Index: lib/CodeGen/CGExprScalar.cpp
===================================================================
--- lib/CodeGen/CGExprScalar.cpp
+++ 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.EmitObjCIsOSVersionAtLeast(Args);
+  }
+
   Value *VisitArraySubscriptExpr(ArraySubscriptExpr *E);
   Value *VisitShuffleVectorExpr(ShuffleVectorExpr *E);
   Value *VisitConvertVectorExpr(ConvertVectorExpr *E);
Index: lib/CodeGen/CGObjC.cpp
===================================================================
--- lib/CodeGen/CGObjC.cpp
+++ lib/CodeGen/CGObjC.cpp
@@ -3399,5 +3399,21 @@
   return Val;
 }
 
+llvm::Value *
+CodeGenFunction::EmitObjCIsOSVersionAtLeast(ArrayRef<llvm::Value *> Args) {
+  assert(Args.size() == 3 && "Expected 3 argument here!");
+
+  if (!CGM.ObjCIsOSVersionAtLeastFn) {
+    llvm::FunctionType *FTy =
+        llvm::FunctionType::get(Int32Ty, {Int32Ty, Int32Ty, Int32Ty}, false);
+    CGM.ObjCIsOSVersionAtLeastFn =
+        CGM.CreateRuntimeFunction(FTy, "_IsOSVersionAtLeast");
+  }
+
+  llvm::Value *CallRes =
+      EmitNounwindRuntimeCall(CGM.ObjCIsOSVersionAtLeastFn, Args);
+
+  return Builder.CreateICmpNE(CallRes, llvm::Constant::getNullValue(Int32Ty));
+}
 
 CGObjCRuntime::~CGObjCRuntime() {}
Index: lib/CodeGen/CodeGenFunction.h
===================================================================
--- lib/CodeGen/CodeGenFunction.h
+++ lib/CodeGen/CodeGenFunction.h
@@ -2476,6 +2476,8 @@
   void EmitObjCAtSynchronizedStmt(const ObjCAtSynchronizedStmt &S);
   void EmitObjCAutoreleasePoolStmt(const ObjCAutoreleasePoolStmt &S);
 
+  llvm::Value *EmitObjCIsOSVersionAtLeast(ArrayRef<llvm::Value *> Args);
+
   void EmitCoroutineBody(const CoroutineBodyStmt &S);
   RValue EmitCoroutineIntrinsic(const CallExpr *E, unsigned int IID);
 
Index: lib/CodeGen/CodeGenModule.h
===================================================================
--- lib/CodeGen/CodeGenModule.h
+++ 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 *ObjCIsOSVersionAtLeastFn = nullptr;
+
   InstrProfStats &getPGOStats() { return PGOStats; }
   llvm::IndexedInstrProfReader *getPGOReader() const { return PGOReader.get(); }
 
Index: test/CodeGenObjC/availability-check.m
===================================================================
--- test/CodeGenObjC/availability-check.m
+++ test/CodeGenObjC/availability-check.m
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.11 -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -xc -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, *))
+    ;
+  // This check should be folded: our deployment target is 10.11.
+  // CHECK-NOT: call i32 @_IsOSVersionAtLeast
+  if (__builtin_available(macos 10.11, *))
+    ;
+}
+
+// CHECK: declare i32 @_IsOSVersionAtLeast(i32, i32, i32)