Index: lib/asan/asan_rtl.cc
===================================================================
--- lib/asan/asan_rtl.cc
+++ lib/asan/asan_rtl.cc
@@ -420,6 +420,8 @@
   __asan_option_detect_stack_use_after_return =
       flags()->detect_stack_use_after_return;
 
+  __sanitizer::InitializePlatformEarly();
+
   // Re-exec ourselves if we need to set additional env or command line args.
   MaybeReexec();
 
Index: lib/sanitizer_common/sanitizer_common.h
===================================================================
--- lib/sanitizer_common/sanitizer_common.h
+++ lib/sanitizer_common/sanitizer_common.h
@@ -896,6 +896,7 @@
   bool IsMemoryAccess() const;
 };
 
+void InitializePlatformEarly();
 void MaybeReexec();
 
 template <typename Fn>
Index: lib/sanitizer_common/sanitizer_fuchsia.cc
===================================================================
--- lib/sanitizer_common/sanitizer_fuchsia.cc
+++ lib/sanitizer_common/sanitizer_fuchsia.cc
@@ -89,6 +89,7 @@
   *stack_top = *stack_bottom + size;
 }
 
+void InitializePlatformEarly() {}
 void MaybeReexec() {}
 void CheckASLR() {}
 void PlatformPrepareForSandboxing(__sanitizer_sandbox_arguments *args) {}
Index: lib/sanitizer_common/sanitizer_linux.cc
===================================================================
--- lib/sanitizer_common/sanitizer_linux.cc
+++ lib/sanitizer_common/sanitizer_linux.cc
@@ -186,7 +186,11 @@
                           offset / 4096);
 #endif
 }
-#endif // !SANITIZER_S390 && !SANITIZER_OPENBSD
+uptr internal_mmap_shadow(void *addr, uptr length, int prot, int flags, int fd,
+                          OFF_T offset) {
+  return internal_mmap(addr, length, prot, flags, fd, offset);
+}
+#endif  // !SANITIZER_S390 && !SANITIZER_OPENBSD
 
 #if !SANITIZER_OPENBSD
 uptr internal_munmap(void *addr, uptr length) {
@@ -1950,6 +1954,10 @@
 
 void SignalContext::InitPcSpBp() { GetPcSpBp(context, &pc, &sp, &bp); }
 
+void InitializePlatformEarly() {
+  // Do nothing.
+}
+
 void MaybeReexec() {
   // No need to re-exec on Linux.
 }
Index: lib/sanitizer_common/sanitizer_linux_s390.cc
===================================================================
--- lib/sanitizer_common/sanitizer_linux_s390.cc
+++ lib/sanitizer_common/sanitizer_linux_s390.cc
@@ -55,6 +55,11 @@
 # endif
 }
 
+uptr internal_mmap_shadow(void *addr, uptr length, int prot, int flags, int fd,
+                          OFF_T offset) {
+  return internal_mmap(addr, length, prot, flags, fd, offset);
+}
+
 uptr internal_clone(int (*fn)(void *), void *child_stack, int flags, void *arg,
                     int *parent_tidptr, void *newtls, int *child_tidptr) {
   if (!fn || !child_stack)
Index: lib/sanitizer_common/sanitizer_mac.h
===================================================================
--- lib/sanitizer_common/sanitizer_mac.h
+++ lib/sanitizer_common/sanitizer_mac.h
@@ -40,6 +40,10 @@
   MACOS_VERSION_MOUNTAIN_LION,
   MACOS_VERSION_MAVERICKS,
   MACOS_VERSION_YOSEMITE,
+  MACOS_VERSION_EL_CAPITAN,
+  MACOS_VERSION_SIERRA,
+  MACOS_VERSION_HIGH_SIERRA,
+  MACOS_VERSION_HIGH_SIERRA_DOT_RELEASE_4,
   MACOS_VERSION_UNKNOWN_NEWER
 };
 
Index: lib/sanitizer_common/sanitizer_mac.cc
===================================================================
--- lib/sanitizer_common/sanitizer_mac.cc
+++ lib/sanitizer_common/sanitizer_mac.cc
@@ -115,6 +115,21 @@
   return (uptr)mmap(addr, length, prot, flags, fd, offset);
 }
 
