Index: compiler-rt/trunk/include/sanitizer/hwasan_interface.h
===================================================================
--- compiler-rt/trunk/include/sanitizer/hwasan_interface.h
+++ compiler-rt/trunk/include/sanitizer/hwasan_interface.h
@@ -19,6 +19,12 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
+  // Initialize shadow but not the rest of the runtime.
+  // Does not call libc unless there is an error.
+  // Can be called multiple times, or not at all (in which case shadow will
+  // be initialized in compiler-inserted __hwasan_init() call).
+  void __hwasan_shadow_init(void);
+
   // This function may be optionally provided by user and should return
   // a string containing HWASan runtime options. See asan_flags.h for details.
   const char* __hwasan_default_options(void);
Index: compiler-rt/trunk/lib/hwasan/hwasan.h
===================================================================
--- compiler-rt/trunk/lib/hwasan/hwasan.h
+++ compiler-rt/trunk/lib/hwasan/hwasan.h
@@ -61,6 +61,7 @@
 
 bool ProtectRange(uptr beg, uptr end);
 bool InitShadow();
+void MadviseShadow();
 char *GetProcSelfMaps();
 void InitializeInterceptors();
 
Index: compiler-rt/trunk/lib/hwasan/hwasan.cc
===================================================================
--- compiler-rt/trunk/lib/hwasan/hwasan.cc
+++ compiler-rt/trunk/lib/hwasan/hwasan.cc
@@ -57,6 +57,7 @@
 }
 
 int hwasan_inited = 0;
+int hwasan_shadow_inited = 0;
 bool hwasan_init_is_running;
 
 int hwasan_report_count = 0;
@@ -161,6 +162,22 @@
 
 uptr __hwasan_shadow_memory_dynamic_address;  // Global interface symbol.
 
+void __hwasan_shadow_init() {
+  if (hwasan_shadow_inited) return;
+  if (!InitShadow()) {
+    Printf("FATAL: HWAddressSanitizer cannot mmap the shadow memory.\n");
+    if (HWASAN_FIXED_MAPPING) {
+      Printf("FATAL: Make sure to compile with -fPIE and to link with -pie.\n");
+      Printf("FATAL: Disabling ASLR is known to cause this error.\n");
+      Printf("FATAL: If running under GDB, try "
+             "'set disable-randomization off'.\n");
+    }
+    DumpProcessMap();
+    Die();
+  }
+  hwasan_shadow_inited = 1;
+}
+
 void __hwasan_init() {
   CHECK(!hwasan_init_is_running);
   if (hwasan_inited) return;
@@ -178,17 +195,8 @@
   __sanitizer_set_report_path(common_flags()->log_path);
 
   DisableCoreDumperIfNecessary();
-  if (!InitShadow()) {
-    Printf("FATAL: HWAddressSanitizer cannot mmap the shadow memory.\n");
-    if (HWASAN_FIXED_MAPPING) {
-      Printf("FATAL: Make sure to compile with -fPIE and to link with -pie.\n");
-      Printf("FATAL: Disabling ASLR is known to cause this error.\n");
-      Printf("FATAL: If running under GDB, try "
-             "'set disable-randomization off'.\n");
-    }
-    DumpProcessMap();
-    Die();
-  }
+  __hwasan_shadow_init();
+  MadviseShadow();
 
   InitializeInterceptors();
   InstallDeadlySignalHandlers(HwasanOnDeadlySignal);
Index: compiler-rt/trunk/lib/hwasan/hwasan_interface_internal.h
===================================================================
--- compiler-rt/trunk/lib/hwasan/hwasan_interface_internal.h
+++ compiler-rt/trunk/lib/hwasan/hwasan_interface_internal.h
@@ -20,6 +20,9 @@
 extern "C" {
 
 SANITIZER_INTERFACE_ATTRIBUTE
+void __hwasan_shadow_init();
+
+SANITIZER_INTERFACE_ATTRIBUTE
 void __hwasan_init();
 
 using __sanitizer::uptr;
Index: compiler-rt/trunk/lib/hwasan/hwasan_linux.cc
===================================================================
--- compiler-rt/trunk/lib/hwasan/hwasan_linux.cc
+++ compiler-rt/trunk/lib/hwasan/hwasan_linux.cc
@@ -51,8 +51,6 @@
         size);
     Abort();
   }
-  if (common_flags()->no_huge_pages_for_shadow) NoHugePagesInRegion(beg, size);
-  if (common_flags()->use_madv_dontdump) DontDumpShadowMemory(beg, size);
 }
 
 static void ProtectGap(uptr addr, uptr size) {
@@ -219,6 +217,19 @@
   return true;
 }
 
+static void MadviseShadowRegion(uptr beg, uptr end) {
+  uptr size = end - beg + 1;
+  if (common_flags()->no_huge_pages_for_shadow)
+    NoHugePagesInRegion(beg, size);
+  if (common_flags()->use_madv_dontdump)
+    DontDumpShadowMemory(beg, size);
+}
+
+void MadviseShadow() {
+  MadviseShadowRegion(kLowShadowStart, kLowShadowEnd);
+  MadviseShadowRegion(kHighShadowStart, kHighShadowEnd);
+}
+
 bool MemIsApp(uptr p) {
   CHECK(GetTagFromPointer(p) == 0);
   return p >= kHighMemStart || (p >= kLowMemStart && p <= kLowMemEnd);