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
@@ -80,6 +80,7 @@
                           uptr *tls_addr, uptr *tls_size);
 
 // Memory management
+extern bool use_xnu_fast_mmap;
 void *MmapOrDie(uptr size, const char *mem_type, bool raw_report = false);
 INLINE void *MmapOrDieQuietly(uptr size, const char *mem_type) {
   return MmapOrDie(size, mem_type, /*raw_report*/ true);
@@ -896,6 +897,7 @@
   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
@@ -86,6 +86,7 @@
   *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
@@ -1950,6 +1950,10 @@
 
 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_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
@@ -496,23 +496,33 @@
   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;
   }
 }
 
@@ -655,6 +665,16 @@
 
 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_posix_libcdep.cc
===================================================================
--- lib/sanitizer_common/sanitizer_posix_libcdep.cc
+++ lib/sanitizer_common/sanitizer_posix_libcdep.cc
@@ -326,10 +326,16 @@
 }
 #endif
 
+// XNU on Darwin provides a mmap flag that optimizes allocation/deallocation of
+// giant memory regions.
+#define kXnuFastMmapFd 0x4
+bool use_xnu_fast_mmap = false;
+
 void *MmapFixedNoReserve(uptr fixed_addr, uptr size, const char *name) {
   int fd = name ? GetNamedMappingFd(name, size) : -1;
   unsigned flags = MAP_PRIVATE | MAP_FIXED | MAP_NORESERVE;
   if (fd == -1) flags |= MAP_ANON;
+  if (fd == -1 && use_xnu_fast_mmap) fd = kXnuFastMmapFd;
 
   uptr PageSize = GetPageSizeCached();
   uptr p = internal_mmap((void *)(fixed_addr & ~(PageSize - 1)),
@@ -382,6 +388,7 @@
   int fd = name ? GetNamedMappingFd(name, size) : -1;
   unsigned flags = MAP_PRIVATE | MAP_FIXED | MAP_NORESERVE;
   if (fd == -1) flags |= MAP_ANON;
+  if (fd == -1 && use_xnu_fast_mmap) fd = kXnuFastMmapFd;
 
   return (void *)internal_mmap((void *)fixed_addr, size, PROT_NONE, flags, fd,
                                0);
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
@@ -1023,6 +1023,10 @@
   // 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
@@ -357,7 +357,9 @@
   CheckASLR();
   InitializeFlags(&ctx->flags, options);
   AvoidCVE_2016_2143();
+  __sanitizer::InitializePlatformEarly();
   InitializePlatformEarly();
+
 #if !SANITIZER_GO
   // Re-exec ourselves if we need to set additional env or command line args.
   MaybeReexec();