[libc++][C++17] Elementary string conversions for integral types

Summary:
Major QoI considerations:

- The facility is backported to C++14, same as libstdc++.
- Efforts have been made to minimize the header dependencies.
- The design is friendly to the uses of MSVC intrinsics (`__emulu`, `_umul128`, `_BitScanForward`, `_BitScanForward64`) but not implemented; future contributions are welcome.

Thanks to Milo Yip for contributing the implementation of `__u64toa` and `__u32toa`.

References:
 https://wg21.link/p0067r5
 https://wg21.link/p0682r1

Reviewers: mclow.lists, EricWF

Reviewed By: mclow.lists

Subscribers: ldionne, Quuxplusone, christof, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D41458

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@338479 91177308-0d34-0410-b5e6-96231b3b80d8

10 files changed, 1402 insertions, 0 deletions

diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index a45b80160..d9def18d7 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -32,6 +32,7 @@ set(files
   cerrno
   cfenv
   cfloat
+  charconv
   chrono
   cinttypes
   ciso646
diff --git a/include/charconv b/include/charconv
new file mode 100644
index 000000000..7cb790e1b
--- /dev/null
+++ b/include/charconv
@@ -0,0 +1,610 @@
+// -*- C++ -*-
+//===------------------------------ charconv ------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_CHARCONV
+#define _LIBCPP_CHARCONV
+
+/*
+    charconv synopsis
+
+namespace std {
+
+  // floating-point format for primitive numerical conversion
+  enum class chars_format {
+    scientific = unspecified,
+    fixed = unspecified,
+    hex = unspecified,
+    general = fixed | scientific
+  };
+
+  // 23.20.2, primitive numerical output conversion
+  struct to_chars_result {
+    char* ptr;
+    errc ec;
+  };
+
+  to_chars_result to_chars(char* first, char* last, see below value,
+                           int base = 10);
+
+  to_chars_result to_chars(char* first, char* last, float value);
+  to_chars_result to_chars(char* first, char* last, double value);
+  to_chars_result to_chars(char* first, char* last, long double value);
+
+  to_chars_result to_chars(char* first, char* last, float value,
+                           chars_format fmt);
+  to_chars_result to_chars(char* first, char* last, double value,
+                           chars_format fmt);
+  to_chars_result to_chars(char* first, char* last, long double value,
+                           chars_format fmt);
+
+  to_chars_result to_chars(char* first, char* last, float value,
+                           chars_format fmt, int precision);
+  to_chars_result to_chars(char* first, char* last, double value,
+                           chars_format fmt, int precision);
+  to_chars_result to_chars(char* first, char* last, long double value,
+                           chars_format fmt, int precision);
+
+  // 23.20.3, primitive numerical input conversion
+  struct from_chars_result {
+    const char* ptr;
+    errc ec;
+  };
+
+  from_chars_result from_chars(const char* first, const char* last,
+                               see below& value, int base = 10);
+
+  from_chars_result from_chars(const char* first, const char* last,
+                               float& value,
+                               chars_format fmt = chars_format::general);
+  from_chars_result from_chars(const char* first, const char* last,
+                               double& value,
+                               chars_format fmt = chars_format::general);
+  from_chars_result from_chars(const char* first, const char* last,
+                               long double& value,
+                               chars_format fmt = chars_format::general);
+
+} // namespace std
+
+*/
+
+#include <__errc>
+#include <type_traits>
+#include <limits>
+#include <stdint.h>
+#include <string.h>
+#include <math.h>
+
+#include <__debug>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 11
+
+enum class _LIBCPP_ENUM_VIS chars_format
+{
+    scientific = 0x1,
+    fixed = 0x2,
+    hex = 0x4,
+    general = fixed | scientific
+};
+
+struct _LIBCPP_TYPE_VIS to_chars_result
+{
+    char* ptr;
+    errc ec;
+};
+
+struct _LIBCPP_TYPE_VIS from_chars_result
+{
+    const char* ptr;
+    errc ec;
+};
+
+void to_chars(char*, char*, bool, int = 10) = delete;
+void from_chars(const char*, const char*, bool, int = 10) = delete;
+
+namespace __itoa
+{
+
+static constexpr uint64_t __pow10_64[] = {
+    UINT64_C(0),
+    UINT64_C(10),
+    UINT64_C(100),
+    UINT64_C(1000),
+    UINT64_C(10000),
+    UINT64_C(100000),
+    UINT64_C(1000000),
+    UINT64_C(10000000),
+    UINT64_C(100000000),
+    UINT64_C(1000000000),
+    UINT64_C(10000000000),
+    UINT64_C(100000000000),
+    UINT64_C(1000000000000),
+    UINT64_C(10000000000000),
+    UINT64_C(100000000000000),
+    UINT64_C(1000000000000000),
+    UINT64_C(10000000000000000),
+    UINT64_C(100000000000000000),
