diff --git a/lib/IRGen/SwiftTargetInfo.cpp b/lib/IRGen/SwiftTargetInfo.cpp
index 47ecc2288cd21..ea5464ecf4e9c 100644
--- a/lib/IRGen/SwiftTargetInfo.cpp
+++ b/lib/IRGen/SwiftTargetInfo.cpp
@@ -54,6 +54,10 @@ static void configureARM64(IRGenModule &IGM, const llvm::Triple &triple,
 
   // arm64 requires ISA-masking.
   target.ObjCUseISAMask = true;
+
+  // arm64 tops out at 56 effective bits of address space and reserves the high
+  // half for the kernel.
+  target.SwiftRetainIgnoresNegativeValues = true;
 }
 
 /// Configures target-specific information for x86-64 platforms.
@@ -75,6 +79,10 @@ static void configureX86_64(IRGenModule &IGM, const llvm::Triple &triple,
 
   // x86-64 requires ISA-masking.
   target.ObjCUseISAMask = true;
+  
+  // x86-64 only has 48 effective bits of address space and reserves the high
+  // half for the kernel.
+  target.SwiftRetainIgnoresNegativeValues = true;
 }
 
 /// Configures target-specific information for 32-bit x86 platforms.
diff --git a/lib/IRGen/SwiftTargetInfo.h b/lib/IRGen/SwiftTargetInfo.h
index 755047371822a..f9de7ef4c904c 100644
--- a/lib/IRGen/SwiftTargetInfo.h
+++ b/lib/IRGen/SwiftTargetInfo.h
@@ -95,6 +95,10 @@ class SwiftTargetInfo {
   /// The value stored in a Builtin.once predicate to indicate that an
   /// initialization has already happened, if known.
   Optional<int64_t> OnceDonePredicateValue = None;
+  
+  /// True if `swift_retain` and `swift_release` are no-ops when passed
+  /// "negative" pointer values.
+  bool SwiftRetainIgnoresNegativeValues = false;
 };
 
 }
diff --git a/stdlib/public/runtime/HeapObject.cpp b/stdlib/public/runtime/HeapObject.cpp
index 402c9d4a1f6c9..3797b6c74c3b4 100644
--- a/stdlib/public/runtime/HeapObject.cpp
+++ b/stdlib/public/runtime/HeapObject.cpp
@@ -43,6 +43,18 @@
 
 using namespace swift;
 
+/// Returns true if the pointer passed to a native retain or release is valid.
+/// If false, the operation should immediately return.
+static inline bool isValidPointerForNativeRetain(const void *p) {
+#if defined(__x86_64__) || defined(__arm64__)
+  // On these platforms, the upper half of address space is reserved for the
+  // kernel, so we can assume that pointer values in this range are invalid.
+  return (intptr_t)p > 0;
+#else
+  return p != nullptr;
+#endif
+}
+
 HeapObject *swift::swift_allocObject(HeapMetadata const *metadata,
                                      size_t requiredSize,
                                      size_t requiredAlignmentMask)
@@ -185,7 +197,7 @@ OpaqueValue *swift::swift_projectBox(HeapObject *o) {
   // for boxes of empty type. The address of an empty value is always undefined,
   // so we can just return nil back in this case.
   if (!o)
-    return reinterpret_cast<OpaqueValue*>(o);
+    return nullptr;
   auto metadata = static_cast<const GenericBoxHeapMetadata *>(o->metadata);
   return metadata->project(o);
 }
@@ -206,7 +218,7 @@ void swift::swift_nonatomic_retain(HeapObject *object) {
 SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 void SWIFT_RT_ENTRY_IMPL(swift_nonatomic_retain)(HeapObject *object) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.incrementNonAtomic(1);
 }
 
@@ -217,7 +229,7 @@ void swift::swift_nonatomic_release(HeapObject *object) {
 SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 void SWIFT_RT_ENTRY_IMPL(swift_nonatomic_release)(HeapObject *object) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.decrementAndMaybeDeinitNonAtomic(1);
 }
 
@@ -225,7 +237,7 @@ SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 void SWIFT_RT_ENTRY_IMPL(swift_retain)(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.increment(1);
 }
 
@@ -238,7 +250,7 @@ SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 void SWIFT_RT_ENTRY_IMPL(swift_retain_n)(HeapObject *object, uint32_t n)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.increment(n);
 }
 
@@ -251,7 +263,7 @@ SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 void SWIFT_RT_ENTRY_IMPL(swift_nonatomic_retain_n)(HeapObject *object, uint32_t n)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.incrementNonAtomic(n);
 }
 
