Index: libcxx/trunk/include/math.h
===================================================================
--- libcxx/trunk/include/math.h
+++ libcxx/trunk/include/math.h
@@ -1553,6 +1553,40 @@
 typename std::enable_if<std::is_integral<_A1>::value, double>::type
 trunc(_A1 __lcpp_x) _NOEXCEPT {return ::trunc((double)__lcpp_x);}
 
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _IntT, class _FloatT,
+    bool _FloatBigger = (numeric_limits<_FloatT>::digits > numeric_limits<_IntT>::digits),
+    int _Bits = (numeric_limits<_IntT>::digits - numeric_limits<_FloatT>::digits)>
+_LIBCPP_INLINE_VISIBILITY
+_LIBCPP_CONSTEXPR _IntT __max_representable_int_for_float() _NOEXCEPT {
+  static_assert(is_floating_point<_FloatT>::value, "must be a floating point type");
+  static_assert(is_integral<_IntT>::value, "must be an integral type");
+  static_assert(numeric_limits<_FloatT>::radix == 2, "FloatT has incorrect radix");
+  static_assert(_IsSame<_FloatT, float>::value || _IsSame<_FloatT, double>::value
+               || _IsSame<_FloatT,long double>::value, "unsupported floating point type");
+  return _FloatBigger ? numeric_limits<_IntT>::max() :  (numeric_limits<_IntT>::max() >> _Bits << _Bits);
+}
+
+// Convert a floating point number to the specified integral type after
+// clamping to the integral types representable range.
+//
+// The behavior is undefined if `__r` is NaN.
+template <class _IntT, class _RealT>
+_LIBCPP_INLINE_VISIBILITY
+_IntT __clamp_to_integral(_RealT __r) _NOEXCEPT {
+  using _Lim = std::numeric_limits<_IntT>;
+  const _IntT _MaxVal = std::__max_representable_int_for_float<_IntT, _RealT>();
+  if (__r >= ::nextafter(static_cast<_RealT>(_MaxVal), INFINITY)) {
+    return _Lim::max();
+  } else if (__r <= _Lim::lowest()) {
+    return _Lim::min();
+  }
+  return static_cast<_IntT>(__r);
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
 } // extern "C++"
 
 #endif // __cplusplus
Index: libcxx/trunk/test/libcxx/numerics/clamp_to_integral.pass.cpp
===================================================================
--- libcxx/trunk/test/libcxx/numerics/clamp_to_integral.pass.cpp
+++ libcxx/trunk/test/libcxx/numerics/clamp_to_integral.pass.cpp
@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// __clamp_to_integral<IntT>(RealT)
+
+// Test the conversion function that truncates floating point types to the
+// closest representable value for the specified integer type, or
+// numeric_limits<IntT>::max()/min() if the value isn't representable.
+
+#include <limits>
+#include <cassert>
+#include <cmath>
+
+template <class IntT>
+void test() {
+  typedef std::numeric_limits<IntT> Lim;
+  const bool MaxIsRepresentable = sizeof(IntT) < 8;
+  const bool IsSigned = std::is_signed<IntT>::value;
+  struct TestCase {
+    double Input;
+    IntT Expect;
+    bool IsRepresentable;
+  } TestCases[] = {
+      {0, 0, true},
+      {1, 1, true},
+      {IsSigned ? static_cast<IntT>(-1) : 0,
+       IsSigned ? static_cast<IntT>(-1) : 0, true},
+      {Lim::lowest(), Lim::lowest(), true},
+      {static_cast<double>(Lim::max()), Lim::max(), MaxIsRepresentable},
+      {static_cast<double>(Lim::max()) + 1, Lim::max(), false},
+      {static_cast<double>(Lim::max()) + 1024, Lim::max(), false},
+      {nextafter(static_cast<double>(Lim::max()), INFINITY), Lim::max(), false},
+  };
+  for (TestCase TC : TestCases) {
+    auto res = std::__clamp_to_integral<IntT>(TC.Input);
+    assert(res == TC.Expect);
+    if (TC.IsRepresentable) {
+      auto other = static_cast<IntT>(std::trunc(TC.Input));
+      assert(res == other);
+    } else
+      assert(res == Lim::min() || res == Lim::max());
+  }
+}
+
+template <class IntT>
+void test_float() {
+  typedef std::numeric_limits<IntT> Lim;
+  const bool MaxIsRepresentable = sizeof(IntT) < 4;
+  ((void)MaxIsRepresentable);
+  const bool IsSigned = std::is_signed<IntT>::value;
+  struct TestCase {
+    float Input;
+    IntT Expect;
+    bool IsRepresentable;
+  } TestCases[] = {
+      {0, 0, true},
+      {1, 1, true},
+      {IsSigned ? static_cast<IntT>(-1) : 0,
+       IsSigned ? static_cast<IntT>(-1) : 0, true},
+      {Lim::lowest(), Lim::lowest(), true},
+      {static_cast<float>(Lim::max()), Lim::max(), MaxIsRepresentable },
+       {nextafter(static_cast<float>(Lim::max()), INFINITY), Lim::max(), false},
+  };
+  for (TestCase TC : TestCases) {
+    auto res = std::__clamp_to_integral<IntT>(TC.Input);
+    assert(res == TC.Expect);
+    if (TC.IsRepresentable) {
+      auto other = static_cast<IntT>(std::trunc(TC.Input));
+      assert(res == other);
+    } else
+      assert(res == Lim::min() || res == Lim::max());
+  }
+}
+
+int main() {
+  test<short>();
+  test<unsigned short>();
+  test<int>();
+  test<unsigned>();
+  test<long long>();
+  test<unsigned long long>();
+  test_float<short>();
+  test_float<int>();
+  test_float<long long>();
+}