+    UINT64_C(1000000000000000000),
+    UINT64_C(10000000000000000000),
+};
+
+static constexpr uint32_t __pow10_32[] = {
+    UINT32_C(0),          UINT32_C(10),       UINT32_C(100),
+    UINT32_C(1000),       UINT32_C(10000),    UINT32_C(100000),
+    UINT32_C(1000000),    UINT32_C(10000000), UINT32_C(100000000),
+    UINT32_C(1000000000),
+};
+
+_LIBCPP_FUNC_VIS char* __u64toa(uint64_t __value, char* __buffer);
+_LIBCPP_FUNC_VIS char* __u32toa(uint32_t __value, char* __buffer);
+
+template <typename _Tp, typename = void>
+struct _LIBCPP_HIDDEN __traits_base
+{
+    using type = uint64_t;
+
+#if !defined(_LIBCPP_COMPILER_MSVC)
+    static _LIBCPP_INLINE_VISIBILITY int __width(_Tp __v)
+    {
+        auto __t = (64 - __builtin_clzll(__v | 1)) * 1233 >> 12;
+        return __t - (__v < __pow10_64[__t]) + 1;
+    }
+#endif
+
+    static _LIBCPP_INLINE_VISIBILITY char* __convert(_Tp __v, char* __p)
+    {
+        return __u64toa(__v, __p);
+    }
+
+    static _LIBCPP_INLINE_VISIBILITY auto& __pow() { return __pow10_64; }
+};
+
+template <typename _Tp>
+struct _LIBCPP_HIDDEN
+    __traits_base<_Tp, decltype(void(uint32_t{declval<_Tp>()}))>
+{
+    using type = uint32_t;
+
+#if !defined(_LIBCPP_COMPILER_MSVC)
+    static _LIBCPP_INLINE_VISIBILITY int __width(_Tp __v)
+    {
+        auto __t = (32 - __builtin_clz(__v | 1)) * 1233 >> 12;
+        return __t - (__v < __pow10_32[__t]) + 1;
+    }
+#endif
+
+    static _LIBCPP_INLINE_VISIBILITY char* __convert(_Tp __v, char* __p)
+    {
+        return __u32toa(__v, __p);
+    }
+
+    static _LIBCPP_INLINE_VISIBILITY auto& __pow() { return __pow10_32; }
+};
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY bool
+__mul_overflowed(unsigned char __a, _Tp __b, unsigned char& __r)
+{
+    auto __c = __a * __b;
+    __r = __c;
+    return __c > (numeric_limits<unsigned char>::max)();
+}
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY bool
+__mul_overflowed(unsigned short __a, _Tp __b, unsigned short& __r)
+{
+    auto __c = __a * __b;
+    __r = __c;
+    return __c > (numeric_limits<unsigned short>::max)();
+}
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY bool
+__mul_overflowed(_Tp __a, _Tp __b, _Tp& __r)
+{
+    static_assert(is_unsigned<_Tp>::value, "");
+#if !defined(_LIBCPP_COMPILER_MSVC)
+    return __builtin_mul_overflow(__a, __b, &__r);
+#else
+    bool __did = __b && ((numeric_limits<_Tp>::max)() / __b) < __a;
+    __r = __a * __b;
+    return __did;
+#endif
+}
+
+template <typename _Tp, typename _Up>
+inline _LIBCPP_INLINE_VISIBILITY bool
+__mul_overflowed(_Tp __a, _Up __b, _Tp& __r)
+{
+    return __mul_overflowed(__a, static_cast<_Tp>(__b), __r);
+}
+
+template <typename _Tp>
+struct _LIBCPP_HIDDEN __traits : __traits_base<_Tp>
+{
+    static constexpr int digits = numeric_limits<_Tp>::digits10 + 1;
+    using __traits_base<_Tp>::__pow;
+    using typename __traits_base<_Tp>::type;
+
+    // precondition: at least one non-zero character available
+    static _LIBCPP_INLINE_VISIBILITY char const*
+    __read(char const* __p, char const* __ep, type& __a, type& __b)
+    {
+        type __cprod[digits];
+        int __j = digits - 1;
+        int __i = digits;
+        do
+        {
+            if (!('0' <= *__p && *__p <= '9'))
+                break;
+            __cprod[--__i] = *__p++ - '0';
+        } while (__p != __ep && __i != 0);
+
+        __a = __inner_product(__cprod + __i + 1, __cprod + __j, __pow() + 1,
+                              __cprod[__i]);
+        if (__mul_overflowed(__cprod[__j], __pow()[__j - __i], __b))
+            --__p;
+        return __p;
+    }
+
+    template <typename _It1, typename _It2, class _Up>
+    static _LIBCPP_INLINE_VISIBILITY _Up
+    __inner_product(_It1 __first1, _It1 __last1, _It2 __first2, _Up __init)
+    {
+        for (; __first1 < __last1; ++__first1, ++__first2)
+            __init = __init + *__first1 * *__first2;
+        return __init;
+    }
+};
+
+}  // namespace __itoa
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY _Tp
+__complement(_Tp __x)
+{
+    static_assert(is_unsigned<_Tp>::value, "cast to unsigned first");
+    return _Tp(~__x + 1);
+}
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY auto
+__to_unsigned(_Tp __x)
+{
+    return static_cast<make_unsigned_t<_Tp>>(__x);
+}
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY to_chars_result
+__to_chars_itoa(char* __first, char* __last, _Tp __value, true_type)
+{
+    auto __x = __to_unsigned(__value);
+    if (__value < 0 && __first != __last)
+    {
+        *__first++ = '-';
+        __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)
+{
+    using __tx = __itoa::__traits<_Tp>;
+    auto __diff = __last - __first;
+
+#if !defined(_LIBCPP_COMPILER_MSVC)
+    if (__tx::digits <= __diff || __tx::__width(__value) <= __diff)
+        return {__tx::__convert(__value, __first), {}};
+    else
