Index: libcxx/include/charconv
===================================================================
--- libcxx/include/charconv
+++ libcxx/include/charconv
@@ -94,6 +94,8 @@
 namespace __itoa {
 _LIBCPP_FUNC_VIS char* __u64toa(uint64_t __value, char* __buffer);
 _LIBCPP_FUNC_VIS char* __u32toa(uint32_t __value, char* __buffer);
+_LIBCPP_FUNC_VIS wchar_t* __u64toa(uint64_t __value, wchar_t* __buffer);
+_LIBCPP_FUNC_VIS wchar_t* __u32toa(uint32_t __value, wchar_t* __buffer);
 }
 
 #ifndef _LIBCPP_CXX03_LANG
@@ -124,6 +126,21 @@
 namespace __itoa
 {
 
+template <typename _Cp>
+inline _LIBCPP_INLINE_VISIBILITY _Cp __minus();
+
+template <>
+inline _LIBCPP_INLINE_VISIBILITY char __minus()
+{
+    return '-';
+}
+
+template <>
+inline _LIBCPP_INLINE_VISIBILITY wchar_t __minus()
+{
+    return L'-';
+}
+
 static _LIBCPP_CONSTEXPR uint64_t __pow10_64[] = {
     UINT64_C(0),
     UINT64_C(10),
@@ -167,7 +184,8 @@
     }
 #endif
 
-    static _LIBCPP_INLINE_VISIBILITY char* __convert(_Tp __v, char* __p)
+    template <typename _Cp>
+    static _LIBCPP_INLINE_VISIBILITY _Cp* __convert(_Tp __v, _Cp* __p)
     {
         return __u64toa(__v, __p);
     }
@@ -189,7 +207,8 @@
     }
 #endif
 
-    static _LIBCPP_INLINE_VISIBILITY char* __convert(_Tp __v, char* __p)
+    template <typename _Cp>
+    static _LIBCPP_INLINE_VISIBILITY _Cp* __convert(_Tp __v, _Cp* __p)
     {
         return __u32toa(__v, __p);
     }
@@ -291,23 +310,36 @@
     return static_cast<typename make_unsigned<_Tp>::type>(__x);
 }
 
-template <typename _Tp>
+template <typename _Cp>
+struct _LIBCPP_HIDDEN __to_chars_itoa_result
+{
+    _Cp* ptr;
+    errc ec;
+};
+
 inline _LIBCPP_INLINE_VISIBILITY to_chars_result
-__to_chars_itoa(char* __first, char* __last, _Tp __value, true_type)
+__make_to_chars_result(__to_chars_itoa_result<char> __x)
+{
+    return {__x.ptr, __x.ec};
+}
+
+template <typename _Tp, typename _Cp>
+inline _LIBCPP_INLINE_VISIBILITY __to_chars_itoa_result<_Cp>
+__to_chars_itoa(_Cp* __first, _Cp* __last, _Tp __value, true_type)
 {
     auto __x = __to_unsigned(__value);
     if (__value < 0 && __first != __last)
     {
-        *__first++ = '-';
+        *__first++ = __itoa::__minus<_Cp>();
         __x = __complement(__x);
     }
 
     return __to_chars_itoa(__first, __last, __x, false_type());
 }
 
-template <typename _Tp>
-inline _LIBCPP_INLINE_VISIBILITY to_chars_result
-__to_chars_itoa(char* __first, char* __last, _Tp __value, false_type)
+template <typename _Tp, typename _Cp>
+inline _LIBCPP_INLINE_VISIBILITY __to_chars_itoa_result<_Cp>
+__to_chars_itoa(_Cp* __first, _Cp* __last, _Tp __value, false_type)
 {
     using __tx = __itoa::__traits<_Tp>;
     auto __diff = __last - __first;
@@ -322,12 +354,12 @@
         return {__tx::__convert(__value, __first), {}};
     else
     {
-        char __buf[__tx::digits];
+        _Cp __buf[__tx::digits];
         auto __p = __tx::__convert(__value, __buf);
         auto __len = __p - __buf;
         if (__len <= __diff)
         {
-            memcpy(__first, __buf, __len);
+            memcpy(__first, __buf, __len * sizeof(_Cp));
             return {__first + __len, {}};
         }
         else
@@ -357,7 +389,10 @@
                     false_type)
 {
     if (__base == 10)
-        return __to_chars_itoa(__first, __last, __value, false_type());
+    {
+        const auto res = __to_chars_itoa(__first, __last, __value, false_type());
+        return __make_to_chars_result(res);
+    }
 
