Index: lib/tsan/rtl/tsan_debugging.cc
===================================================================
--- lib/tsan/rtl/tsan_debugging.cc
+++ lib/tsan/rtl/tsan_debugging.cc
@@ -83,6 +83,13 @@
 }
 
 SANITIZER_INTERFACE_ATTRIBUTE
+int __tsan_get_report_tag(void *report, uptr *tag) {
+  const ReportDesc *rep = (ReportDesc *)report;
+  *tag = rep->tag;
+  return 1;
+}
+
+SANITIZER_INTERFACE_ATTRIBUTE
 int __tsan_get_report_stack(void *report, uptr idx, void **trace,
                             uptr trace_size) {
   const ReportDesc *rep = (ReportDesc *)report;
Index: lib/tsan/rtl/tsan_interface.h
===================================================================
--- lib/tsan/rtl/tsan_interface.h
+++ lib/tsan/rtl/tsan_interface.h
@@ -117,6 +117,19 @@
                            int *unique_tid_count, void **sleep_trace,
                            uptr trace_size);
 
+/// Retrieves the "tag" from a report (for external-race report types). External
+/// races can be associated with a tag which give them more meaning. For example
+/// tag value '1' means "Swift access race". Tag value '0' indicated a plain
+/// external race.
+///
+/// \param report opaque pointer to the current report (obtained as argument in
+///               __tsan_on_report, or from __tsan_get_current_report)
+/// \param [out] tag points to storage that will be filled with the tag value
+///
+/// \returns non-zero value on success, zero on failure
+SANITIZER_INTERFACE_ATTRIBUTE
+int __tsan_get_report_tag(void *report, uptr *tag);
+
 // Returns information about stack traces included in the report.
 SANITIZER_INTERFACE_ATTRIBUTE
 int __tsan_get_report_stack(void *report, uptr idx, void **trace,
Index: test/tsan/Darwin/external-swift-debugging.cc
===================================================================
--- test/tsan/Darwin/external-swift-debugging.cc
+++ test/tsan/Darwin/external-swift-debugging.cc
@@ -0,0 +1,68 @@
+// RUN: %clangxx_tsan %s -o %t
+// RUN: %deflake %run %t 2>&1 | FileCheck %s
+
+#include <thread>
+
+#import "../test.h"
+
+
+extern "C" {
+int __tsan_get_report_data(void *report, const char **description, int *count,
+                           int *stack_count, int *mop_count, int *loc_count,
+                           int *mutex_count, int *thread_count,
+                           int *unique_tid_count, void **sleep_trace,
+                           unsigned long trace_size);
+int __tsan_get_report_tag(void *report, unsigned long *tag);
+}
+
+__attribute__((no_sanitize("thread"), noinline))
+void ExternalWrite(void *addr) {
+  void *kSwiftAccessRaceTag = (void *)0x1;
+  __tsan_external_write(addr, nullptr, kSwiftAccessRaceTag);
+}
+
+int main(int argc, char *argv[]) {
+  barrier_init(&barrier, 2);
+  fprintf(stderr, "Start.\n");
+  // CHECK: Start.
+  
+  void *opaque_object = malloc(16);
+  std::thread t1([opaque_object] {
+    ExternalWrite(opaque_object);
+    barrier_wait(&barrier);
+  });
+  std::thread t2([opaque_object] {
+    barrier_wait(&barrier);
+    ExternalWrite(opaque_object);
+  });
+  // CHECK: WARNING: ThreadSanitizer: Swift access race
+  // CHECK: Modifying access of Swift variable at {{.*}} by thread {{.*}}
+  // CHECK: Previous modifying access of Swift variable at {{.*}} by thread {{.*}}
+  // CHECK: SUMMARY: ThreadSanitizer: Swift access race
+  t1.join();
+  t2.join();
+
+  fprintf(stderr, "Done.\n");
+}
+
+extern "C"
+void __tsan_on_report(void *report) {
+  const char *description;
+  int count;
+  int stack_count, mop_count, loc_count, mutex_count, thread_count,
+      unique_tid_count;
+  void *sleep_trace[16] = {0};
+  __tsan_get_report_data(report, &description, &count, &stack_count, &mop_count,
+                         &loc_count, &mutex_count, &thread_count,
+                         &unique_tid_count, sleep_trace, 16);
+  fprintf(stderr, "report type = '%s', count = %d\n", description, count);
+  // CHECK: report type = 'external-race', count = 0
+
+  unsigned long tag;
+  __tsan_get_report_tag(report, &tag);
+  fprintf(stderr, "tag = %ld\n", tag);
+  // CHECK: tag = 1
+}
+
+// CHECK: Done.
+// CHECK: ThreadSanitizer: reported 1 warnings