+        return {__last, errc::value_too_large};
+#else
+    if (__tx::digits <= __diff)
+        return {__tx::__convert(__value, __first), {}};
+    else
+    {
+        char __buf[__tx::digits];
+        auto __p = __tx::__convert(__value, __buf);
+        auto __len = __p - __buf;
+        if (__len <= __diff)
+        {
+            memcpy(__first, __buf, __len);
+            return {__first + __len, {}};
+        }
+        else
+            return {__last, errc::value_too_large};
+    }
+#endif
+}
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY to_chars_result
+__to_chars_integral(char* __first, char* __last, _Tp __value, int __base,
+                    true_type)
+{
+    auto __x = __to_unsigned(__value);
+    if (__value < 0 && __first != __last)
+    {
+        *__first++ = '-';
+        __x = __complement(__x);
+    }
+
+    return __to_chars_integral(__first, __last, __x, __base, false_type());
+}
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY to_chars_result
+__to_chars_integral(char* __first, char* __last, _Tp __value, int __base,
+                    false_type)
+{
+    if (__base == 10)
+        return __to_chars_itoa(__first, __last, __value, false_type());
+
+    auto __p = __last;
+    while (__p != __first)
+    {
+        auto __c = __value % __base;
+        __value /= __base;
+        *--__p = "0123456789abcdefghijklmnopqrstuvwxyz"[__c];
+        if (__value == 0)
+            break;
+    }
+
+    auto __len = __last - __p;
+    if (__value != 0 || !__len)
+        return {__last, errc::value_too_large};
+    else
+    {
+        memmove(__first, __p, __len);
+        return {__first + __len, {}};
+    }
+}
+
+template <typename _Tp, enable_if_t<is_integral<_Tp>::value, int> = 0>
+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>());
+}
+
+template <typename _Tp, enable_if_t<is_integral<_Tp>::value, int> = 0>
+inline _LIBCPP_INLINE_VISIBILITY to_chars_result
+to_chars(char* __first, char* __last, _Tp __value, int __base)
+{
+    _LIBCPP_ASSERT(2 <= __base && __base <= 36, "base not in [2, 36]");
+    return __to_chars_integral(__first, __last, __value, __base,
+                               is_signed<_Tp>());
+}
+
+template <typename _It, typename _Tp, typename _Fn, typename... _Ts>
+inline _LIBCPP_INLINE_VISIBILITY from_chars_result
+__sign_combinator(_It __first, _It __last, _Tp& __value, _Fn __f, _Ts... __args)
+{
+    using __tl = numeric_limits<_Tp>;
+    decltype(__to_unsigned(__value)) __x;
+
+    bool __neg = (__first != __last && *__first == '-');
+    auto __r = __f(__neg ? __first + 1 : __first, __last, __x, __args...);
+    switch (__r.ec)
+    {
+    case errc::invalid_argument:
+        return {__first, __r.ec};
+    case errc::result_out_of_range:
+        return __r;
+    default:
+        break;
+    }
+
+    if (__neg)
+    {
+        if (__x <= __complement(__to_unsigned(__tl::min())))
+        {
+            __x = __complement(__x);
+            memcpy(&__value, &__x, sizeof(__x));
+            return __r;
+        }
+    }
+    else
+    {
+        if (__x <= (__tl::max)())
+        {
+            __value = __x;
+            return __r;
+        }
+    }
+
+    return {__r.ptr, errc::result_out_of_range};
+}
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY bool
+__in_pattern(_Tp __c)
+{
+    return '0' <= __c && __c <= '9';
+}
+
+struct _LIBCPP_HIDDEN __in_pattern_result
+{
+    bool __ok;
+    int __val;
+
+    explicit _LIBCPP_INLINE_VISIBILITY operator bool() const { return __ok; }
+};
+
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY __in_pattern_result
+__in_pattern(_Tp __c, int __base)
+{
+    if (__base <= 10)
+        return {'0' <= __c && __c < '0' + __base, __c - '0'};
+    else if (__in_pattern(__c))
+        return {true, __c - '0'};
+    else if ('a' <= __c && __c < 'a' + __base - 10)
+        return {true, __c - 'a' + 10};
+    else
+        return {'A' <= __c && __c < 'A' + __base - 10, __c - 'A' + 10};
+}
+
+template <typename _It, typename _Tp, typename _Fn, typename... _Ts>
+inline _LIBCPP_INLINE_VISIBILITY from_chars_result
+__subject_seq_combinator(_It __first, _It __last, _Tp& __value, _Fn __f,
+                         _Ts... __args)
+{
+    auto __find_non_zero = [](_It __first, _It __last) {
+        for (; __first != __last; ++__first)
+            if (*__first != '0')
+                break;
+        return __first;
+    };
+
+    auto __p = __find_non_zero(__first, __last);
+    if (__p == __last || !__in_pattern(*__p, __args...))
+    {
+        if (__p == __first)
+            return {__first, errc::invalid_argument};
+        else
+        {
+            __value = 0;
+            return {__p, {}};
+        }
+    }
+
+    auto __r = __f(__p, __last, __value, __args...);
+    if (__r.ec == errc::result_out_of_range)
+    {
+        for (; __r.ptr != __last; ++__r.ptr)
+        {
+            if (!__in_pattern(*__r.ptr, __args...))
