|9)}}, i1 false){{$}}
+  // CHECK1-SAME:                         i8 0, i64 {{(5|9)}}, i1 false), !Swift.isSwiftLLDBpreinit
   // CHECK1: #dbg_declare(ptr %val.debug, !{{.*}}, !DIExpression{{.*}},
   // CHECK1-SAME:              ![[DBG0:.*]])
   // CHECK1-LABEL: define {{.*}}@"$s4main1gyySSSgF"
diff --git a/test/DebugInfo/uninitialized.swift b/test/DebugInfo/uninitialized.swift
index 77df41a31e0ae..0196d02ede5ad 100644
--- a/test/DebugInfo/uninitialized.swift
+++ b/test/DebugInfo/uninitialized.swift
@@ -9,7 +9,7 @@ public func f() {
   // CHECK: %[[OBJ:.*]] = alloca ptr, align
   // CHECK: #dbg_declare(ptr %[[OBJ]],
   // CHECK: void @llvm.memset.{{.*}}(ptr align {{(4|8)}} %[[OBJ]], i8 0,
-  // CHECK-SAME:                    ){{$}}
+  // CHECK-SAME:                    ), !Swift.isSwiftLLDBpreinit
   // OPT-NOT: @llvm.memset
   // OPT: ret
 }
@@ -21,7 +21,7 @@ public func g() {
   // CHECK: %[[DICT:.*]] = alloca
   // CHECK: #dbg_declare(ptr %[[DICT]],
   // CHECK: void @llvm.memset.{{.*}}(ptr align {{(4|8)}} %[[DICT]], i8 0,
-  // CHECK-SAME:                    ){{$}}
+  // CHECK-SAME:                    ), !Swift.isSwiftLLDBpreinit
   // OPT-NOT: @llvm.memset
   // OPT: ret
 }