Index: lib/builtins/CMakeLists.txt
===================================================================
--- lib/builtins/CMakeLists.txt
+++ lib/builtins/CMakeLists.txt
@@ -132,6 +132,7 @@
   negvdi2.c
   negvsi2.c
   negvti2.c
+  os_version_check.c
   paritydi2.c
   paritysi2.c
   parityti2.c
Index: lib/builtins/os_version_check.c
===================================================================
--- lib/builtins/os_version_check.c
+++ lib/builtins/os_version_check.c
@@ -0,0 +1,115 @@
+/* ===-- os_version_check.c - OS version checking  -------------------------===
+ *
+ *                     The LLVM Compiler Infrastructure
+ *
+ * This file is dual licensed under the MIT and the University of Illinois Open
+ * Source Licenses. See LICENSE.TXT for details.
+ *
+ * ===----------------------------------------------------------------------===
+ *
+ * This file implements the function __isOSVersionAtLeast, used by
+ * Objective-C's @available
+ *
+ * ===----------------------------------------------------------------------===
+ */
+
+#ifdef __APPLE__
+
+#include <CoreFoundation/CoreFoundation.h>
+#include <Dispatch/Dispatch.h>
+#include <TargetConditionals.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* These three variables hold the host's OS version. */
+static int32_t GlobalMajor, GlobalMinor, GlobalSubminor;
+static dispatch_once_t DispatchOnceCounter;
+
+/* Find and parse the SystemVersion.plist file. */
+static void parseSystemVersionPList(void *Unused) {
+  (void)Unused;
+
+  char *PListPath = "/System/Library/CoreServices/SystemVersion.plist";
+
+#if TARGET_OS_SIMULATOR
+  char *PListPathPrefix = getenv("IPHONE_SIMULATOR_ROOT");
+  if (!PListPathPrefix)
+    return;
+  char FullPath[strlen(PListPathPrefix) + strlen(PListPath) + 1];
+  strcpy(FullPath, PListPathPrefix);
+  strcat(FullPath, PListPath);
+  PListPath = FullPath;
+#endif
+  FILE *PropertyList = fopen(PListPath, "r");
+  if (!PropertyList)
+    return;
+
+  /* Dynamically allocated stuff. */
+  CFDictionaryRef PListRef = NULL;
+  CFDataRef FileContentsRef = NULL;
+  UInt8 *PListBuf = NULL;
+
+  fseek(PropertyList, 0, SEEK_END);
+  long PListFileSize = ftell(PropertyList);
+  if (PListFileSize < 0)
+    goto Fail;
+  rewind(PropertyList);
+
+  PListBuf = malloc((size_t)PListFileSize);
+  if (!PListBuf)
+    goto Fail;
+
+  size_t NumRead = fread(PListBuf, 1, (size_t)PListFileSize, PropertyList);
+  if (NumRead != (size_t)PListFileSize)
+    goto Fail;
+
+  /* Get the file buffer into CF's format. We pass in a null allocator here *
+   * because we free PListBuf ourselves */
+  FileContentsRef = CFDataCreateWithBytesNoCopy(
+      NULL, PListBuf, (CFIndex)NumRead, kCFAllocatorNull);
+  if (!FileContentsRef)
+    goto Fail;
+
+  if (!&CFPropertyListCreateWithData)
+    PListRef = CFPropertyListCreateWithData(
+        NULL, FileContentsRef, kCFPropertyListImmutable, NULL, NULL);
+  else
+    PListRef = CFPropertyListCreateFromXMLData(NULL, FileContentsRef,
+                                               kCFPropertyListImmutable, NULL);
+  if (!PListRef)
+    goto Fail;
+
+  CFTypeRef OpaqueValue =
+      CFDictionaryGetValue(PListRef, CFSTR("ProductVersion"));
+  if (!OpaqueValue || CFGetTypeID(OpaqueValue) != CFStringGetTypeID())
+    goto Fail;
+
+  char VersionStr[32];
+  if (!CFStringGetCString((CFStringRef)OpaqueValue, VersionStr,
+                          sizeof(VersionStr), kCFStringEncodingUTF8))
+    goto Fail;
+  sscanf(VersionStr, "%d.%d.%d", &GlobalMajor, &GlobalMinor, &GlobalSubminor);
+
+Fail:
+  if (PListRef)
+    CFRelease(PListRef);
+  if (FileContentsRef)
+    CFRelease(FileContentsRef);
+  free(PListBuf);
+  fclose(PropertyList);
+}
+
+int32_t __isOSVersionAtLeast(int32_t Major, int32_t Minor, int32_t Subminor) {
+  /* Populate the global version variables, if they haven't already. */
+  dispatch_once_f(&DispatchOnceCounter, NULL, parseSystemVersionPList);
+
+  if (Major < GlobalMajor) return 1;
+  if (Major > GlobalMajor) return 0;
+  if (Minor < GlobalMinor) return 1;
+  if (Minor > GlobalMinor) return 0;
+  return Subminor <= GlobalSubminor;
+}
+
+#endif