diff --git a/src/base/platform/platform-posix.cc b/src/base/platform/platform-posix.cc
index 9759febf2a..97453b83a6 100644
--- a/src/base/platform/platform-posix.cc
+++ b/src/base/platform/platform-posix.cc
@@ -1130,7 +1130,47 @@ static pthread_key_t LocalKeyToPthreadKey(Thread::LocalStorageKey local_key) {
 #endif
 }
 
-#if defined(V8_FAST_TLS_SUPPORTED) && defined(DEBUG)
+
+#ifdef V8_FAST_TLS_SUPPORTED
+
+static std::atomic<bool> tls_base_offset_initialized{false};
+intptr_t kMacTlsBaseOffset = 0;
+
+// It's safe to do the initialization more that once, but it has to be
+// done at least once.
+static void InitializeTlsBaseOffset() {
+  const size_t kBufferSize = 128;
+  char buffer[kBufferSize];
+  size_t buffer_size = kBufferSize;
+  int ctl_name[] = { CTL_KERN , KERN_OSRELEASE };
+  if (sysctl(ctl_name, 2, buffer, &buffer_size, nullptr, 0) != 0) {
+    FATAL("V8 failed to get kernel version");
+  }
+  // The buffer now contains a string of the form XX.YY.ZZ, where
+  // XX is the major kernel version component.
+  // Make sure the buffer is 0-terminated.
+  buffer[kBufferSize - 1] = '\0';
+  char* period_pos = strchr(buffer, '.');
+  *period_pos = '\0';
+  int kernel_version_major = static_cast<int>(strtol(buffer, nullptr, 10));
+  // The constants below are taken from pthreads.s from the XNU kernel
+  // sources archive at www.opensource.apple.com.
+  if (kernel_version_major < 11) {
+    // 8.x.x (Tiger), 9.x.x (Leopard), 10.x.x (Snow Leopard) have the
+    // same offsets.
+#if V8_HOST_ARCH_IA32
+    kMacTlsBaseOffset = 0x48;
+#else
+    kMacTlsBaseOffset = 0x60;
+#endif
+  } else {
+    // 11.x.x (Lion) changed the offset.
+    kMacTlsBaseOffset = 0;
+  }
+
+  tls_base_offset_initialized.store(true, std::memory_order_release);
+}
+
 
 static void CheckFastTls(Thread::LocalStorageKey key) {
   void* expected = reinterpret_cast<void*>(0x1234CAFE);
@@ -1142,20 +1182,30 @@ static void CheckFastTls(Thread::LocalStorageKey key) {
   Thread::SetThreadLocal(key, nullptr);
 }
 
-#endif  // defined(V8_FAST_TLS_SUPPORTED) && defined(DEBUG)
+#endif  // V8_FAST_TLS_SUPPORTED
+
 
 Thread::LocalStorageKey Thread::CreateThreadLocalKey() {
+#ifdef V8_FAST_TLS_SUPPORTED
+  bool check_fast_tls = false;
+  if (!tls_base_offset_initialized.load(std::memory_order_acquire)) {
+    check_fast_tls = true;
+    InitializeTlsBaseOffset();
+  }
+#endif
   pthread_key_t key;
   int result = pthread_key_create(&key, nullptr);
   DCHECK_EQ(0, result);
   USE(result);
   LocalStorageKey local_key = PthreadKeyToLocalKey(key);
-#if defined(V8_FAST_TLS_SUPPORTED) && defined(DEBUG)
-  CheckFastTls(local_key);
+#ifdef V8_FAST_TLS_SUPPORTED
+  // If we just initialized fast TLS support, make sure it works.
+  if (check_fast_tls) CheckFastTls(local_key);
 #endif
   return local_key;
 }
 
+
 void Thread::DeleteThreadLocalKey(LocalStorageKey key) {
   pthread_key_t pthread_key = LocalKeyToPthreadKey(key);
   int result = pthread_key_delete(pthread_key);
diff --git a/src/base/platform/platform.h b/src/base/platform/platform.h
index 95c0952371..f67d5c079a 100644
--- a/src/base/platform/platform.h
+++ b/src/base/platform/platform.h
@@ -71,7 +71,9 @@ namespace base {
 
 #define V8_FAST_TLS_SUPPORTED 1
 
-V8_INLINE intptr_t InternalGetExistingThreadLocal(intptr_t index) {
+V8_INLINE intptr_t InternalGetExistingThreadLocal(intptr_t index);
+
+inline intptr_t InternalGetExistingThreadLocal(intptr_t index) {
   const intptr_t kTibInlineTlsOffset = 0xE10;
   const intptr_t kTibExtraTlsOffset = 0xF94;
   const intptr_t kMaxInlineSlots = 64;
@@ -89,8 +91,6 @@ V8_INLINE intptr_t InternalGetExistingThreadLocal(intptr_t index) {
                                                   (index - kMaxInlineSlots));
 }
 
-// Not possible on ARM64, the register holding the base pointer is not stable
-// across major releases.
 #elif defined(__APPLE__) && (V8_HOST_ARCH_IA32 || V8_HOST_ARCH_X64)
 
 // tvOS simulator does not use intptr_t as TLS key.
@@ -98,14 +98,20 @@ V8_INLINE intptr_t InternalGetExistingThreadLocal(intptr_t index) {
 
 #define V8_FAST_TLS_SUPPORTED 1
 
-V8_INLINE intptr_t InternalGetExistingThreadLocal(intptr_t index) {
+extern V8_BASE_EXPORT intptr_t kMacTlsBaseOffset;
+
+V8_INLINE intptr_t InternalGetExistingThreadLocal(intptr_t index);
+
+inline intptr_t InternalGetExistingThreadLocal(intptr_t index) {
   intptr_t result;
 #if V8_HOST_ARCH_IA32
-  asm("movl %%gs:(,%1,4), %0;"
-      : "=r"(result)  // Output must be a writable register.
-      : "r"(index));
+  asm("movl %%gs:(%1,%2,4), %0;"
+      :"=r"(result)  // Output must be a writable register.
+      :"r"(kMacTlsBaseOffset), "r"(index));
 #else
-  asm("movq %%gs:(,%1,8), %0;" : "=r"(result) : "r"(index));
+  asm("movq %%gs:(%1,%2,8), %0;"
+      :"=r"(result)
+      :"r"(kMacTlsBaseOffset), "r"(index));
 #endif
   return result;
 }