Index: lib/tsan/rtl/tsan_debugging.cc
===================================================================
--- lib/tsan/rtl/tsan_debugging.cc
+++ lib/tsan/rtl/tsan_debugging.cc
@@ -25,6 +25,7 @@
   if (typ == ReportTypeUseAfterFree) return "heap-use-after-free";
   if (typ == ReportTypeVptrUseAfterFree) return "heap-use-after-free-vptr";
   if (typ == ReportTypeExternalRace) return "external-race";
+  if (typ == ReportTypeSwiftAccessRace) return "swift-access-race";
   if (typ == ReportTypeThreadLeak) return "thread-leak";
   if (typ == ReportTypeMutexDestroyLocked) return "locked-mutex-destroy";
   if (typ == ReportTypeMutexDoubleLock) return "mutex-double-lock";
Index: lib/tsan/rtl/tsan_external.cc
===================================================================
--- lib/tsan/rtl/tsan_external.cc
+++ lib/tsan/rtl/tsan_external.cc
@@ -16,11 +16,8 @@
 
 #define CALLERPC ((uptr)__builtin_return_address(0))
 
-const uptr kMaxTag = 128;  // Limited to 65,536, since MBlock only stores tags
-                           // as 16-bit values, see tsan_defs.h.
-
-const char *registered_tags[kMaxTag];
-static atomic_uint32_t used_tags{1};  // Tag 0 means "no tag". NOLINT
+const char *registered_tags[kExternalTagMax];
+static atomic_uint32_t used_tags{kExternalTagFirstUserAvailable};
 
 const char *GetObjectTypeFromTag(uptr tag) {
   if (tag == 0) return nullptr;
@@ -33,7 +30,7 @@
 SANITIZER_INTERFACE_ATTRIBUTE
 void *__tsan_external_register_tag(const char *object_type) {
   uptr new_tag = atomic_fetch_add(&used_tags, 1, memory_order_relaxed);
-  CHECK_LT(new_tag, kMaxTag);
+  CHECK_LT(new_tag, kExternalTagMax);
   registered_tags[new_tag] = internal_strdup(object_type);
   return (void *)new_tag;
 }
@@ -57,9 +54,9 @@
   CHECK_LT(tag, atomic_load(&used_tags, memory_order_relaxed));
   ThreadState *thr = cur_thread();
   thr->external_tag = (uptr)tag;
-  FuncEntry(thr, (uptr)caller_pc);
+  if (caller_pc) FuncEntry(thr, (uptr)caller_pc);
   MemoryRead(thr, CALLERPC, (uptr)addr, kSizeLog8);
-  FuncExit(thr);
+  if (caller_pc) FuncExit(thr);
   thr->external_tag = 0;
 }
 
@@ -68,9 +65,9 @@
   CHECK_LT(tag, atomic_load(&used_tags, memory_order_relaxed));
   ThreadState *thr = cur_thread();
   thr->external_tag = (uptr)tag;
-  FuncEntry(thr, (uptr)caller_pc);
+  if (caller_pc) FuncEntry(thr, (uptr)caller_pc);
   MemoryWrite(thr, CALLERPC, (uptr)addr, kSizeLog8);
-  FuncExit(thr);
+  if (caller_pc) FuncExit(thr);
   thr->external_tag = 0;
 }
 }  // extern "C"
Index: lib/tsan/rtl/tsan_report.h
===================================================================
--- lib/tsan/rtl/tsan_report.h
+++ lib/tsan/rtl/tsan_report.h
@@ -25,6 +25,7 @@
   ReportTypeUseAfterFree,
   ReportTypeVptrUseAfterFree,
   ReportTypeExternalRace,
+  ReportTypeSwiftAccessRace,
   ReportTypeThreadLeak,
   ReportTypeMutexDestroyLocked,
   ReportTypeMutexDoubleLock,
Index: lib/tsan/rtl/tsan_report.cc
===================================================================
--- lib/tsan/rtl/tsan_report.cc
+++ lib/tsan/rtl/tsan_report.cc
@@ -92,6 +92,8 @@
     return "heap-use-after-free (virtual call vs free)";
   if (typ == ReportTypeExternalRace)
     return "race on a library object";
+  if (typ == ReportTypeSwiftAccessRace)
+    return "Swift access race";
   if (typ == ReportTypeThreadLeak)
     return "thread leak";
   if (typ == ReportTypeMutexDestroyLocked)
@@ -158,13 +160,21 @@
   return first ? (write ? "Mutating" : "Read-only")
                : (write ? "Previous mutating" : "Previous read-only");
 }
+  
+static const char *SwiftMopDesc(bool first) {
+  return first ? "Modifying access" : "Previous modifying access";
+}
 
