diff --git a/lib/Checker/OSAtomicChecker.cpp b/lib/Checker/OSAtomicChecker.cpp
new file mode 100644
index 0000000..7f4aeca
--- /dev/null
+++ b/lib/Checker/OSAtomicChecker.cpp
@@ -0,0 +1,197 @@
+//=== OSAtomicChecker.cpp - OSAtomic functions evaluator --------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This checker evaluates OSAtomic functions.
+//
+//===----------------------------------------------------------------------===//
+
+#include "GRExprEngineInternalChecks.h"
+#include "clang/Checker/PathSensitive/Checker.h"
+#include "clang/Basic/Builtins.h"
+#include "llvm/ADT/StringSwitch.h"
+
+using namespace clang;
+
+namespace {
+
+class OSAtomicChecker : public Checker {
+public:
+  static void *getTag() { static int tag = 0; return &tag; }
+  virtual bool EvalCallExpr(CheckerContext &C, const CallExpr *CE);
+
+private:
+  bool EvalOSAtomicCompareAndSwap(CheckerContext &C, const CallExpr *CE);
+};
+
+}
+
+void clang::RegisterOSAtomicChecker(GRExprEngine &Eng) {
+  Eng.registerCheck(new OSAtomicChecker());
+}
+
+bool OSAtomicChecker::EvalCallExpr(CheckerContext &C,const CallExpr *CE) {
+  const GRState *state = C.getState();
+  const Expr *Callee = CE->getCallee();
+  SVal L = state->getSVal(Callee);
+
+  const FunctionDecl* FD = L.getAsFunctionDecl();
+  if (!FD)
+    return false;
+
+  const IdentifierInfo *II = FD->getIdentifier();
+  if (!II)
+    return false;
+  
+  llvm::StringRef FName(II->getName());
+
+  // Check for compare and swap.
+  if (FName.startswith("OSAtomicCompareAndSwap") ||
+      FName.startswith("objc_atomicCompareAndSwap"))
+    return EvalOSAtomicCompareAndSwap(C, CE);
+
+  // FIXME: Other atomics.
+  return false;
+}
+
+bool OSAtomicChecker::EvalOSAtomicCompareAndSwap(CheckerContext &C, 
+                                                 const CallExpr *CE) {
+  // Not enough arguments to match OSAtomicCompareAndSwap?
+  if (CE->getNumArgs() != 3)
+    return false;
+
+  ASTContext &Ctx = C.getASTContext();
+  const Expr *oldValueExpr = CE->getArg(0);
+  QualType oldValueType = Ctx.getCanonicalType(oldValueExpr->getType());
+
+  const Expr *newValueExpr = CE->getArg(1);
+  QualType newValueType = Ctx.getCanonicalType(newValueExpr->getType());
+
+  // Do the types of 'oldValue' and 'newValue' match?
+  if (oldValueType != newValueType)
+    return false;
+
+  const Expr *theValueExpr = CE->getArg(2);
+  const PointerType *theValueType=theValueExpr->getType()->getAs<PointerType>();
+
+  // theValueType not a pointer?
+  if (!theValueType)
+    return false;
+
+  QualType theValueTypePointee =
+    Ctx.getCanonicalType(theValueType->getPointeeType()).getUnqualifiedType();
+
+  // The pointee must match newValueType and oldValueType.
+  if (theValueTypePointee != newValueType)
+    return false;
+
+  static unsigned magic_load = 0;
+  static unsigned magic_store = 0;
+
+  const void *OSAtomicLoadTag = &magic_load;
+  const void *OSAtomicStoreTag = &magic_store;
+
+  // Load 'theValue'.
+  GRExprEngine &Engine = C.getEngine();
+  const GRState *state = C.getState();
+  ExplodedNodeSet Tmp;
+  SVal location = state->getSVal(theValueExpr);
+  // Here we should use the value type of the region as the load type.
