[Bug libstdc++/98384] [11 Regression] new test case 20_util/to_chars/long_double.cc in r11-6249 fails on powerpc64 BE

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98384

--- Comment #19 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:70aa0e6eef9d65744f37adc2a3cffef1a8217dc1

commit r11-7367-g70aa0e6eef9d65744f37adc2a3cffef1a8217dc1
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Feb 24 12:24:43 2021 -0500

    libstdc++: Robustify long double std::to_chars testcase [PR98384]

    The long double std::to_chars testcase currently verifies the
    correctness of its output by comparing it to that of printf, so if
    there's a mismatch between to_chars and printf, the test FAILs.  This
    works well for the scientific, fixed and general formatting modes,
    because the corresponding printf conversion specifiers (%e, %f and %g)
    are rigidly specified.

    But this doesn't work well for the hex formatting mode because the
    corresponding printf conversion specifier %a is more flexibly specified.
    For instance, the hexadecimal forms 0x1p+0, 0x2p-1, 0x4p-2 and 0x8p-3
    are all equivalent and valid outputs of the %a specifier for the number 1.
    The apparent freedom here is the choice of leading hex digit -- the
    standard just requires that the leading hex digit is nonzero for
    normalized numbers.

    Currently, our hexadecimal formatting implementation uses 0/1/2 as the
    leading hex digit for floating point types that have an implicit leading
    mantissa bit which in practice means all supported floating point types
    except x86 long double.  The latter type has a 64 bit mantissa with an
    explicit leading mantissa bit, and for this type our implementation uses
    the most significant four bits of the mantissa as leading hex digit.
    This seems to be consistent with most printf implementations, but not
    all, as PR98384 illustrates.

    In order to avoid false-positive FAILs due to arbitrary disagreement
    between to_chars and printf about the choice of leading hex digit, this
    patch makes the testcase's verification via printf conditional on the
    leading hex digits first agreeing.  An additional verification step is
    also added: round-tripping the output of to_chars through from_chars
    should recover the value exactly.

    libstdc++-v3/ChangeLog:

            PR libstdc++/98384
            * testsuite/20_util/to_chars/long_double.cc: Include <optional>.
            (test01): Simplify verifying the nearby values by using a
            2-iteration loop and a dedicated output buffer to check that the
            nearby values are different.  Factor out the printf-based
            verification into a local function, and check that the leading
            hex digits agree before comparing to the output of printf.  Also
            verify the output by round-tripping it through from_chars.