     auto __p = __last;
     while (__p != __first)
@@ -383,7 +418,8 @@
 inline _LIBCPP_INLINE_VISIBILITY to_chars_result
 to_chars(char* __first, char* __last, _Tp __value)
 {
-    return __to_chars_itoa(__first, __last, __value, is_signed<_Tp>());
+    const auto res = __to_chars_itoa(__first, __last, __value, is_signed<_Tp>());
+    return __make_to_chars_result(res);
 }
 
 template <typename _Tp, typename enable_if<is_integral<_Tp>::value, int>::type = 0>
Index: libcxx/src/charconv.cpp
===================================================================
--- libcxx/src/charconv.cpp
+++ libcxx/src/charconv.cpp
@@ -30,38 +30,79 @@
     '9', '0', '9', '1', '9', '2', '9', '3', '9', '4', '9', '5', '9', '6', '9',
     '7', '9', '8', '9', '9'};
 
-template <typename T>
-inline _LIBCPP_INLINE_VISIBILITY char*
-append1(char* buffer, T i)
+static constexpr wchar_t cDigitsLutW[200] = {
+    L'0', L'0', L'0', L'1', L'0', L'2', L'0', L'3', L'0', L'4', L'0', L'5',
+    L'0', L'6', L'0', L'7', L'0', L'8', L'0', L'9', L'1', L'0', L'1', L'1',
+    L'1', L'2', L'1', L'3', L'1', L'4', L'1', L'5', L'1', L'6', L'1', L'7',
+    L'1', L'8', L'1', L'9', L'2', L'0', L'2', L'1', L'2', L'2', L'2', L'3',
+    L'2', L'4', L'2', L'5', L'2', L'6', L'2', L'7', L'2', L'8', L'2', L'9',
+    L'3', L'0', L'3', L'1', L'3', L'2', L'3', L'3', L'3', L'4', L'3', L'5',
+    L'3', L'6', L'3', L'7', L'3', L'8', L'3', L'9', L'4', L'0', L'4', L'1',
+    L'4', L'2', L'4', L'3', L'4', L'4', L'4', L'5', L'4', L'6', L'4', L'7',
+    L'4', L'8', L'4', L'9', L'5', L'0', L'5', L'1', L'5', L'2', L'5', L'3',
+    L'5', L'4', L'5', L'5', L'5', L'6', L'5', L'7', L'5', L'8', L'5', L'9',
+    L'6', L'0', L'6', L'1', L'6', L'2', L'6', L'3', L'6', L'4', L'6', L'5',
+    L'6', L'6', L'6', L'7', L'6', L'8', L'6', L'9', L'7', L'0', L'7', L'1',
+    L'7', L'2', L'7', L'3', L'7', L'4', L'7', L'5', L'7', L'6', L'7', L'7',
+    L'7', L'8', L'7', L'9', L'8', L'0', L'8', L'1', L'8', L'2', L'8', L'3',
+    L'8', L'4', L'8', L'5', L'8', L'6', L'8', L'7', L'8', L'8', L'8', L'9',
+    L'9', L'0', L'9', L'1', L'9', L'2', L'9', L'3', L'9', L'4', L'9', L'5',
+    L'9', L'6', L'9', L'7', L'9', L'8', L'9', L'9'};
+
+template <typename C>
+struct itoa_char_traits;
+
+template <>
+struct itoa_char_traits<char>
+{
+    static constexpr _LIBCPP_INLINE_VISIBILITY char zero() { return '0'; }
+
+    static constexpr _LIBCPP_INLINE_VISIBILITY const char*
+    digitlut(int i) { return cDigitsLut + i; }
+};
+
+template <>
+struct itoa_char_traits<wchar_t>
 {
-    *buffer = '0' + static_cast<char>(i);
+    static constexpr _LIBCPP_INLINE_VISIBILITY wchar_t zero() { return L'0'; }
+
+    static constexpr _LIBCPP_INLINE_VISIBILITY const wchar_t*
+    digitlut(int i) { return cDigitsLutW + i; }
+};
+
+template <typename T, typename C>
+inline _LIBCPP_INLINE_VISIBILITY C*
+append1(C* buffer, T i)
+{
+    *buffer = itoa_char_traits<C>::zero() + static_cast<C>(i);
     return buffer + 1;
 }
 
