Index: cfe/trunk/lib/CodeGen/CGExprScalar.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CGExprScalar.cpp
+++ cfe/trunk/lib/CodeGen/CGExprScalar.cpp
@@ -300,6 +300,24 @@
     return V;
   }
 
+  Value *VisitObjCAvailabilityCheckExpr(ObjCAvailabilityCheckExpr *E) {
+    VersionTuple Version = E->getVersion();
+
+    // If we're checking for a platform older than our minimum deployment
+    // target, we can fold the check away.
+    if (Version <= CGF.CGM.getTarget().getPlatformMinVersion())
+      return llvm::ConstantInt::get(Builder.getInt1Ty(), 1);
+
+    Optional<unsigned> Min = Version.getMinor(), SMin = Version.getSubminor();
+    llvm::Value *Args[] = {
+        llvm::ConstantInt::get(CGF.CGM.Int32Ty, Version.getMajor()),
+        llvm::ConstantInt::get(CGF.CGM.Int32Ty, Min ? *Min : 0),
+        llvm::ConstantInt::get(CGF.CGM.Int32Ty, SMin ? *SMin : 0),
+    };
+
+    return CGF.EmitBuiltinAvailable(Args);
+  }
+
   Value *VisitArraySubscriptExpr(ArraySubscriptExpr *E);
   Value *VisitShuffleVectorExpr(ShuffleVectorExpr *E);
   Value *VisitConvertVectorExpr(ConvertVectorExpr *E);
Index: cfe/trunk/lib/CodeGen/CGObjC.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CGObjC.cpp
+++ cfe/trunk/lib/CodeGen/CGObjC.cpp
@@ -3399,5 +3399,21 @@
   return Val;
 }
 
+llvm::Value *
+CodeGenFunction::EmitBuiltinAvailable(ArrayRef<llvm::Value *> Args) {
+  assert(Args.size() == 3 && "Expected 3 argument here!");
+
+  if (!CGM.IsOSVersionAtLeastFn) {
+    llvm::FunctionType *FTy =
+        llvm::FunctionType::get(Int32Ty, {Int32Ty, Int32Ty, Int32Ty}, false);
+    CGM.IsOSVersionAtLeastFn =
+        CGM.CreateRuntimeFunction(FTy, "__isOSVersionAtLeast");
+  }
+
+  llvm::Value *CallRes =
+      EmitNounwindRuntimeCall(CGM.IsOSVersionAtLeastFn, Args);
+
+  return Builder.CreateICmpNE(CallRes, llvm::Constant::getNullValue(Int32Ty));
+}
 
 CGObjCRuntime::~CGObjCRuntime() {}
Index: cfe/trunk/lib/CodeGen/CodeGenFunction.h
===================================================================
--- cfe/trunk/lib/CodeGen/CodeGenFunction.h
+++ cfe/trunk/lib/CodeGen/CodeGenFunction.h
@@ -3169,6 +3169,8 @@
 public:
   llvm::Value *EmitMSVCBuiltinExpr(MSVCIntrin BuiltinID, const CallExpr *E);
 
+  llvm::Value *EmitBuiltinAvailable(ArrayRef<llvm::Value *> Args);
+
   llvm::Value *EmitObjCProtocolExpr(const ObjCProtocolExpr *E);
   llvm::Value *EmitObjCStringLiteral(const ObjCStringLiteral *E);
   llvm::Value *EmitObjCBoxedExpr(const ObjCBoxedExpr *E);
Index: cfe/trunk/lib/CodeGen/CodeGenModule.h
===================================================================
--- cfe/trunk/lib/CodeGen/CodeGenModule.h
+++ cfe/trunk/lib/CodeGen/CodeGenModule.h
@@ -546,6 +546,10 @@
     return *ObjCData;
   }
 
+  // Version checking function, used to implement ObjC's @available:
+  // i32 @__isOSVersionAtLeast(i32, i32, i32)
+  llvm::Constant *IsOSVersionAtLeastFn = nullptr;
+
   InstrProfStats &getPGOStats() { return PGOStats; }
   llvm::IndexedInstrProfReader *getPGOReader() const { return PGOReader.get(); }
 
Index: cfe/trunk/test/CodeGenObjC/availability-check.m
===================================================================
--- cfe/trunk/test/CodeGenObjC/availability-check.m
+++ cfe/trunk/test/CodeGenObjC/availability-check.m
@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.11 -emit-llvm -o - %s | FileCheck %s
+
+void use_at_available() {
+  // CHECK: call i32 @__isOSVersionAtLeast(i32 10, i32 12, i32 0)
+  // CHECK-NEXT: icmp ne
+  if (__builtin_available(macos 10.12, *))
+    ;
+
+  // CHECK: call i32 @__isOSVersionAtLeast(i32 10, i32 12, i32 0)
+  // CHECK-NEXT: icmp ne
+  if (@available(macos 10.12, *))
+    ;
+
+  // CHECK: call i32 @__isOSVersionAtLeast(i32 10, i32 12, i32 42)
+  // CHECK-NEXT: icmp ne
+  if (__builtin_available(ios 10, macos 10.12.42, *))
+    ;
+
+  // CHECK-NOT: call i32 @__isOSVersionAtLeast
+  // CHECK: br i1 true
+  if (__builtin_available(ios 10, *))
+    ;
+
+  // This check should be folded: our deployment target is 10.11.
+  // CHECK-NOT: call i32 @__isOSVersionAtLeast
+  // CHECK: br i1 true
+  if (__builtin_available(macos 10.11, *))
+    ;
+}
+
+// CHECK: declare i32 @__isOSVersionAtLeast(i32, i32, i32)