+  QualType LoadTy;
+  if (const TypedRegion *TR =
+      dyn_cast_or_null<TypedRegion>(location.getAsRegion())) {
+    LoadTy = TR->getValueType(Ctx);
+  }
+  Engine.EvalLoad(Tmp, const_cast<Expr *>(theValueExpr), C.getPredecessor(), 
+                  state, location, OSAtomicLoadTag, LoadTy);
+
+  if (Tmp.empty()) {
+    // If no nodes were generated, other checkers must generated sinks. But 
+    // since the builder state was restored, we set it manually to prevent 
+    // auto transition.
+    // FIXME: there should be a better approach.
+    C.getNodeBuilder().BuildSinks = true;
+    return true;
+  }
+ 
+  for (ExplodedNodeSet::iterator I = Tmp.begin(), E = Tmp.end();
+       I != E; ++I) {
+
+    ExplodedNode *N = *I;
+    const GRState *stateLoad = N->getState();
+    SVal theValueVal_untested = stateLoad->getSVal(theValueExpr);
+    SVal oldValueVal_untested = stateLoad->getSVal(oldValueExpr);
+
+    // FIXME: Issue an error.
+    if (theValueVal_untested.isUndef() || oldValueVal_untested.isUndef()) {
+      return false;
+    }
+    
+    DefinedOrUnknownSVal theValueVal =
+      cast<DefinedOrUnknownSVal>(theValueVal_untested);
+    DefinedOrUnknownSVal oldValueVal =
+      cast<DefinedOrUnknownSVal>(oldValueVal_untested);
+
+    SValuator &SVator = Engine.getSValuator();
+
+    // Perform the comparison.
+    DefinedOrUnknownSVal Cmp = SVator.EvalEQ(stateLoad,theValueVal,oldValueVal);
+
+    const GRState *stateEqual = stateLoad->Assume(Cmp, true);
+
+    // Were they equal?
+    if (stateEqual) {
+      // Perform the store.
+      ExplodedNodeSet TmpStore;
+      SVal val = stateEqual->getSVal(newValueExpr);
+
+      // Handle implicit value casts.
+      if (const TypedRegion *R =
+          dyn_cast_or_null<TypedRegion>(location.getAsRegion())) {
+        val = SVator.EvalCast(val,R->getValueType(Ctx),newValueExpr->getType());
+      }
+
+      Engine.EvalStore(TmpStore, NULL, const_cast<Expr *>(theValueExpr), N, 
+                       stateEqual, location, val, OSAtomicStoreTag);
+
+      if (TmpStore.empty()) {
+        // If no nodes were generated, other checkers must generated sinks. But 
+        // since the builder state was restored, we set it manually to prevent 
+        // auto transition.
+        // FIXME: there should be a better approach.
+        C.getNodeBuilder().BuildSinks = true;
+        return true;
+      }
+
+      // Now bind the result of the comparison.
+      for (ExplodedNodeSet::iterator I2 = TmpStore.begin(),
+           E2 = TmpStore.end(); I2 != E2; ++I2) {
+        ExplodedNode *predNew = *I2;
+        const GRState *stateNew = predNew->getState();
+        // Check for 'void' return type if we have a bogus function prototype.
+        SVal Res = UnknownVal();
+        QualType T = CE->getType();
+        if (!T->isVoidType())
+          Res = Engine.getValueManager().makeTruthVal(true, T);
+        C.GenerateNode(stateNew->BindExpr(CE, Res), predNew);
+      }
+    }
+
+    // Were they not equal?
+    if (const GRState *stateNotEqual = stateLoad->Assume(Cmp, false)) {
+      // Check for 'void' return type if we have a bogus function prototype.
+      SVal Res = UnknownVal();
+      QualType T = CE->getType();
+      if (!T->isVoidType())
+        Res = Engine.getValueManager().makeTruthVal(false, CE->getType());
+      C.GenerateNode(stateNotEqual->BindExpr(CE, Res), N);
+    }
+  }
+
+  return true;
+}