+// XNU on Darwin provides a mmap flag that optimizes allocation/deallocation of
+// giant memory regions (i.e. shadow memory regions).
+#define kXnuFastMmapFd 0x4
+static bool use_xnu_fast_mmap = false;
+
+uptr internal_mmap_shadow(void *addr, size_t length, int prot, int flags,
+                          int fd, u64 offset) {
+  if (fd == -1) {
+    int fd = VM_MAKE_TAG(VM_MEMORY_SANITIZER);
+    if (use_xnu_fast_mmap) fd |= kXnuFastMmapFd;
+  }
+  if (&__mmap) return (uptr)__mmap(addr, length, prot, flags, fd, offset);
+  return (uptr)mmap(addr, length, prot, flags, fd, offset);
+}
+
 uptr internal_munmap(void *addr, uptr length) {
   if (&__munmap) return __munmap(addr, length);
   return munmap(addr, length);
@@ -502,23 +517,33 @@
   CHECK_NE(sysctl(mib, 2, 0, &len, 0, 0), -1);
   CHECK_LT(len, maxlen);
   CHECK_NE(sysctl(mib, 2, version, &len, 0, 0), -1);
-  switch (version[0]) {
-    case '9': return MACOS_VERSION_LEOPARD;
-    case '1': {
-      switch (version[1]) {
-        case '0': return MACOS_VERSION_SNOW_LEOPARD;
-        case '1': return MACOS_VERSION_LION;
-        case '2': return MACOS_VERSION_MOUNTAIN_LION;
-        case '3': return MACOS_VERSION_MAVERICKS;
-        case '4': return MACOS_VERSION_YOSEMITE;
-        default:
-          if (IsDigit(version[1]))
-            return MACOS_VERSION_UNKNOWN_NEWER;
-          else
-            return MACOS_VERSION_UNKNOWN;
-      }
-    }
-    default: return MACOS_VERSION_UNKNOWN;
+
+  // Expect <major>.<minor>(.<patch>)
+  CHECK_GE(len, 3);
+  const char *p = version;
+  int major = internal_simple_strtoll(p, &p, /*base=*/10);
+  if (*p != '.') return MACOS_VERSION_UNKNOWN;
+  p += 1;
+  int minor = internal_simple_strtoll(p, &p, /*base=*/10);
+  if (*p != '.') return MACOS_VERSION_UNKNOWN;
+
+  switch (major) {
+    case 9: return MACOS_VERSION_LEOPARD;
+    case 10: return MACOS_VERSION_SNOW_LEOPARD;
+    case 11: return MACOS_VERSION_LION;
+    case 12: return MACOS_VERSION_MOUNTAIN_LION;
+    case 13: return MACOS_VERSION_MAVERICKS;
+    case 14: return MACOS_VERSION_YOSEMITE;
+    case 15: return MACOS_VERSION_EL_CAPITAN;
+    case 16: return MACOS_VERSION_SIERRA;
+    case 17:
+      // Not a typo, 17.5 Darwin Kernel Version maps to High Sierra 10.13.4.
+      if (minor >= 5)
+        return MACOS_VERSION_HIGH_SIERRA_DOT_RELEASE_4;
+      return MACOS_VERSION_HIGH_SIERRA;
+    default:
+      if (major < 9) return MACOS_VERSION_UNKNOWN;
+      return MACOS_VERSION_UNKNOWN_NEWER;
   }
 }
 
@@ -661,6 +686,16 @@
 
 void SignalContext::InitPcSpBp() { GetPcSpBp(context, &pc, &sp, &bp); }
 
+void InitializePlatformEarly() {
+  // Only use xnu_fast_mmap when on x86_64 and the OS supports it.
+  use_xnu_fast_mmap =
+#if defined(__x86_64__)
+      GetMacosVersion() >= MACOS_VERSION_HIGH_SIERRA_DOT_RELEASE_4;
+#else
+      false;
+#endif
+}
+
 #if !SANITIZER_GO
 static const char kDyldInsertLibraries[] = "DYLD_INSERT_LIBRARIES";
 LowLevelAllocator allocator_for_env;
Index: lib/sanitizer_common/sanitizer_openbsd.cc
===================================================================
--- lib/sanitizer_common/sanitizer_openbsd.cc
+++ lib/sanitizer_common/sanitizer_openbsd.cc
@@ -45,6 +45,11 @@
   return (uptr)mmap(addr, length, prot, flags, fd, offset);
 }
 