@@ -264,7 +276,7 @@ SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 void SWIFT_RT_ENTRY_IMPL(swift_release)(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.decrementAndMaybeDeinit(1);
 }
 
@@ -277,7 +289,7 @@ SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 void SWIFT_RT_ENTRY_IMPL(swift_release_n)(HeapObject *object, uint32_t n)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.decrementAndMaybeDeinit(n);
 }
 
@@ -294,7 +306,7 @@ SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 void SWIFT_RT_ENTRY_IMPL(swift_nonatomic_release_n)(HeapObject *object, uint32_t n)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.decrementAndMaybeDeinitNonAtomic(n);
 }
 
@@ -308,7 +320,7 @@ size_t swift::swift_unownedRetainCount(HeapObject *object) {
 
 void swift::swift_unownedRetain(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (!object)
+  if (!isValidPointerForNativeRetain(object))
     return;
 
   object->refCounts.incrementUnowned(1);
@@ -316,7 +328,7 @@ void swift::swift_unownedRetain(HeapObject *object)
 
 void swift::swift_unownedRelease(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (!object)
+  if (!isValidPointerForNativeRetain(object))
     return;
 
   // Only class objects can be unowned-retained and unowned-released.
@@ -334,7 +346,7 @@ void swift::swift_unownedRelease(HeapObject *object)
 
 void swift::swift_unownedRetain_n(HeapObject *object, int n)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (!object)
+  if (!isValidPointerForNativeRetain(object))
     return;
 
   object->refCounts.incrementUnowned(n);
@@ -342,7 +354,7 @@ void swift::swift_unownedRetain_n(HeapObject *object, int n)
 
 void swift::swift_unownedRelease_n(HeapObject *object, int n)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (!object)
+  if (!isValidPointerForNativeRetain(object))
     return;
 
   // Only class objects can be unowned-retained and unowned-released.
@@ -359,7 +371,7 @@ void swift::swift_unownedRelease_n(HeapObject *object, int n)
 
 HeapObject *swift::swift_tryPin(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  assert(object);
+  assert(isValidPointerForNativeRetain(object));
 
   // Try to set the flag.  If this succeeds, the caller will be
   // responsible for clearing it.
@@ -373,7 +385,7 @@ HeapObject *swift::swift_tryPin(HeapObject *object)
 
 void swift::swift_unpin(HeapObject *object)
   SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.decrementAndUnpinAndMaybeDeinit();
 }
 
@@ -398,7 +410,7 @@ HeapObject *swift::swift_nonatomic_tryPin(HeapObject *object)
 
 void swift::swift_nonatomic_unpin(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (object)
+  if (isValidPointerForNativeRetain(object))
     object->refCounts.decrementAndUnpinAndMaybeDeinitNonAtomic();
 }
 
@@ -406,7 +418,7 @@ SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 HeapObject *SWIFT_RT_ENTRY_IMPL(swift_tryRetain)(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (!object)
+  if (!isValidPointerForNativeRetain(object))
     return nullptr;
 
   if (object->refCounts.tryIncrement()) return object;
@@ -421,14 +433,14 @@ bool swift_isDeallocating(HeapObject *object) {
 SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 bool SWIFT_RT_ENTRY_IMPL(swift_isDeallocating)(HeapObject *object) {
-  if (!object)
+  if (!isValidPointerForNativeRetain(object))
     return false;
   return object->refCounts.isDeiniting();
 }
 
 void swift::swift_unownedRetainStrong(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (!object)
+  if (!isValidPointerForNativeRetain(object))
     return;
   assert(object->refCounts.getUnownedCount() &&
          "object is not currently unowned-retained");
@@ -439,7 +451,7 @@ void swift::swift_unownedRetainStrong(HeapObject *object)
 
 void swift::swift_unownedRetainStrongAndRelease(HeapObject *object)
     SWIFT_CC(RegisterPreservingCC_IMPL) {
-  if (!object)
+  if (!isValidPointerForNativeRetain(object))
     return;
   assert(object->refCounts.getUnownedCount() &&
          "object is not currently unowned-retained");
@@ -454,7 +466,7 @@ void swift::swift_unownedRetainStrongAndRelease(HeapObject *object)
 }
 
 void swift::swift_unownedCheck(HeapObject *object) {
-  if (!object) return;
+  if (!isValidPointerForNativeRetain(object)) return;
   assert(object->refCounts.getUnownedCount() &&
          "object is not currently unowned-retained");