[libcxx][libc] Hand in Hand PoC with from_chars

Implements std::from_chars for float and double.

The implementation uses LLVM-libc to do the real parsing. Since this is the first time libc++
uses LLVM-libc there is a bit of additional infrastructure code. The patch is based on the
[RFC] Project Hand In Hand (LLVM-libc/libc++ code sharing)
https://discourse.llvm.org/t/rfc-project-hand-in-hand-llvm-libc-libc-code-sharing/77701

Author: michaelrj-google

libc/shared/fp_bits.h

@@ -0,0 +1,22 @@
+//===-- Floating point number utils -----------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_FP_BITS_H
+#define LLVM_LIBC_SHARED_FP_BITS_H
+
+#include "src/__support/FPUtil/FPBits.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using fputil::FPBits;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_FP_BITS_H

libc/shared/str_to_float.h

@@ -0,0 +1,27 @@
+//===-- String to float conversion utils ------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_STR_TO_FLOAT_H
+#define LLVM_LIBC_SHARED_STR_TO_FLOAT_H
+
+#include "src/__support/str_to_float.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using internal::ExpandedFloat;
+using internal::FloatConvertReturn;
+using internal::RoundDirection;
+
+using internal::binary_exp_to_float;
+using internal::decimal_exp_to_float;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_STR_TO_FLOAT_H

libc/shared/str_to_integer.h

@@ -0,0 +1,24 @@
+//===-- String to int conversion utils --------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_STR_TO_INTEGER_H
+#define LLVM_LIBC_SHARED_STR_TO_INTEGER_H
+
+#include "src/__support/str_to_integer.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using LIBC_NAMESPACE::StrToNumResult;
+
+using internal::strtointeger;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_STR_TO_INTEGER_H

libc/src/__support/FPUtil/FPBits.h

@@ -6,6 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This file is shared with libc++. You should also be careful when adding
+// dependencies to this file, since it needs to build for all libc++ targets.
+// -----------------------------------------------------------------------------
+
 #ifndef LLVM_LIBC_SRC___SUPPORT_FPUTIL_FPBITS_H
 #define LLVM_LIBC_SRC___SUPPORT_FPUTIL_FPBITS_H
 
@@ -795,6 +801,12 @@ template <typename T> LIBC_INLINE static constexpr FPType get_fp_type() {
     static_assert(cpp::always_false<UnqualT>, "Unsupported type");
 }
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This interface is shared with libc++, if you change this interface you need
+// to update it in both libc and libc++. You should also be careful when adding
+// dependencies to this file, since it needs to build for all libc++ targets.
+// -----------------------------------------------------------------------------
 // A generic class to manipulate C++ floating point formats.
 // It derives its functionality to FPRepImpl above.
 template <typename T>

libc/src/__support/high_precision_decimal.h

@@ -6,6 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This file is shared with libc++. You should also be careful when adding
+// dependencies to this file, since it needs to build for all libc++ targets.
+// -----------------------------------------------------------------------------
+
 #ifndef LLVM_LIBC_SRC___SUPPORT_HIGH_PRECISION_DECIMAL_H
 #define LLVM_LIBC_SRC___SUPPORT_HIGH_PRECISION_DECIMAL_H
 
@@ -23,6 +29,11 @@ struct LShiftTableEntry {
   char const *power_of_five;
 };
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This interface is shared with libc++, if you change this interface you need
+// to update it in both libc and libc++.
+// -----------------------------------------------------------------------------
 // This is used in both this file and in the main str_to_float.h.
 // TODO: Figure out where to put this.
 enum class RoundDirection { Up, Down, Nearest };

libc/src/__support/str_to_float.h

@@ -6,6 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This file is shared with libc++. You should also be careful when adding
+// dependencies to this file, since it needs to build for all libc++ targets.
+// -----------------------------------------------------------------------------
+
 #ifndef LLVM_LIBC_SRC___SUPPORT_STR_TO_FLOAT_H
 #define LLVM_LIBC_SRC___SUPPORT_STR_TO_FLOAT_H
 
@@ -32,11 +38,21 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace internal {
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This interface is shared with libc++, if you change this interface you need
+// to update it in both libc and libc++.
+// -----------------------------------------------------------------------------
 template <class T> struct ExpandedFloat {
   typename fputil::FPBits<T>::StorageType mantissa;
   int32_t exponent;
 };
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This interface is shared with libc++, if you change this interface you need
+// to update it in both libc and libc++.
+// -----------------------------------------------------------------------------
 template <class T> struct FloatConvertReturn {
   ExpandedFloat<T> num = {0, 0};
   int error = 0;
@@ -637,6 +653,11 @@ template <> LIBC_INLINE constexpr int32_t get_lower_bound<double>() {
   return -(309 + 15 + 20);
 }
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This interface is shared with libc++, if you change this interface you need
+// to update it in both libc and libc++.
+// -----------------------------------------------------------------------------
 // Takes a mantissa and base 10 exponent and converts it into its closest
 // floating point type T equivalient. First we try the Eisel-Lemire algorithm,
 // then if that fails then we fall back to a more accurate algorithm for
@@ -716,6 +737,11 @@ LIBC_INLINE FloatConvertReturn<T> decimal_exp_to_float(
   return output;
 }
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This interface is shared with libc++, if you change this interface you need
+// to update it in both libc and libc++.
+// -----------------------------------------------------------------------------
 // Takes a mantissa and base 2 exponent and converts it into its closest
 // floating point type T equivalient. Since the exponent is already in the right
 // form, this is mostly just shifting and rounding. This is used for hexadecimal