+                break;
+        }
+    }
+
+    return __r;
+}
+
+template <typename _Tp, enable_if_t<is_unsigned<_Tp>::value, int> = 0>
+inline _LIBCPP_INLINE_VISIBILITY from_chars_result
+__from_chars_atoi(const char* __first, const char* __last, _Tp& __value)
+{
+    using __tx = __itoa::__traits<_Tp>;
+    using __output_type = typename __tx::type;
+
+    return __subject_seq_combinator(
+        __first, __last, __value,
+        [](const char* __first, const char* __last,
+           _Tp& __value) -> from_chars_result {
+            __output_type __a, __b;
+            auto __p = __tx::__read(__first, __last, __a, __b);
+            if (__p == __last || !__in_pattern(*__p))
+            {
+                __output_type __m = (numeric_limits<_Tp>::max)();
+                if (__m >= __a && __m - __a >= __b)
+                {
+                    __value = __a + __b;
+                    return {__p, {}};
+                }
+            }
+            return {__p, errc::result_out_of_range};
+        });
+}
+
+template <typename _Tp, enable_if_t<is_signed<_Tp>::value, int> = 0>
+inline _LIBCPP_INLINE_VISIBILITY from_chars_result
+__from_chars_atoi(const char* __first, const char* __last, _Tp& __value)
+{
+    using __t = decltype(__to_unsigned(__value));
+    return __sign_combinator(__first, __last, __value, __from_chars_atoi<__t>);
+}
+
+template <typename _Tp, enable_if_t<is_unsigned<_Tp>::value, int> = 0>
+inline _LIBCPP_INLINE_VISIBILITY from_chars_result
+__from_chars_integral(const char* __first, const char* __last, _Tp& __value,
+                      int __base)
+{
+    if (__base == 10)
+        return __from_chars_atoi(__first, __last, __value);
+
+    return __subject_seq_combinator(
+        __first, __last, __value,
+        [](const char* __p, const char* __last, _Tp& __value,
+           int __base) -> from_chars_result {
+            using __tl = numeric_limits<_Tp>;
+            auto __digits = __tl::digits / log2f(float(__base));
+            _Tp __a = __in_pattern(*__p++, __base).__val, __b = 0;
+
+            for (int __i = 1; __p != __last; ++__i, ++__p)
+            {
+                if (auto __c = __in_pattern(*__p, __base))
+                {
+                    if (__i < __digits - 1)
+                        __a = __a * __base + __c.__val;
+                    else
+                    {
+                        if (!__itoa::__mul_overflowed(__a, __base, __a))
+                            ++__p;
+                        __b = __c.__val;
+                        break;
+                    }
+                }
+                else
+                    break;
+            }
+
+            if (__p == __last || !__in_pattern(*__p, __base))
+            {
+                if ((__tl::max)() - __a >= __b)
+                {
+                    __value = __a + __b;
+                    return {__p, {}};
+                }
+            }
+            return {__p, errc::result_out_of_range};
+        },
+        __base);
+}
+
+template <typename _Tp, enable_if_t<is_signed<_Tp>::value, int> = 0>
+inline _LIBCPP_INLINE_VISIBILITY from_chars_result
+__from_chars_integral(const char* __first, const char* __last, _Tp& __value,
+                      int __base)
+{
+    using __t = decltype(__to_unsigned(__value));
+    return __sign_combinator(__first, __last, __value,
+                             __from_chars_integral<__t>, __base);
+}
+
+template <typename _Tp, enable_if_t<is_integral<_Tp>::value, int> = 0>
+inline _LIBCPP_INLINE_VISIBILITY from_chars_result
+from_chars(const char* __first, const char* __last, _Tp& __value)
+{
+    return __from_chars_atoi(__first, __last, __value);
+}
+
+template <typename _Tp, enable_if_t<is_integral<_Tp>::value, int> = 0>
+inline _LIBCPP_INLINE_VISIBILITY from_chars_result
+from_chars(const char* __first, const char* __last, _Tp& __value, int __base)
+{
+    _LIBCPP_ASSERT(2 <= __base && __base <= 36, "base not in [2, 36]");
+    return __from_chars_integral(__first, __last, __value, __base);
+}
+
+#endif  // _LIBCPP_STD_VER > 11
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif  // _LIBCPP_CHARCONV
diff --git a/include/module.modulemap b/include/module.modulemap
index 3b66390fb..089505586 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -235,6 +235,10 @@ module std [system] {
     export *
   }
   // No submodule for cassert. It fundamentally needs repeated, textual inclusion.
