Index: lib/asan/asan_internal.h
===================================================================
--- lib/asan/asan_internal.h
+++ lib/asan/asan_internal.h
@@ -62,6 +62,11 @@
 
 void AsanInitFromRtl();
 
+#if SANITIZER_WINDOWS64
+// asan_win.cc
+void InitializeExceptionHandlerOnWindows64();
+#endif
+
 // asan_rtl.cc
 void NORETURN ShowStatsAndAbort();
 
@@ -127,7 +132,6 @@
 // Used to avoid infinite recursion in __asan_init().
 extern bool asan_init_is_running;
 extern void (*death_callback)(void);
-
 // These magic values are written to shadow for better error reporting.
 const int kAsanHeapLeftRedzoneMagic = 0xfa;
 const int kAsanHeapRightRedzoneMagic = 0xfb;
Index: lib/asan/asan_rtl.cc
===================================================================
--- lib/asan/asan_rtl.cc
+++ lib/asan/asan_rtl.cc
@@ -420,6 +420,11 @@
   SetCanPoisonMemory(flags()->poison_heap);
   SetMallocContextSize(common_flags()->malloc_context_size);
 
+#if SANITIZER_WINDOWS64
+  // On Win64, we map memory on demand with access violation handler.
+  InitializeExceptionHandlerOnWindows64();
+#endif
+
   InitializeHighMemEnd();
 
   // Make sure we are not statically linked.
Index: lib/asan/asan_win.cc
===================================================================
--- lib/asan/asan_win.cc
+++ lib/asan/asan_win.cc
@@ -24,6 +24,7 @@
 #include "asan_report.h"
 #include "asan_stack.h"
 #include "asan_thread.h"
+#include "asan_mapping.h"
 #include "sanitizer_common/sanitizer_libc.h"
 #include "sanitizer_common/sanitizer_mutex.h"
 
@@ -215,6 +216,76 @@
   UNIMPLEMENTED();
 }
 
+static SIZE_T page_size = 0;
+static SIZE_T alloc_granularity = 0;
+
+#if SANITIZER_WINDOWS64
+// Exception handler for dealing with shadow memory.
+static LONG CALLBACK
+ShadowExceptionHandler(PEXCEPTION_POINTERS exception_pointers) {
+  // One-time init.
+  if (page_size == 0) {
+    SYSTEM_INFO system_info = {};
+    ::GetSystemInfo(&system_info);
+    page_size = system_info.dwPageSize;
+    alloc_granularity = system_info.dwAllocationGranularity;
+  }
+  // Only handle access violations.
+  if (exception_pointers->ExceptionRecord->ExceptionCode !=
+      EXCEPTION_ACCESS_VIOLATION) {
+    return EXCEPTION_CONTINUE_SEARCH;
+  }
+
+  // Only handle access violations that land within the shadow memory.
+  LPVOID addr = reinterpret_cast<LPVOID>(
+      exception_pointers->ExceptionRecord->ExceptionInformation[1]);
+
+  // Check valid shadow range.
+  if (!AddrIsInShadow((uptr)addr)) {
+    return EXCEPTION_CONTINUE_SEARCH;
+  }
+
+  // This is an access violation while trying to read from the shadow. Commit
+  // the relevant page and let execution continue.
+
+  // Determine the address of the page that is being accessed.
+  LPVOID page = reinterpret_cast<LPVOID>(reinterpret_cast<uintptr_t>(addr) &
+                                         ~(page_size - 1));
+
+  // Query the existing page.
+  MEMORY_BASIC_INFORMATION mem_info = {};
+  if (::VirtualQuery(page, &mem_info, sizeof(mem_info)) == 0)
+    return EXCEPTION_CONTINUE_SEARCH;
+
+  // If the memory isn't part of any reservation, then reserve the chunk of
+  // pages.
+  if (mem_info.AllocationBase == 0) {
+    LPVOID chunk = reinterpret_cast<LPVOID>(reinterpret_cast<uintptr_t>(addr) &
+                                            ~(alloc_granularity - 1));
+    auto result =
+        ::VirtualAlloc(chunk, alloc_granularity, MEM_RESERVE, PAGE_READONLY);
+    if (result != chunk) return EXCEPTION_CONTINUE_SEARCH;
+  }
+
+  // Commit the page.
+  auto result = ::VirtualAlloc(page, page_size, MEM_COMMIT, PAGE_READWRITE);
+  if (result != page) return EXCEPTION_CONTINUE_SEARCH;
+
+  // The page mapping succeeded, so continue execution as usual.
+  return EXCEPTION_CONTINUE_EXECUTION;
+}
+
+void InitializeExceptionHandlerOnWindows64() {
+  // Install our exception handler.
+  auto handler = AddVectoredExceptionHandler(TRUE, &ShadowExceptionHandler);
+  if (handler == NULL) {
+    // It is critical this handler must work.
+    __debugbreak();
+  }
+}
+#endif
+
+
 static LPTOP_LEVEL_EXCEPTION_FILTER default_seh_handler;
 
 static long WINAPI SEHHandler(EXCEPTION_POINTERS *info) {