Index: lib/asan/asan_interface_internal.h
===================================================================
--- lib/asan/asan_interface_internal.h
+++ lib/asan/asan_interface_internal.h
@@ -79,6 +79,10 @@
   SANITIZER_INTERFACE_ATTRIBUTE
   void __asan_after_dynamic_init();
 
+  // Calls real memset with no mapping or checks.
+  SANITIZER_INTERFACE_ATTRIBUTE
+  void __asan_memset_real(uptr addr, u8 value, uptr size);
+
   // These two functions are used by instrumented code in the
   // use-after-scope mode. They mark memory for local variables as
   // unaddressable when they leave scope and addressable before the
Index: lib/asan/asan_poisoning.cc
===================================================================
--- lib/asan/asan_poisoning.cc
+++ lib/asan/asan_poisoning.cc
@@ -314,6 +314,10 @@
   }
 }
 
+void __asan_memset_real(uptr addr, u8 c, uptr size) {
+  REAL(memset)((void *)addr, c, size);
+}
+
 void __asan_poison_stack_memory(uptr addr, uptr size) {
   if (!__asan_option_detect_stack_use_after_scope) return;
   VReport(1, "poisoning: %p %zx\n", (void *)addr, size);
Index: lib/asan/asan_rtl.cc
===================================================================
--- lib/asan/asan_rtl.cc
+++ lib/asan/asan_rtl.cc
@@ -265,44 +265,123 @@
   // __asan_report_* functions are noreturn, so we need a switch to prevent
   // the compiler from removing any of them.
   switch (fake_condition) {
-    case 1: __asan_report_load1(0); break;
-    case 2: __asan_report_load2(0); break;
-    case 3: __asan_report_load4(0); break;
-    case 4: __asan_report_load8(0); break;
-    case 5: __asan_report_load16(0); break;
-    case 6: __asan_report_load_n(0, 0); break;
-    case 7: __asan_report_store1(0); break;
-    case 8: __asan_report_store2(0); break;
-    case 9: __asan_report_store4(0); break;
-    case 10: __asan_report_store8(0); break;
-    case 11: __asan_report_store16(0); break;
-    case 12: __asan_report_store_n(0, 0); break;
-    case 13: __asan_report_exp_load1(0, 0); break;
-    case 14: __asan_report_exp_load2(0, 0); break;
-    case 15: __asan_report_exp_load4(0, 0); break;
-    case 16: __asan_report_exp_load8(0, 0); break;
-    case 17: __asan_report_exp_load16(0, 0); break;
-    case 18: __asan_report_exp_load_n(0, 0, 0); break;
-    case 19: __asan_report_exp_store1(0, 0); break;
-    case 20: __asan_report_exp_store2(0, 0); break;
-    case 21: __asan_report_exp_store4(0, 0); break;
-    case 22: __asan_report_exp_store8(0, 0); break;
-    case 23: __asan_report_exp_store16(0, 0); break;
-    case 24: __asan_report_exp_store_n(0, 0, 0); break;
-    case 25: __asan_register_globals(nullptr, 0); break;
-    case 26: __asan_unregister_globals(nullptr, 0); break;
-    case 27: __asan_set_death_callback(nullptr); break;
-    case 28: __asan_set_error_report_callback(nullptr); break;
-    case 29: __asan_handle_no_return(); break;
-    case 30: __asan_address_is_poisoned(nullptr); break;
-    case 31: __asan_poison_memory_region(nullptr, 0); break;
-    case 32: __asan_unpoison_memory_region(nullptr, 0); break;
-    case 34: __asan_before_dynamic_init(nullptr); break;
-    case 35: __asan_after_dynamic_init(); break;
-    case 36: __asan_poison_stack_memory(0, 0); break;
-    case 37: __asan_unpoison_stack_memory(0, 0); break;
-    case 38: __asan_region_is_poisoned(0, 0); break;
-    case 39: __asan_describe_address(0); break;
+    case 1:
+      __asan_report_load1(0);
+      break;
+    case 2:
+      __asan_report_load2(0);
+      break;
+    case 3:
+      __asan_report_load4(0);
+      break;
+    case 4:
+      __asan_report_load8(0);
+      break;
+    case 5:
+      __asan_report_load16(0);
+      break;
+    case 6:
+      __asan_report_load_n(0, 0);
+      break;
+    case 7:
+      __asan_report_store1(0);
+      break;
+    case 8:
+      __asan_report_store2(0);
+      break;
+    case 9:
+      __asan_report_store4(0);
+      break;
+    case 10:
+      __asan_report_store8(0);
+      break;
+    case 11:
+      __asan_report_store16(0);
+      break;
+    case 12:
+      __asan_report_store_n(0, 0);
+      break;
+    case 13:
+      __asan_report_exp_load1(0, 0);
+      break;
+    case 14:
+      __asan_report_exp_load2(0, 0);
+      break;
+    case 15:
+      __asan_report_exp_load4(0, 0);
+      break;
+    case 16:
+      __asan_report_exp_load8(0, 0);
+      break;
+    case 17:
+      __asan_report_exp_load16(0, 0);
+      break;
+    case 18:
+      __asan_report_exp_load_n(0, 0, 0);
+      break;
+    case 19:
+      __asan_report_exp_store1(0, 0);
+      break;
+    case 20:
+      __asan_report_exp_store2(0, 0);
+      break;
+    case 21:
+      __asan_report_exp_store4(0, 0);
+      break;
+    case 22:
+      __asan_report_exp_store8(0, 0);
+      break;
+    case 23:
+      __asan_report_exp_store16(0, 0);
+      break;
+    case 24:
+      __asan_report_exp_store_n(0, 0, 0);
+      break;
+    case 25:
+      __asan_register_globals(nullptr, 0);
+      break;
+    case 26:
+      __asan_unregister_globals(nullptr, 0);
+      break;
+    case 27:
+      __asan_set_death_callback(nullptr);
+      break;
+    case 28:
+      __asan_set_error_report_callback(nullptr);
+      break;
+    case 29:
+      __asan_handle_no_return();
+      break;
+    case 30:
+      __asan_address_is_poisoned(nullptr);
+      break;
+    case 31:
+      __asan_poison_memory_region(nullptr, 0);
+      break;
+    case 32:
+      __asan_unpoison_memory_region(nullptr, 0);
+      break;
+    case 34:
+      __asan_before_dynamic_init(nullptr);
+      break;
+    case 35:
+      __asan_after_dynamic_init();
+      break;
+    case 36:
+      __asan_poison_stack_memory(0, 0);
+      break;
+    case 37:
+      __asan_unpoison_stack_memory(0, 0);
+      break;
+    case 38:
+      __asan_region_is_poisoned(0, 0);
+      break;
+    case 39:
+      __asan_describe_address(0);
+      break;
+    case 40:
+      __asan_memset_real(0, 0, 0);
+      break;
   }
 }
 
Index: lib/asan/asan_win_dll_thunk.cc
===================================================================
--- lib/asan/asan_win_dll_thunk.cc
+++ lib/asan/asan_win_dll_thunk.cc
@@ -261,6 +261,8 @@
 INTERFACE_FUNCTION(__asan_memset);
 INTERFACE_FUNCTION(__asan_memmove);
 
+INTERFACE_FUNCTION(__asan_memset_real);
+
 INTERFACE_FUNCTION(__asan_alloca_poison);
 INTERFACE_FUNCTION(__asan_allocas_unpoison);