+  module charconv {
+    header "charconv"
+    export *
+  }
   module chrono {
     header "chrono"
     export *
diff --git a/src/charconv.cpp b/src/charconv.cpp
new file mode 100644
index 000000000..ec241db74
--- /dev/null
+++ b/src/charconv.cpp
@@ -0,0 +1,233 @@
+//===------------------------- charconv.cpp -------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "charconv"
+#include <string.h>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace __itoa
+{
+
+static constexpr char cDigitsLut[200] = {
+    '0', '0', '0', '1', '0', '2', '0', '3', '0', '4', '0', '5', '0', '6', '0',
+    '7', '0', '8', '0', '9', '1', '0', '1', '1', '1', '2', '1', '3', '1', '4',
+    '1', '5', '1', '6', '1', '7', '1', '8', '1', '9', '2', '0', '2', '1', '2',
+    '2', '2', '3', '2', '4', '2', '5', '2', '6', '2', '7', '2', '8', '2', '9',
+    '3', '0', '3', '1', '3', '2', '3', '3', '3', '4', '3', '5', '3', '6', '3',
+    '7', '3', '8', '3', '9', '4', '0', '4', '1', '4', '2', '4', '3', '4', '4',
+    '4', '5', '4', '6', '4', '7', '4', '8', '4', '9', '5', '0', '5', '1', '5',
+    '2', '5', '3', '5', '4', '5', '5', '5', '6', '5', '7', '5', '8', '5', '9',
+    '6', '0', '6', '1', '6', '2', '6', '3', '6', '4', '6', '5', '6', '6', '6',
+    '7', '6', '8', '6', '9', '7', '0', '7', '1', '7', '2', '7', '3', '7', '4',
+    '7', '5', '7', '6', '7', '7', '7', '8', '7', '9', '8', '0', '8', '1', '8',
+    '2', '8', '3', '8', '4', '8', '5', '8', '6', '8', '7', '8', '8', '8', '9',
+    '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)
+{
+    *buffer = '0' + static_cast<char>(i);
+    return buffer + 1;
+}
+
+template <typename T>
+inline _LIBCPP_INLINE_VISIBILITY char*
+append2(char* buffer, T i)
+{
+    memcpy(buffer, &cDigitsLut[(i)*2], 2);
+    return buffer + 2;
+}
+
+template <typename T>
+inline _LIBCPP_INLINE_VISIBILITY char*
+append3(char* buffer, T i)
+{
+    return append2(append1(buffer, (i) / 100), (i) % 100);
+}
+
+template <typename T>
+inline _LIBCPP_INLINE_VISIBILITY char*
+append4(char* buffer, T i)
+{
+    return append2(append2(buffer, (i) / 100), (i) % 100);
+}
+
+char*
+__u32toa(uint32_t value, char* buffer)
+{
+    if (value < 10000)
+    {
+        if (value < 100)
+        {
+            if (value < 10)
+                buffer = append1(buffer, value);
+            else
+                buffer = append2(buffer, value);
+        }
+        else
+        {
+            if (value < 1000)
+                buffer = append3(buffer, value);
+            else
+                buffer = append4(buffer, value);
+        }
+    }
+    else if (value < 100000000)
+    {
+        // value = bbbbcccc
+        const uint32_t b = value / 10000;
+        const uint32_t c = value % 10000;
+
+        if (value < 1000000)
+        {
+            if (value < 100000)
+                buffer = append1(buffer, b);
+            else
+                buffer = append2(buffer, b);
+        }
+        else
+        {
+            if (value < 10000000)
+                buffer = append3(buffer, b);
+            else
+                buffer = append4(buffer, b);
+        }
+
+        buffer = append4(buffer, c);
+    }
+    else
+    {
+        // value = aabbbbcccc in decimal
+        const uint32_t a = value / 100000000;  // 1 to 42
+        value %= 100000000;
+
+        if (a < 10)
+            buffer = append1(buffer, a);
+        else
+            buffer = append2(buffer, a);
+
+        buffer = append4(buffer, value / 10000);
+        buffer = append4(buffer, value % 10000);
+    }
+
+    return buffer;
+}
+
+char*
+__u64toa(uint64_t value, char* buffer)
+{
+    if (value < 100000000)
+    {
+        uint32_t v = static_cast<uint32_t>(value);
+        if (v < 10000)
+        {
+            if (v < 100)
+            {
+                if (v < 10)
+                    buffer = append1(buffer, v);
+                else
+                    buffer = append2(buffer, v);
+            }
+            else
+            {
+                if (v < 1000)
+                    buffer = append3(buffer, v);
+                else
+                    buffer = append4(buffer, v);
+            }
+        }
+        else
+        {
+            // value = bbbbcccc
+            const uint32_t b = v / 10000;
+            const uint32_t c = v % 10000;
+
+            if (v < 1000000)
+            {
+                if (v < 100000)
+                    buffer = append1(buffer, b);
+                else
+                    buffer = append2(buffer, b);
+            }
+            else
+            {
+                if (v < 10000000)
+                    buffer = append3(buffer, b);
+                else
+                    buffer = append4(buffer, b);
+            }
+
+            buffer = append4(buffer, c);
+        }
+    }
+    else if (value < 10000000000000000)
+    {
+        const uint32_t v0 = static_cast<uint32_t>(value / 100000000);
+        const uint32_t v1 = static_cast<uint32_t>(value % 100000000);
+
+        const uint32_t b0 = v0 / 10000;
+        const uint32_t c0 = v0 % 10000;
+
+        if (v0 < 1000000)
+        {
+            if (v0 < 100000)
+                buffer = append1(buffer, b0);
+            else
+                buffer = append2(buffer, b0);
+        }
+        else
+        {
+            if (v0 < 10000000)
+                buffer = append3(buffer, b0);
+            else
+                buffer = append4(buffer, b0);
+        }
+
+        buffer = append4(buffer, c0);
+        buffer = append4(buffer, v1 / 10000);
+        buffer = append4(buffer, v1 % 10000);
+    }
+    else
+    {
+        const uint32_t a =
+            static_cast<uint32_t>(value / 10000000000000000);  // 1 to 1844
+        value %= 10000000000000000;
+
+        if (a < 100)
+        {
+            if (a < 10)
+                buffer = append1(buffer, a);
+            else
+                buffer = append2(buffer, a);
+        }
+        else
+        {
+            if (a < 1000)
+                buffer = append3(buffer, a);
+            else
+                buffer = append4(buffer, a);
+        }
+
+        const uint32_t v0 = static_cast<uint32_t>(value / 100000000);
+        const uint32_t v1 = static_cast<uint32_t>(value % 100000000);
+        buffer = append4(buffer, v0 / 10000);
+        buffer = append4(buffer, v0 % 10000);
+        buffer = append4(buffer, v1 / 10000);
+        buffer = append4(buffer, v1 % 10000);
+    }
+
+    return buffer;
+}
+
+}  // namespace __itoa
+
+_LIBCPP_END_NAMESPACE_STD
diff --git a/test/libcxx/double_include.sh.cpp b/test/libcxx/double_include.sh.cpp
index c98cc8065..8d08db4fd 100644
--- a/test/libcxx/double_include.sh.cpp
+++ b/test/libcxx/double_include.sh.cpp
@@ -34,6 +34,7 @@
 #include <cerrno>
 #include <cfenv>
 #include <cfloat>
+#include <charconv>
 #include <chrono>
 #include <cinttypes>
 #include <ciso646>
diff --git a/test/std/utilities/charconv/charconv.from.chars/integral.bool.fail.cpp b/test/std/utilities/charconv/charconv.from.chars/integral.bool.fail.cpp
new file mode 100644
index 000000000..d21b638d0
--- /dev/null
+++ b/test/std/utilities/charconv/charconv.from.chars/integral.bool.fail.cpp
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11
+// <charconv>
+
+// In
+//
+// from_chars_result from_chars(const char* first, const char* last,
+//                              Integral& value, int base = 10)
+//
+// Integral cannot be bool.
