diff --git a/tools/clang/include/clang/Basic/BuiltinsX86.def b/tools/clang/include/clang/Basic/BuiltinsX86.def
index cfa38fb..5de5792 100644
--- a/tools/clang/include/clang/Basic/BuiltinsX86.def
+++ b/tools/clang/include/clang/Basic/BuiltinsX86.def
@@ -746,4 +746,6 @@ BUILTIN(__builtin_ia32_xend, "v", "")
 BUILTIN(__builtin_ia32_xabort, "vIc", "")
 BUILTIN(__builtin_ia32_xtest, "i", "")
 
+BUILTIN(__factorial, "UiUi", "n")
+
 #undef BUILTIN
diff --git a/tools/clang/lib/CodeGen/CGBuiltin.cpp b/tools/clang/lib/CodeGen/CGBuiltin.cpp
index 7ad4492..1660f18 100644
--- a/tools/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/tools/clang/lib/CodeGen/CGBuiltin.cpp
@@ -3256,6 +3256,44 @@ Value *CodeGenFunction::EmitX86BuiltinExpr(unsigned BuiltinID,
     Value *F = CGM.getIntrinsic(Intrinsic::x86_avx2_vbroadcasti128);
     return Builder.CreateCall(F, Builder.CreateBitCast(VecTmp, Int8PtrTy));
   }
+  case X86::BI__factorial: {
+    Value *ArgValue = EmitScalarExpr(E->getArg(0));
+    llvm::Type* a_t = ArgValue->getType();
+    BasicBlock* preloop = Builder.GetInsertBlock();
+    Function* curFun = preloop->getParent();
+    BasicBlock* loop = BasicBlock::Create(getGlobalContext(), "fact_loop", curFun);
+    BasicBlock* after = BasicBlock::Create(getGlobalContext(), "finished", curFun);
+ 
+    // Temporarily move the insert point to the top of the function to ensure
+    // that the alloca is in the entry block.
+    IRBuilder<> TmpB(&curFun->getEntryBlock(), curFun->getEntryBlock().begin());
+    Value* tmp = TmpB.CreateAlloca(a_t, 0, "tmp");
+    Value* arg = TmpB.CreateAlloca(a_t, 0, "arg");
+    // back to the plot.
+    unsigned bs = a_t->getIntegerBitWidth();
+    ConstantInt* one = Builder.getInt(APInt(bs, 1));
+    Value *in_test = Builder.CreateICmpULE(ArgValue, one, "nothing.to.do");
+    Value *init = Builder.CreateSelect(in_test, one, ArgValue, "init.res");
+    Builder.CreateStore(init, arg);
+    Builder.CreateStore(ArgValue, tmp);
+    Builder.CreateCondBr(in_test, after, loop);
+    
+    // factorial loop.
+    Builder.SetInsertPoint(loop);
+    Value *dec = Builder.CreateLoad(tmp, "tmp.load");
+    Value *dm1 = Builder.CreateSub(dec, one, "tmp.dec");
+    Builder.CreateStore(dm1, tmp);
+    Value *fact = Builder.CreateLoad(arg, "fact.load");
+    Value *prod = Builder.CreateMul(fact, dm1, "fact.prod");
+    Builder.CreateStore(prod, arg);
+    Value *out_test = Builder.CreateICmpULE(dm1, one, "done");
+    Builder.CreateCondBr(out_test, after, loop);
+    
+    // out.
+    Builder.SetInsertPoint(after);
+    Value *res = Builder.CreateLoad(arg, "result");
+    return res;
+    }
   }
 }