[Bug libstdc++/121404] New: to_chars with hex format produces same output for different long double inputs on powerpc64le

[cassio.neri at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121404

            Bug ID: 121404
           Summary: to_chars with hex format produces same output for
                    different long double inputs on powerpc64le
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cassio.neri at gmail dot com
  Target Milestone: ---

I added this test to trunk (be377ef9ce3eb870ffd18ac02dabe32260dfcb6f):

// { dg-do run { target c++14 } }
// { dg-require-string-conversions "" }

#include <charconv>
#include <cstdio>
#include <iostream>
#include <testsuite_hooks.h>

int main() {

        long double const x1 = 1.0l + 1.0e-17l;
        long double const x2 = 1.0l + .99999999999999899e-17l;

        VERIFY(x1 != x2);

        char str1[80] = {};
        auto const [ptr1, ec1] = std::to_chars(str1, str1 + sizeof(str1) - 1,
x1, std::chars_format::hex);
        auto const size1 = ptr1 - str1;
        VERIFY(ec1 == std::errc{});

        char str2[80] = {};
        auto const [ptr2, ec2] = std::to_chars(str2, str2 + sizeof(str2) - 1,
x2, std::chars_format::hex);
        auto const size2 = ptr2 - str2;
        VERIFY(ec2 == std::errc{});

        std::printf("%La\n%La\n", x1, x2);
        std::cerr << str1 << '\n' << str2 << '\n';
        VERIFY(size1 != size2 || memcmp(str1, str2, size1) != 0);
}

Only the last VERIFY failed so that x1 and x2 are different but str1 and str2
are equal strings. Furtheremore, the results should be the same obtained by
printf with "a" conversion specifier but without "0x". It turns out that printf
produces different strings for x1 and x2. The output is:

0x1.00000000000000b877aa3236a4b8p+0
0x1.00000000000000b877aa3236a48p+0
1.00000000000000b877aa3236a48p+0
1.00000000000000b877aa3236a48p+0
/home/cassio/gcc/src/libstdc++-v3/testsuite/20_util/to_chars/6.cc:28: int
main(): Assertion 'size1 != size2 || memcmp(str1, str2, size1) != 0' failed.
FAIL: 20_util/to_chars/6.cc  -std=gnu++17 execution test

FWIW: As I mentioned, in bug 121403, I'm working on to_chars to replace Ryu
with Tejú Jaguá. However, for hex format to_chars doesn't use Ryu so replacing
it won't help. The problem is elsewhere.