-template <typename T>
-inline _LIBCPP_INLINE_VISIBILITY char*
-append2(char* buffer, T i)
+template <typename T, typename C>
+inline _LIBCPP_INLINE_VISIBILITY C*
+append2(C* buffer, T i)
 {
-    memcpy(buffer, &cDigitsLut[(i)*2], 2);
+    memcpy(buffer, itoa_char_traits<C>::digitlut(i * 2), 2 * sizeof(C));
     return buffer + 2;
 }
 
-template <typename T>
-inline _LIBCPP_INLINE_VISIBILITY char*
-append3(char* buffer, T i)
+template <typename T, typename C>
+inline _LIBCPP_INLINE_VISIBILITY C*
+append3(C* buffer, T i)
 {
     return append2(append1(buffer, (i) / 100), (i) % 100);
 }
 
-template <typename T>
-inline _LIBCPP_INLINE_VISIBILITY char*
-append4(char* buffer, T i)
+template <typename T, typename C>
+inline _LIBCPP_INLINE_VISIBILITY C*
+append4(C* buffer, T i)
 {
     return append2(append2(buffer, (i) / 100), (i) % 100);
 }
 
-char*
-__u32toa(uint32_t value, char* buffer)
+template <typename C>
+C*
+__u32toa_t(uint32_t value, C* buffer)
 {
     if (value < 10000)
     {
@@ -121,8 +162,9 @@
     return buffer;
 }
 
-char*
-__u64toa(uint64_t value, char* buffer)
+template <typename C>
+C*
+__u64toa_t(uint64_t value, C* buffer)
 {
     if (value < 100000000)
     {
@@ -227,6 +269,26 @@
     return buffer;
 }
 
+char* __u32toa(uint32_t value, char* buffer)
+{
+    return __u32toa_t(value, buffer);
+}
+
+wchar_t* __u32toa(uint32_t value, wchar_t* buffer)
+{
+    return __u32toa_t(value, buffer);
+}
+
+char* __u64toa(uint64_t value, char* buffer)
+{
+    return __u64toa_t(value, buffer);
+}
+
+wchar_t* __u64toa(uint64_t value, wchar_t* buffer)
+{
+    return __u64toa_t(value, buffer);
+}
+
 }  // namespace __itoa
 
 _LIBCPP_END_NAMESPACE_STD
Index: libcxx/src/string.cpp
===================================================================
--- libcxx/src/string.cpp
+++ libcxx/src/string.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "string"
+#include "charconv"
 #include "cstdlib"
 #include "cwchar"
 #include "cerrno"
@@ -348,33 +349,6 @@
 
 // as_string
 
-template<typename S, typename P, typename V >
-inline
-S
-as_string(P sprintf_like, S s, const typename S::value_type* fmt, V a)
-{
-    typedef typename S::size_type size_type;
-    size_type available = s.size();
-    while (true)
-    {
-        int status = sprintf_like(&s[0], available + 1, fmt, a);
-        if ( status >= 0 )
-        {
-            size_type used = static_cast<size_type>(status);
-            if ( used <= available )
-            {
-                s.resize( used );
-                break;
-            }
-            available = used; // Assume this is advice of how much space we need.
-        }
-        else
-            available = available * 2 + 1;
-        s.resize(available);
-    }
-    return s;
-}
-
 template <class S, class V, bool = is_floating_point<V>::value>
 struct initial_string;
 
@@ -417,6 +391,34 @@
     }
 };
 
