diff --git a/compiler-rt/lib/dfsan/dfsan.cpp b/compiler-rt/lib/dfsan/dfsan.cpp
--- a/compiler-rt/lib/dfsan/dfsan.cpp
+++ b/compiler-rt/lib/dfsan/dfsan.cpp
@@ -158,6 +158,12 @@
   }
 }
 
+static void ReportUnsupportedFast16(const char *func) {
+  Report("FATAL: DataFlowSanitizer: %s is unsupported in fast16labels mode\n",
+         func);
+  Die();
+}
+
 // Resolves the union of two unequal labels.  Nonequality is a precondition for
 // this function (the instrumentation pass inlines the equality test).
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
@@ -253,8 +259,10 @@
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
 dfsan_label dfsan_create_label(const char *desc, void *userdata) {
+  if (flags().fast16labels)
+    ReportUnsupportedFast16("dfsan_create_label");
   dfsan_label label =
-    atomic_fetch_add(&__dfsan_last_label, 1, memory_order_relaxed) + 1;
+      atomic_fetch_add(&__dfsan_last_label, 1, memory_order_relaxed) + 1;
   dfsan_check_label(label);
   __dfsan_label_info[label].l1 = __dfsan_label_info[label].l2 = 0;
   __dfsan_label_info[label].desc = desc;
@@ -311,11 +319,15 @@
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
 const struct dfsan_label_info *dfsan_get_label_info(dfsan_label label) {
+  if (flags().fast16labels)
+    ReportUnsupportedFast16("dfsan_get_label_info");
   return &__dfsan_label_info[label];
 }
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE int
 dfsan_has_label(dfsan_label label, dfsan_label elem) {
+  if (flags().fast16labels)
+    return label & elem;
   if (label == elem)
     return true;
   const dfsan_label_info *info = dfsan_get_label_info(label);
@@ -328,6 +340,8 @@
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE dfsan_label
 dfsan_has_label_with_desc(dfsan_label label, const char *desc) {
+  if (flags().fast16labels)
+    ReportUnsupportedFast16("dfsan_has_label_with_desc");
   const dfsan_label_info *info = dfsan_get_label_info(label);
   if (info->l1 != 0) {
     return dfsan_has_label_with_desc(info->l1, desc) ||
@@ -347,9 +361,11 @@
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE void
 dfsan_dump_labels(int fd) {
+  if (flags().fast16labels)
+    ReportUnsupportedFast16("dfsan_dump_labels");
+
   dfsan_label last_label =
       atomic_load(&__dfsan_last_label, memory_order_relaxed);
-
   for (uptr l = 1; l <= last_label; ++l) {
     char buf[64];
     internal_snprintf(buf, sizeof(buf), "%u %u %u ", l,
diff --git a/compiler-rt/test/dfsan/fast16labels.c b/compiler-rt/test/dfsan/fast16labels.c
--- a/compiler-rt/test/dfsan/fast16labels.c
+++ b/compiler-rt/test/dfsan/fast16labels.c
@@ -1,4 +1,13 @@
-// RUN: %clang_dfsan %s -o %t && DFSAN_OPTIONS=fast16labels=1 %run %t
+// RUN: %clang_dfsan %s -o %t
+// RUN: DFSAN_OPTIONS=fast16labels=1 %run %t
+// RUN: DFSAN_OPTIONS=fast16labels=1 not %run %t dfsan_create_label 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CREATE-LABEL
+// RUN: DFSAN_OPTIONS=fast16labels=1 not %run %t dfsan_get_label_info 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=GET-LABEL-INFO
+// RUN: DFSAN_OPTIONS=fast16labels=1 not %run %t dfsan_has_label_with_desc \
+// RUN:   2>&1 | FileCheck %s --check-prefix=HAS-LABEL-WITH-DESC
+// RUN: DFSAN_OPTIONS=fast16labels=1:dump_labels_at_exit=/dev/stdout not %run \
+// RUN:   %t 2>&1 | FileCheck %s --check-prefix=DUMP-LABELS
 //
 // Tests DFSAN_OPTIONS=fast16labels=1
 //
@@ -6,12 +15,27 @@
 
 #include <assert.h>
 #include <stdio.h>
+#include <string.h>
 
 int foo(int a, int b) {
   return a + b;
 }
 
-int main() {
+int main(int argc, char *argv[]) {
+  // Death tests for unsupported API usage.
+  const char *command = (argc < 2) ? "" : argv[1];
+  // CREATE-LABEL: FATAL: DataFlowSanitizer: dfsan_create_label is unsupported
+  if (strcmp(command, "dfsan_create_label") == 0)
+    dfsan_create_label("", NULL);
+  // GET-LABEL-INFO: FATAL: DataFlowSanitizer: dfsan_get_label_info is unsupported
+  if (strcmp(command, "dfsan_get_label_info") == 0)
+    dfsan_get_label_info(1);
+  // HAS-LABEL-WITH-DESC: FATAL: DataFlowSanitizer: dfsan_has_label_with_desc is unsupported
+  if (strcmp(command, "dfsan_has_label_with_desc") == 0)
+    dfsan_has_label_with_desc(1, "");
+  // DUMP-LABELS: FATAL: DataFlowSanitizer: dfsan_dump_labels is unsupported
+
+  // Supported usage.
   int a = 10;
   int b = 20;
   dfsan_set_label(8, &a, sizeof(a));
@@ -22,4 +46,7 @@
   dfsan_label l = dfsan_get_label(c);
   printf("C: 0x%x\n", l);
   assert(l == 520);  // OR of the other two labels.
+  assert(dfsan_has_label(l, 8));
+  assert(dfsan_has_label(l, 512));
+  assert(!dfsan_has_label(l, 1));
 }