+
+#include <charconv>
+
+int main()
+{
+    using std::from_chars;
+    char buf[] = "01001";
+    bool lv;
+
+    from_chars(buf, buf + sizeof(buf), lv);      // expected-error {{call to deleted function}}
+    from_chars(buf, buf + sizeof(buf), lv, 16);  // expected-error {{call to deleted function}}
+}
diff --git a/test/std/utilities/charconv/charconv.from.chars/integral.pass.cpp b/test/std/utilities/charconv/charconv.from.chars/integral.pass.cpp
new file mode 100644
index 000000000..e5a93b497
--- /dev/null
+++ b/test/std/utilities/charconv/charconv.from.chars/integral.pass.cpp
@@ -0,0 +1,181 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11
+// <charconv>
+
+// from_chars_result from_chars(const char* first, const char* last,
+//                              Integral& value, int base = 10)
+
+#include "charconv_test_helpers.h"
+
+template <typename T>
+struct test_basics : roundtrip_test_base<T>
+{
+    using roundtrip_test_base<T>::test;
+
+    void operator()()
+    {
+        test(0);
+        test(42);
+        test(32768);
+        test(0, 10);
+        test(42, 10);
+        test(32768, 10);
+        test(0xf, 16);
+        test(0xdeadbeaf, 16);
+        test(0755, 8);
+
+        for (int b = 2; b < 37; ++b)
+        {
+            using xl = std::numeric_limits<T>;
+
+            test(1, b);
+            test(xl::lowest(), b);
+            test((xl::max)(), b);
+            test((xl::max)() / 2, b);
+        }
+
+        using std::from_chars;
+        std::from_chars_result r;
+        T x;
+
+        {
+            char s[] = "001x";
+
+            // the expected form of the subject sequence is a sequence of
+            // letters and digits representing an integer with the radix
+            // specified by base (C11 7.22.1.4/3)
+            r = from_chars(s, s + sizeof(s), x);
+            assert(r.ec == std::errc{});
+            assert(r.ptr == s + 3);
+            assert(x == 1);
+        }
+
+        {
+            char s[] = "0X7BAtSGHDkEIXZg ";
+
+            // The letters from a (or A) through z (or Z) are ascribed the
+            // values 10 through 35; (C11 7.22.1.4/3)
+            r = from_chars(s, s + sizeof(s), x, 36);
+            assert(r.ec == std::errc::result_out_of_range);
+            // The member ptr of the return value points to the first character
+            // not matching the pattern
+            assert(r.ptr == s + sizeof(s) - 2);
+            assert(x == 1);
+
+            // no "0x" or "0X" prefix shall appear if the value of base is 16
+            r = from_chars(s, s + sizeof(s), x, 16);
+            assert(r.ec == std::errc{});
+            assert(r.ptr == s + 1);
+            assert(x == 0);
+
+            // only letters and digits whose ascribed values are less than that
+            // of base are permitted. (C11 7.22.1.4/3)
+            r = from_chars(s + 2, s + sizeof(s), x, 12);
+            // If the parsed value is not in the range representable by the type
+            // of value,
+            if (!fits_in<T>(1150))
+            {
+                // value is unmodified and
+                assert(x == 0);
+                // the member ec of the return value is equal to
+                // errc::result_out_of_range
+                assert(r.ec == std::errc::result_out_of_range);
+            }
+            else
+            {
+                // Otherwise, value is set to the parsed value,
+                assert(x == 1150);
+                // and the member ec is value-initialized.
+                assert(r.ec == std::errc{});
+            }
+            assert(r.ptr == s + 5);
+        }
+    }
+};
+
+template <typename T>
+struct test_signed : roundtrip_test_base<T>
+{
+    using roundtrip_test_base<T>::test;
+
+    void operator()()
+    {
+        test(-1);
+        test(-12);
+        test(-1, 10);
+        test(-12, 10);
+        test(-21734634, 10);
+        test(-2647, 2);
+        test(-0xcc1, 16);
+
+        for (int b = 2; b < 37; ++b)
+        {
+            using xl = std::numeric_limits<T>;
+
+            test(0, b);
+            test(xl::lowest(), b);
+            test((xl::max)(), b);
+        }
+
+        using std::from_chars;
+        std::from_chars_result r;
+        T x;
+
+        {
+            // If the pattern allows for an optional sign,
+            // but the string has no digit characters following the sign,
+            char s[] = "- 9+12";
+            r = from_chars(s, s + sizeof(s), x);
+            // no characters match the pattern.
