[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

1 files changed, 233 insertions, 0 deletions

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