libc/src/__support/str_to_integer.h

@@ -6,6 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This file is shared with libc++. You should also be careful when adding
+// dependencies to this file, since it needs to build for all libc++ targets.
+// -----------------------------------------------------------------------------
+
 #ifndef LLVM_LIBC_SRC___SUPPORT_STR_TO_INTEGER_H
 #define LLVM_LIBC_SRC___SUPPORT_STR_TO_INTEGER_H
 
@@ -73,6 +79,11 @@ LIBC_INLINE int infer_base(const char *__restrict src, size_t src_len) {
   return 10;
 }
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This interface is shared with libc++, if you change this interface you need
+// to update it in both libc and libc++.
+// -----------------------------------------------------------------------------
 // Takes a pointer to a string and the base to convert to. This function is used
 // as the backend for all of the string to int functions.
 template <class T>

libc/src/__support/str_to_num_result.h

@@ -6,6 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This file is shared with libc++. You should also be careful when adding
+// dependencies to this file, since it needs to build for all libc++ targets.
+// -----------------------------------------------------------------------------
+
 #ifndef LLVM_LIBC_SRC___SUPPORT_STR_TO_NUM_RESULT_H
 #define LLVM_LIBC_SRC___SUPPORT_STR_TO_NUM_RESULT_H
 
@@ -16,6 +22,11 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
+// -----------------------------------------------------------------------------
+//                               **** WARNING ****
+// This interface is shared with libc++, if you change this interface you need
+// to update it in both libc and libc++.
+// -----------------------------------------------------------------------------
 template <typename T> struct StrToNumResult {
   T value;
   int error;

libcxx/docs/Status/Cxx17Papers.csv

@@ -71,7 +71,7 @@
 "`P0394R4 <https://wg21.link/P0394R4>`__","Hotel Parallelifornia: terminate() for Parallel Algorithms Exception Handling","2016-06 (Oulu)","|Complete|","17.0",""
 "","","","","",""
 "`P0003R5 <https://wg21.link/P0003R5>`__","Removing Deprecated Exception Specifications from C++17","2016-11 (Issaquah)","|Complete|","5.0",""
-"`P0067R5 <https://wg21.link/P0067R5>`__","Elementary string conversions, revision 5","2016-11 (Issaquah)","|Partial|","","``std::(to|from)_chars`` for integrals has been available since version 7.0. ``std::to_chars`` for ``float`` and ``double`` since version 14.0 ``std::to_chars`` for ``long double`` uses the implementation for ``double``."
+"`P0067R5 <https://wg21.link/P0067R5>`__","Elementary string conversions, revision 5","2016-11 (Issaquah)","|Partial|","","``std::(to|from)_chars`` for integrals has been available since version 7.0. ``std::to_chars`` for ``float`` and ``double`` since version 14.0 ``std::to_chars`` for ``long double`` uses the implementation for ``double``. ``std::from_chars`` for ``float`` and ``double`` since version 20.0."
 "`P0403R1 <https://wg21.link/P0403R1>`__","Literal suffixes for ``basic_string_view``\ ","2016-11 (Issaquah)","|Complete|","4.0",""
 "`P0414R2 <https://wg21.link/P0414R2>`__","Merging shared_ptr changes from Library Fundamentals to C++17","2016-11 (Issaquah)","|Complete|","11.0",""
 "`P0418R2 <https://wg21.link/P0418R2>`__","Fail or succeed: there is no atomic lattice","2016-11 (Issaquah)","","",""

libcxx/docs/Status/Cxx2cIssues.csv

@@ -78,4 +78,5 @@
 "","","","","",""
 "`LWG3343 <https://wg21.link/LWG3343>`__","Ordering of calls to ``unlock()`` and ``notify_all()`` in Effects element of ``notify_all_at_thread_exit()`` should be reversed","Not Adopted Yet","|Complete|","16.0",""
 "`LWG4139 <https://wg21.link/LWG4139>`__","§[time.zone.leap] recursive constraint in <=>","Not Adopted Yet","|Complete|","20.0",""
+"`LWG3456 <https://wg21.link/LWG3456>`__","Pattern used by std::from_chars is underspecified (option B)",,"Not Yet Adopted","|Complete|","20.0",""
 "","","","","",""