+            assert(r.ptr == s);
+            assert(r.ec == std::errc::invalid_argument);
+        }
+
+        {
+            char s[] = "9+12";
+            r = from_chars(s, s + sizeof(s), x);
+            assert(r.ec == std::errc{});
+            // The member ptr of the return value points to the first character
+            // not matching the pattern,
+            assert(r.ptr == s + 1);
+            assert(x == 9);
+        }
+
+        {
+            char s[] = "12";
+            r = from_chars(s, s + 2, x);
+            assert(r.ec == std::errc{});
+            // or has the value last if all characters match.
+            assert(r.ptr == s + 2);
+            assert(x == 12);
+        }
+
+        {
+            // '-' is the only sign that may appear
+            char s[] = "+30";
+            // If no characters match the pattern,
+            r = from_chars(s, s + sizeof(s), x);
+            // value is unmodified,
+            assert(x == 12);
+            // the member ptr of the return value is first and
+            assert(r.ptr == s);
+            // the member ec is equal to errc::invalid_argument.
+            assert(r.ec == std::errc::invalid_argument);
+        }
+    }
+};
+
+int
+main()
+{
+    run<test_basics>(integrals);
+    run<test_signed>(all_signed);
+}
diff --git a/test/std/utilities/charconv/charconv.to.chars/integral.bool.fail.cpp b/test/std/utilities/charconv/charconv.to.chars/integral.bool.fail.cpp
new file mode 100644
index 000000000..e3d702a3a
--- /dev/null
+++ b/test/std/utilities/charconv/charconv.to.chars/integral.bool.fail.cpp
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11
+// <charconv>
+
+// In
+//
+// to_chars_result to_chars(char* first, char* last, Integral value,
+//                          int base = 10)
+//
+// Integral cannot be bool.
+
+#include <charconv>
+
+int main()
+{
+    using std::to_chars;
+    char buf[10];
+    bool lv = true;
+
+    to_chars(buf, buf + sizeof(buf), false);   // expected-error {{call to deleted function}}
+    to_chars(buf, buf + sizeof(buf), lv, 16);  // expected-error {{call to deleted function}}
+}
diff --git a/test/std/utilities/charconv/charconv.to.chars/integral.pass.cpp b/test/std/utilities/charconv/charconv.to.chars/integral.pass.cpp
new file mode 100644
index 000000000..50ca5b190
--- /dev/null
+++ b/test/std/utilities/charconv/charconv.to.chars/integral.pass.cpp
@@ -0,0 +1,80 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11
+// <charconv>
+
+// to_chars_result to_chars(char* first, char* last, Integral value,
+//                          int base = 10)
+
+#include "charconv_test_helpers.h"
+
+template <typename T>
+struct test_basics : to_chars_test_base<T>
+{
+    using to_chars_test_base<T>::test;
+    using to_chars_test_base<T>::test_value;
+
+    void operator()()
+    {
+        test(0, "0");
+        test(42, "42");
+        test(32768, "32768");
+        test(0, "0", 10);
+        test(42, "42", 10);
+        test(32768, "32768", 10);
+        test(0xf, "f", 16);
+        test(0xdeadbeaf, "deadbeaf", 16);
+        test(0755, "755", 8);
+
+        for (int b = 2; b < 37; ++b)
+        {
+            using xl = std::numeric_limits<T>;
+
+            test_value(1, b);
+            test_value(xl::lowest(), b);
+            test_value((xl::max)(), b);
+            test_value((xl::max)() / 2, b);
+        }
+    }
+};
+
+template <typename T>
+struct test_signed : to_chars_test_base<T>
+{
+    using to_chars_test_base<T>::test;
+    using to_chars_test_base<T>::test_value;
+
+    void operator()()
+    {
+        test(-1, "-1");
+        test(-12, "-12");
+        test(-1, "-1", 10);
+        test(-12, "-12", 10);
+        test(-21734634, "-21734634", 10);
+        test(-2647, "-101001010111", 2);
+        test(-0xcc1, "-cc1", 16);
+
+        for (int b = 2; b < 37; ++b)
+        {
+            using xl = std::numeric_limits<T>;
+
+            test_value(0, b);
+            test_value(xl::lowest(), b);
+            test_value((xl::max)(), b);
+        }
+    }
+};
+
+int
+main()
+{
+    run<test_basics>(integrals);
+    run<test_signed>(all_signed);
+}
diff --git a/test/support/charconv_test_helpers.h b/test/support/charconv_test_helpers.h
new file mode 100644
index 000000000..df1b00a67
--- /dev/null
+++ b/test/support/charconv_test_helpers.h
@@ -0,0 +1,232 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SUPPORT_CHARCONV_TEST_HELPERS_H
+#define SUPPORT_CHARCONV_TEST_HELPERS_H
+
+#include <charconv>
+#include <cassert>
+#include <limits>
+#include <string.h>
+#include <stdlib.h>
+
+#include "test_macros.h"
+
+#if TEST_STD_VER <= 11
+#error This file requires C++14
+#endif
+
+using std::false_type;
+using std::true_type;
+
+template <typename To, typename From>
+constexpr auto
+is_non_narrowing(From a) -> decltype(To{a}, true_type())
+{
+    return {};
+}
+
+template <typename To>
+constexpr auto
+is_non_narrowing(...) -> false_type
+{
+    return {};
+}
+
+template <typename X, typename T>
+constexpr bool
+_fits_in(T, true_type /* non-narrowing*/, ...)