 static void PrintMop(const ReportMop *mop, bool first) {
   Decorator d;
   char thrbuf[kThreadBufSize];
   Printf("%s", d.Access());
   const char *object_type = GetObjectTypeFromTag(mop->external_tag);
-  if (!object_type) {
+  if (mop->external_tag == kExternalTagSwiftModifyingAccess) {
+    Printf("  %s access at %p by %s",
+           SwiftMopDesc(first), (void *)mop->addr,
+           thread_name(thrbuf, mop->tid));
+  } else if (mop->external_tag == kExternalTagNone || !object_type) {
     Printf("  %s of size %d at %p by %s",
            MopDesc(first, mop->write, mop->atomic), mop->size,
            (void *)mop->addr, thread_name(thrbuf, mop->tid));
Index: lib/tsan/rtl/tsan_rtl.h
===================================================================
--- lib/tsan/rtl/tsan_rtl.h
+++ lib/tsan/rtl/tsan_rtl.h
@@ -645,6 +645,14 @@
 bool IsExpectedReport(uptr addr, uptr size);
 void PrintMatchedBenignRaces();
 
+enum ExternalTag : uptr {
+  kExternalTagNone = 0,
+  kExternalTagSwiftModifyingAccess = 1,
+  kExternalTagFirstUserAvailable = 128,
+  kExternalTagMax = 1024,
+  // Don't set kExternalTagMax over 65,536, since MBlock only stores tags
+  // as 16-bit values, see tsan_defs.h.
+};
 const char *GetObjectTypeFromTag(uptr tag);
 
 #if defined(TSAN_DEBUG_OUTPUT) && TSAN_DEBUG_OUTPUT >= 1
Index: lib/tsan/rtl/tsan_rtl_report.cc
===================================================================
--- lib/tsan/rtl/tsan_rtl_report.cc
+++ lib/tsan/rtl/tsan_rtl_report.cc
@@ -625,6 +625,8 @@
     typ = ReportTypeVptrRace;
   else if (freed)
     typ = ReportTypeUseAfterFree;
+  else if (thr->external_tag == kExternalTagSwiftModifyingAccess)
+    typ = ReportTypeSwiftAccessRace;
   else if (thr->external_tag > 0)
     typ = ReportTypeExternalRace;
 
Index: lib/tsan/rtl/tsan_suppressions.cc
===================================================================
--- lib/tsan/rtl/tsan_suppressions.cc
+++ lib/tsan/rtl/tsan_suppressions.cc
@@ -76,6 +76,8 @@
     return kSuppressionRace;
   else if (typ == ReportTypeExternalRace)
     return kSuppressionRace;
+  else if (typ == ReportTypeSwiftAccessRace)
+    return kSuppressionRace;
   else if (typ == ReportTypeThreadLeak)
     return kSuppressionThread;
   else if (typ == ReportTypeMutexDestroyLocked)
Index: test/tsan/Darwin/external-swift.cc
===================================================================
--- test/tsan/Darwin/external-swift.cc
+++ test/tsan/Darwin/external-swift.cc
@@ -0,0 +1,34 @@
+// RUN: %clangxx_tsan %s -o %t
+// RUN: %deflake %run %t 2>&1 | FileCheck %s
+
+#include <thread>
+
+static void *tag = (void *)0x1;
+extern "C" {
+void *__tsan_external_register_tag(const char *object_type);
+void *__tsan_external_assign_tag(void *addr, void *tag);
+void __tsan_external_read(void *addr, void *caller_pc, void *tag);
+void __tsan_external_write(void *addr, void *caller_pc, void *tag);
+}
+
+int main(int argc, char *argv[]) {
+  fprintf(stderr, "Start.\n");
+  void *opaque_object = malloc(16);
+  
+  std::thread t1([opaque_object] {
+    __tsan_external_write(opaque_object, nullptr, tag);
+  });
+  std::thread t2([opaque_object] {
+    __tsan_external_write(opaque_object, nullptr, tag);
+  });
+  t1.join();
+  t2.join();
+  
+  fprintf(stderr, "Stop.\n");
+}
+
+// CHECK: Start.
+// CHECK: WARNING: ThreadSanitizer: Swift access race
+// CHECK: Modifying access at {{.*}} by thread {{.*}}
+// CHECK: Previous modifying access at {{.*}} by thread {{.*}}
+// CHECK: Stop.