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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109772] Memory layout optimization of
 std::chrono::hh_mm_ss is wrong
Date: Mon, 08 May 2023 15:18:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109772

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I had a brain fart with this constraint for choosing the subseconds
representation:

        // True if the maximum constructor argument can be represented in _=
Tp.
        template<typename _Tp>
          static constexpr bool __fits
            =3D duration_values<typename _Duration::rep>::max()
                <=3D duration_values<_Tp>::max();

What matters is not the _Duration type, but the precision type. Given a
_Duration like duration<int64_t, ratio<1, 1024>> we can easily fit the maxi=
mum
fractional seconds value of 1023 in a 32-bit integer. But what we actually =
need
to store is not a count of 1023 [1/1024]s units, but 9990234375
[1/10000000000]s units. And 9990234375 doesn't fit in 32 bits.=