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
@@ -898,6 +898,7 @@
   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
@@ -1966,6 +1966,10 @@
 
 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.cc
===================================================================
--- lib/sanitizer_common/sanitizer_mac.cc
+++ lib/sanitizer_common/sanitizer_mac.cc
@@ -108,9 +108,20 @@
 #define VM_MEMORY_SANITIZER 99
 #endif
 
+// XNU on Darwin provides a mmap flag that optimizes allocation/deallocation of
+// giant memory regions (i.e. shadow memory regions).
+#define kXnuFastMmapFd 0x4
+static size_t kXnuFastMmapThreshold = 2 << 30; // 2 GB
+static bool use_xnu_fast_mmap = false;
+
 uptr internal_mmap(void *addr, size_t length, int prot, int flags,
                    int fd, u64 offset) {
-  if (fd == -1) fd = VM_MAKE_TAG(VM_MEMORY_SANITIZER);
+  if (fd == -1) {
+    fd = VM_MAKE_TAG(VM_MEMORY_SANITIZER);
+    if (length >= kXnuFastMmapThreshold) {
+      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);
 }
@@ -685,6 +696,16 @@
 
 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_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
@@ -1008,6 +1008,10 @@
   // 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();