+template<typename S, typename P, typename V >
+inline
+S
+as_string(P sprintf_like, const typename S::value_type* fmt, V a)
+{
+    S s = initial_string<S, V>()();
+    typedef typename S::size_type size_type;
+    size_type available = s.size();
+    while (true)
+    {
+        int status = sprintf_like(&s[0], available + 1, fmt, a);
+        if ( status >= 0 )
+        {
+            size_type used = static_cast<size_type>(status);
+            if ( used <= available )
+            {
+                s.resize( used );
+                break;
+            }
+            available = used; // Assume this is advice of how much space we need.
+        }
+        else
+            available = available * 2 + 1;
+        s.resize(available);
+    }
+    return s;
+}
+
 typedef int (*wide_printf)(wchar_t* __restrict, size_t, const wchar_t*__restrict, ...);
 
 inline
@@ -430,95 +432,110 @@
 #endif
 }
 
+template<typename S, typename V, typename P>
+inline
+S i_to_string(P sprintf_like, const typename S::value_type* fmt, V v)
+{
+    // try fast conversion using to_chars
+    constexpr size_t bufsize = numeric_limits<V>::digits10 + 1;  // +1 for minus
+    typename S::value_type buf[bufsize];
+    const auto res = __to_chars_itoa(buf, buf + bufsize, v, is_signed<V>());
+    if (res.ec == errc())
+        return S(buf, res.ptr);
+
+    // fallback to slow path on error
+    return as_string<S>(sprintf_like, fmt, v);
+}
+
 }  // unnamed namespace
 
 string to_string(int val)
 {
-    return as_string(snprintf, initial_string<string, int>()(), "%d", val);
+    return i_to_string<string>(snprintf, "%d", val);
 }
 
 string to_string(unsigned val)
 {
-    return as_string(snprintf, initial_string<string, unsigned>()(), "%u", val);
+    return i_to_string<string>(snprintf, "%u", val);
 }
 
 string to_string(long val)
 {
-    return as_string(snprintf, initial_string<string, long>()(), "%ld", val);
+    return i_to_string<string>(snprintf, "%ld", val);
 }
 
 string to_string(unsigned long val)
 {
-    return as_string(snprintf, initial_string<string, unsigned long>()(), "%lu", val);
+    return i_to_string<string>(snprintf, "%lu", val);
 }
 
 string to_string(long long val)
 {
-    return as_string(snprintf, initial_string<string, long long>()(), "%lld", val);
+    return i_to_string<string>(snprintf, "%lld", val);
 }
 
 string to_string(unsigned long long val)
 {
-    return as_string(snprintf, initial_string<string, unsigned long long>()(), "%llu", val);
+    return i_to_string<string>(snprintf, "%llu", val);
 }
 
 string to_string(float val)
 {
-    return as_string(snprintf, initial_string<string, float>()(), "%f", val);
+    return as_string<string>(snprintf, "%f", val);
 }
 
 string to_string(double val)
 {
-    return as_string(snprintf, initial_string<string, double>()(), "%f", val);
+    return as_string<string>(snprintf, "%f", val);
 }
 
 string to_string(long double val)
 {
-    return as_string(snprintf, initial_string<string, long double>()(), "%Lf", val);
+    return as_string<string>(snprintf, "%Lf", val);
 }
 
 wstring to_wstring(int val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, int>()(), L"%d", val);
+    return i_to_string<wstring>(get_swprintf(), L"%d", val);
 }
 
 wstring to_wstring(unsigned val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, unsigned>()(), L"%u", val);
+    return i_to_string<wstring>(get_swprintf(), L"%u", val);
 }
 
 wstring to_wstring(long val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, long>()(), L"%ld", val);
+    return i_to_string<wstring>(get_swprintf(), L"%ld", val);
 }
 
 wstring to_wstring(unsigned long val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, unsigned long>()(), L"%lu", val);
+    return i_to_string<wstring>(get_swprintf(), L"%lu", val);
 }
 
 wstring to_wstring(long long val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, long long>()(), L"%lld", val);
+    return i_to_string<wstring>(get_swprintf(), L"%lld", val);
 }
 
 wstring to_wstring(unsigned long long val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, unsigned long long>()(), L"%llu", val);
+    return i_to_string<wstring>(get_swprintf(), L"%llu", val);
 }
 
 wstring to_wstring(float val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, float>()(), L"%f", val);
+    return as_string<wstring>(get_swprintf(), L"%f", val);
 }
 
 wstring to_wstring(double val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, double>()(), L"%f", val);
+    return as_string<wstring>(get_swprintf(), L"%f", val);
 }
 
 wstring to_wstring(long double val)
 {
-    return as_string(get_swprintf(), initial_string<wstring, long double>()(), L"%Lf", val);
+    return as_string<wstring>(get_swprintf(), L"%Lf", val);
 }
 _LIBCPP_END_NAMESPACE_STD