+{
+    return true;
+}
+
+template <typename X, typename T, typename xl = std::numeric_limits<X>>
+constexpr bool
+_fits_in(T v, false_type, true_type /* T signed*/, true_type /* X signed */)
+{
+    return xl::lowest() <= v && v <= (xl::max)();
+}
+
+template <typename X, typename T, typename xl = std::numeric_limits<X>>
+constexpr bool
+_fits_in(T v, false_type, true_type /* T signed */, false_type /* X unsigned*/)
+{
+    return 0 <= v && std::make_unsigned_t<T>(v) <= (xl::max)();
+}
+
+template <typename X, typename T, typename xl = std::numeric_limits<X>>
+constexpr bool
+_fits_in(T v, false_type, false_type /* T unsigned */, ...)
+{
+    return v <= std::make_unsigned_t<X>((xl::max)());
+}
+
+template <typename X, typename T>
+constexpr bool
+fits_in(T v)
+{
+    return _fits_in<X>(v, is_non_narrowing<X>(v), std::is_signed<T>(),
+                       std::is_signed<X>());
+}
+
+template <typename X>
+struct to_chars_test_base
+{
+    template <typename T, size_t N, typename... Ts>
+    void test(T v, char const (&expect)[N], Ts... args)
+    {
+        using std::to_chars;
+        std::to_chars_result r;
+
+        constexpr size_t len = N - 1;
+        static_assert(len > 0, "expected output won't be empty");
+
+        if (!fits_in<X>(v))
+            return;
+
+        r = to_chars(buf, buf + len - 1, X(v), args...);
+        assert(r.ptr == buf + len - 1);
+        assert(r.ec == std::errc::value_too_large);
+
+        r = to_chars(buf, buf + sizeof(buf), X(v), args...);
+        assert(r.ptr == buf + len);
+        assert(r.ec == std::errc{});
+        assert(memcmp(buf, expect, len) == 0);
+    }
+
+    template <typename... Ts>
+    void test_value(X v, Ts... args)
+    {
+        using std::to_chars;
+        std::to_chars_result r;
+
+        r = to_chars(buf, buf + sizeof(buf), v, args...);
+        assert(r.ec == std::errc{});
+        *r.ptr = '\0';
+
+        auto a = fromchars(buf, r.ptr, args...);
+        assert(v == a);
+
+        auto ep = r.ptr - 1;
+        r = to_chars(buf, ep, v, args...);
+        assert(r.ptr == ep);
+        assert(r.ec == std::errc::value_too_large);
+    }
+
+private:
+    static auto fromchars(char const* p, char const* ep, int base, true_type)
+    {
+        char* last;
+        auto r = strtoll(p, &last, base);
+        assert(last == ep);
+
+        return r;
+    }
+
+    static auto fromchars(char const* p, char const* ep, int base, false_type)
+    {
+        char* last;
+        auto r = strtoull(p, &last, base);
+        assert(last == ep);
+
+        return r;
+    }
+
+    static auto fromchars(char const* p, char const* ep, int base = 10)
+    {
+        return fromchars(p, ep, base, std::is_signed<X>());
+    }
+
+    char buf[100];
+};
+
+template <typename X>
+struct roundtrip_test_base
+{
+    template <typename T, typename... Ts>
+    void test(T v, Ts... args)
+    {
+        using std::from_chars;
+        using std::to_chars;
+        std::from_chars_result r2;
+        std::to_chars_result r;
+        X x = 0xc;
+
+        if (fits_in<X>(v))
+        {
+            r = to_chars(buf, buf + sizeof(buf), v, args...);
+            assert(r.ec == std::errc{});
+
+            r2 = from_chars(buf, r.ptr, x, args...);
+            assert(r2.ptr == r.ptr);
+            assert(x == X(v));
+        }
+        else
+        {
+            r = to_chars(buf, buf + sizeof(buf), v, args...);
+            assert(r.ec == std::errc{});
+
+            r2 = from_chars(buf, r.ptr, x, args...);
+
+            if (std::is_signed<T>::value && v < 0 && std::is_unsigned<X>::value)
+            {
+                assert(x == 0xc);
+                assert(r2.ptr == buf);
+                assert(r.ec == std::errc::invalid_argument);
+            }
+            else
+            {
+                assert(x == 0xc);
+                assert(r2.ptr == r.ptr);
+                assert(r2.ec == std::errc::result_out_of_range);
+            }
+        }
+    }
+
+private:
+    char buf[100];
+};
+
+template <typename... T>
+struct type_list
+{
+};
+
+template <typename L1, typename L2>
+struct type_concat;
+
+template <typename... Xs, typename... Ys>
+struct type_concat<type_list<Xs...>, type_list<Ys...>>
+{
+    using type = type_list<Xs..., Ys...>;
+};
+
+template <typename L1, typename L2>
+using concat_t = typename type_concat<L1, L2>::type;
+
+template <typename L1, typename L2>
+constexpr auto concat(L1, L2) -> concat_t<L1, L2>
+{
+    return {};
+}
+
+auto all_signed = type_list<char, signed char, short, int, long, long long>();
+auto all_unsigned = type_list<unsigned char, unsigned short, unsigned int,
+                              unsigned long, unsigned long long>();
+auto integrals = concat(all_signed, all_unsigned);
+
+template <template <typename> class Fn, typename... Ts>
+void
+run(type_list<Ts...>)
+{
+    int ls[sizeof...(Ts)] = {(Fn<Ts>{}(), 0)...};
+    (void)ls;
+}
+
+#endif  // SUPPORT_CHARCONV_TEST_HELPERS_H