+uptr internal_mmap_shadow(void *addr, uptr length, int prot, int flags, int fd,
+                          OFF_T offset) {
+  return internal_mmap(addr, length, prot, flags, fd, offset);
+}
+
 uptr internal_munmap(void *addr, uptr length) { return munmap(addr, length); }
 
 int internal_mprotect(void *addr, uptr length, int prot) {
Index: lib/sanitizer_common/sanitizer_posix.h
===================================================================
--- lib/sanitizer_common/sanitizer_posix.h
+++ lib/sanitizer_common/sanitizer_posix.h
@@ -38,8 +38,10 @@
 uptr internal_write(fd_t fd, const void *buf, uptr count);
 
 // Memory
-uptr internal_mmap(void *addr, uptr length, int prot, int flags,
-                   int fd, OFF_T offset);
+uptr internal_mmap(void *addr, uptr length, int prot, int flags, int fd,
+                   OFF_T offset);
+uptr internal_mmap_shadow(void *addr, uptr length, int prot, int flags, int fd,
+                          OFF_T offset);
 uptr internal_munmap(void *addr, uptr length);
 int internal_mprotect(void *addr, uptr length, int prot);
 
Index: lib/sanitizer_common/sanitizer_posix_libcdep.cc
===================================================================
--- lib/sanitizer_common/sanitizer_posix_libcdep.cc
+++ lib/sanitizer_common/sanitizer_posix_libcdep.cc
@@ -334,9 +334,9 @@
   if (fd == -1) flags |= MAP_ANON;
 
   uptr PageSize = GetPageSizeCached();
-  uptr p = internal_mmap((void *)(fixed_addr & ~(PageSize - 1)),
-                         RoundUpTo(size, PageSize), PROT_READ | PROT_WRITE,
-                         flags, fd, 0);
+  uptr p = internal_mmap_shadow((void *)(fixed_addr & ~(PageSize - 1)),
+                                RoundUpTo(size, PageSize),
+                                PROT_READ | PROT_WRITE, flags, fd, 0);
   int reserrno;
   if (internal_iserror(p, &reserrno)) {
     Report("ERROR: %s failed to "
@@ -387,8 +387,8 @@
   unsigned flags = MAP_PRIVATE | MAP_FIXED | MAP_NORESERVE;
   if (fd == -1) flags |= MAP_ANON;
 
-  return (void *)internal_mmap((void *)fixed_addr, size, PROT_NONE, flags, fd,
-                               0);
+  return (void *)internal_mmap_shadow((void *)fixed_addr, size, PROT_NONE,
+                                      flags, fd, 0);
 }
 
 void *MmapNoAccess(uptr size) {
Index: lib/sanitizer_common/sanitizer_rtems.cc
===================================================================
--- lib/sanitizer_common/sanitizer_rtems.cc
+++ lib/sanitizer_common/sanitizer_rtems.cc
@@ -95,6 +95,7 @@
   *tls_addr = *tls_size = 0;
 }
 
+void InitializePlatformEarly() {}
 void MaybeReexec() {}
 void CheckASLR() {}
 void DisableCoreDumperIfNecessary() {}
Index: lib/sanitizer_common/sanitizer_win.cc
===================================================================
--- lib/sanitizer_common/sanitizer_win.cc
+++ lib/sanitizer_common/sanitizer_win.cc
@@ -1025,6 +1025,10 @@
   // Do nothing.
 }
 
+void InitializePlatformEarly() {
+  // Do nothing.
+}
+
 void MaybeReexec() {
   // No need to re-exec on Windows.
 }
Index: lib/tsan/rtl/tsan_rtl.cc
===================================================================
--- lib/tsan/rtl/tsan_rtl.cc
+++ lib/tsan/rtl/tsan_rtl.cc
@@ -359,7 +359,9 @@
   CheckASLR();
   InitializeFlags(&ctx->flags, options);
   AvoidCVE_2016_2143();
-  InitializePlatformEarly();
+  __sanitizer::InitializePlatformEarly();
+  __tsan::InitializePlatformEarly();
+
 #if !SANITIZER_GO
   // Re-exec ourselves if we need to set additional env or command line args.
   MaybeReexec();