<charconv>: hex from_chars("0.fffffffffffff8p-1022") is incorrect

[statementreply]

Describe the bug
Hexadecimal floating point from_chars returns incorrect results for numbers just (<= 0.5 ulp) below numeric_limits<T>::min().

The bug also exists in the cl compiler and the strtod family. (DevCom-1093829)

Command-line test case

C:\Users\He\Documents\cpp\test>type charconv.cpp
#include <cassert>
#include <charconv>
#include <cstdio>
#include <cstdlib>
#include <string>
#include <system_error>
using namespace std;

double from_hex_double(const string& s) {
    double x;
    const auto result = from_chars(s.data(), s.data() + s.size(), x, chars_format::hex);

    assert(result.ec != errc::invalid_argument);
    assert(result.ptr == s.data() + s.size());
    return x;
}

int main() {
    constexpr double expected = 0x1p-1022;

#if CHECK_COMPILER_CONV
    static_assert(0x0.fffffffffffff8p-1022 == expected, "0x0.fffffffffffff8p-1022 isn't correctly parsed");
    static_assert(0x1.fffffffffffffp-1023 == expected, "0x1.fffffffffffffp-1023 isn't correctly parsed");
    static_assert(0x3.ffffffffffffep-1024 == expected, "0x3.ffffffffffffep-1024 isn't correctly parsed");
    static_assert(0x7.ffffffffffffcp-1025 == expected, "0x7.ffffffffffffcp-1025 isn't correctly parsed");
    static_assert(0xf.ffffffffffff8p-1026 == expected, "0xf.ffffffffffff8p-1026 isn't correctly parsed");
#endif

    const string cases[] = {
        "0x0.fffffffffffff8p-1022"s,
        "0x1.fffffffffffffp-1023"s,
        "0x3.ffffffffffffep-1024"s,
        "0x7.ffffffffffffcp-1025"s,
        "0xf.ffffffffffff8p-1026"s,
    };

    for (const auto& s : cases) {
        assert(from_hex_double(s.substr(2)) == expected);
    }

    for (const auto& s : cases) {
        assert(strtod(s.c_str(), nullptr) == expected);
    }
}

C:\Users\He\Documents\cpp\test>clang++ -std=c++17 -Wall -Wextra -DCHECK_COMPILER_CONV=1 charconv.cpp

C:\Users\He\Documents\cpp\test>cl /EHsc /std:c++17 /W4 /WX /DCHECK_COMPILER_CONV=1 charconv.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.26.28806 版
版权所有(C) Microsoft Corporation。保留所有权利。

charconv.cpp
charconv.cpp(22): error C2338: 0x0.fffffffffffff8p-1022 isn't correctly parsed
charconv.cpp(23): error C2338: 0x1.fffffffffffffp-1023 isn't correctly parsed
charconv.cpp(25): error C2338: 0x7.ffffffffffffcp-1025 isn't correctly parsed
charconv.cpp(26): error C2338: 0xf.ffffffffffff8p-1026 isn't correctly parsed

C:\Users\He\Documents\cpp\test>cl /EHsc /std:c++17 /W4 /WX /DCHECK_COMPILER_CONV=0 charconv.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.26.28806 版
版权所有(C) Microsoft Corporation。保留所有权利。

charconv.cpp
Microsoft (R) Incremental Linker Version 14.26.28806.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:charconv.exe
charconv.obj

C:\Users\He\Documents\cpp\test>.\charconv.exe
Assertion failed: from_hex_double(s.substr(2)) == expected, file charconv.cpp, line 38

STL version

Microsoft Visual Studio Community 2019
版本 16.6.2

[statementreply]

There is a bug in _Assemble_floating_point_value. I'll create a PR to fix it (should be a single-line change).

The decimal version works by accident 😿

[StephanTLavavej]

Great catch, thank you! I audited this code for months and missed this corner case. (Also thanks for filing the DevCom bug for the UCRT and compiler.) 😺